Heater Cores Purpose and Function

Most of the heat absorbed from the engine by the cooling system is wasted. Some of this heat, however, is recovered by the vehicle heater. Heated coolant is passed through tubes in the small core of the heater. Air is passed across the heater fins and is then sent to the passenger compartment. In some vehicles, the heater and air conditioning work in series to maintain vehicle compartment temperature.

Heater Problem Diagnosis

When the heater does not produce the desired amount of heat, many owners and technicians replace the thermostat before doing any other troubleshooting. It is true that a defective thermostat is the reason for the engine not to reach normal operating temperature, but there are many other causes besides a defective thermostat that can result in lack of heat from the heater. To determine the exact cause, follow this procedure.

A typical heater core installed in a heating, ventilation, and air-conditioning (HVAC) housing assembly.

STEP 1

After the engine has been operated, feel the upper radiator hose. If the engine is up to proper operating temperature, the upper radiator hose should be too hot to hold. The hose should also be pressurized.

1. If the hose is not hot enough, replace the thermostat.
2. If the hose is not pressurized, test or replace the radiator pressure cap if it will not hold the specified pressure.
3. If okay, see step 2.

STEP 2

With the engine running, feel both heater hoses. (The heater should be set to the maximum heat position.) Both hoses should be too hot to hold. If both hoses are warm (not hot) or cool, check the heater control valve for proper operation (if equipped). If one hose is hot and the other (return) is just warm or cool, remove both hoses from the heater core or engine and flush the heater core with water from a garden hose.

STEP 3

If both heater hoses are hot and there is still a lack of heating concern, then the fault is most likely due to an airflow blend door malfunction. Check service information for the exact procedure to follow.

HINT: Heat from the heater that “comes and goes” is most likely the result of low coolant level. Usually with the engine at idle, there is enough coolant flow through the heater. At higher engine speeds, however, the lack of coolant through the heads and block prevents sufficient flow through the heater.

Cooling System Testing

Visual Inspection

Many cooling system faults can be found by performing a thorough visual inspection. Items that can be

inspected visually include:

• Water pump drive belt for tension or faults
• Cooling fan for faults
• Heater and radiator hoses for condition and leaks
• Coolant overflow or surge tank coolant level
• Evidence of coolant loss
• Radiator condition

A heavily corroded radiator from a vehicle that was overheating. A visual inspection discovered that the corrosion had eaten away many of the cooling fins, yet did not leak. This radiator was replaced and it solved the overheating problem.

Pressure Testing

Pressure testing using a hand-operated pressure tester is a quick and easy cooling system test. The radiator cap is removed (engine cold!) and the tester is attached in the place of the radiator cap. By operating the plunger on the pump, the entire cooling system is pressurized.

• CAUTION: Do not pump up the pressure beyond that specified by the vehicle manufacturer. Most systems should not be pressurized beyond 14 PSI (100 kPa). If a greater pressure is used, it may cause the water pump, radiator, heater core, or hoses to fail.

If the cooling system is free from leaks, the pressure should stay and not drop. If the pressure drops, look for evidence of leaks anywhere in the cooling system, including:

1. Heater hoses
2. Radiator hoses
3. Radiator
4. Heater core
5. Cylinder head
6. Core plugs in the side of the block or cylinder head

The pressure cap should be checked for proper operation using a pressure tester as part of the cooling system diagnosis.

Pressure testing should be performed whenever there is a leak or suspected leak. The pressure tester can also be used to test the radiator cap. An adapter is used to connect the pressure tester to the radiator cap. Replace any cap that will not hold pressure.

Pressure testing should be performed whenever there is a leak or suspected leak.

Coolant Dye Leak Testing

One of the best methods to check for a coolant leak is to use a fluorescent dye in the coolant, one that is specifically designed for coolant. Operate the vehicle with the dye in the coolant until the engine reaches normal operating temperature. Use a black light to inspect all areas of the cooling system. When there is a leak, it will be easy to spot because the dye in the coolant will be seen as bright green. Pressure testing the cooling system. A typical hand-operated pressure tester applies pressure equal to the radiator cap pressure. The pressure should hold; if it drops, this indicates a leak somewhere in the cooling system. An adapter is used to attach the pump to the cap to determine if the radiator can hold pressure, and release it when pressure rises above its maximum rated pressure setting.

Use dye specifically made for coolant when checking for leaks using a black light.

Next Steps towards ASE Certification

Now that you’re familiar with Heater Cores: Problem Diagnosis, Inspection, Pressure, and Dye Testing, try out our free Automotive Service Excellence Tests to see how much you know!

Types of Systems

Coolant flows through the engine in one of the following ways.

• Parallel flow system. In the parallel flow system, coolant flows into the block under pressure and then crosses the head gasket to the head through main coolant passages beside each cylinder.
• Series flow system. In the series flow system, the coolant flows around all the cylinders on each bank. All the coolant flows to the rear of the block, where large main coolant passages allow the coolant to flow across the head gasket. The coolant then enters the rear of the heads. In the heads, the coolant flows forward to a crossover passage on the intake manifold outlet at the highest point in the engine cooling passage. This is usually located at the front of the engine. The outlet is either on the heads or in the intake manifold.
• Series-parallel flow system. Some engines use a combination of these two coolant flow systems and call it a seriesparallel flow system. Any steam that develops will go directly to the top of the radiator. In series flow systems, bleed holes or steam slits in the gasket, block, and head perform the function of letting out the steam.

A Chevrolet V-8 Block that shows the large coolant holes and the smaller gas vent or bleed holes that must match the head gasket when the engine is assembled.

Coolant Flow and Head Gasket Design

Most V-type engines use cylinder heads that are interchangeable side to side, but not all engines. Therefore, based on the design of the cooling system and flow through the engine, it is very important to double check that the cylinder head is matched to the block and that the head gasket is installed correctly (end for end) so that all of the cooling passages are open to allow the proper flow of coolant through the system.

Cooling Fans

Electronically Controlled Cooling Fan

Two types of electric cooling fans used on many engines include:

• One two-speed cooling fan
• Two cooling fans (one for normal cooling and one for high heat conditions)

The PCM commands low-speed fans on under the following conditions.

• Engine coolant temperature (ECT) exceeds approximately 223°F (106°C).
• A/C refrigerant pressure exceeds 190 PSI (1,310 kPa).
• After the vehicle is shut off, the engine coolant temperature at key-off is greater than 284°F (140°C) and system voltage is more than 12 volts. The fan(s) will stay on for approximately three minutes.

The PCM commands the high-speed fan on under the following conditions.

• Engine coolant temperature (ECT) reaches 230°F (110°C).
• A/C refrigerant pressure exceeds 240 PSI (1,655 kPa).
• Certain diagnostic trouble codes (DTCs) set.

A typical electric cooling fan assembly showing the radiator and related components.

To prevent a fan from cycling on and off excessively at idle, the fan may not turn off until the ignition switch is moved to the off position or the vehicle speed exceeds approximately 10 mph (16 km/h).

Many rear-wheel-drive vehicles and all transverse engines drive the fan with an electric motor.

• NOTE: Most electric cooling fans are computer controlled. To save energy, most cooling fans are turned off whenever the vehicle is traveling faster than 35 mph (55 km/h). The ram air caused by the vehicle speed is enough to keep the radiator cool. Of course, if the computer senses that the temperature is still too high, the computer will turn on the cooling fan, to “high,” if possible, in an attempt to cool the engine to avoid severe engine damage.

Warning: Some electric cooling fans can come on after the engine is off without warning. Always keep hands and fingers away from the cooling fan blades unless the electrical connector has been disconnected to prevent the fan from coming on. Always follow all warnings and cautions.

Thermostatic Fins

On some rear-wheel-drive vehicles, a thermostatic cooling fan is driven by a belt from the crankshaft. It turns faster as the engine turns faster. Generally, the engine is required to produce more power at higher speeds. Therefore, the cooling system will also transfer more heat. Increased fan speed aids in the required cooling. Engine heat also becomes critical at low engine speeds in traffic where the vehicle moves slowly. The thermostatic fan is designed so that it uses little power at high engine speeds and minimizes noise. Two types of thermostatic fans include:

• Silicone coupling. The silicone coupling fan drive is mounted between the drive pulley and the fan. HINT: When diagnosing an overheating problem, look carefully at the cooling fan. If silicone is leaking, then the fan may not be able to function correctly and should be replaced.
• Thermostatic spring. A second type of thermal fan has a thermostatic spring added to the silicone coupling fan drive. The thermostatic spring operates a valve that allows the fan to freewheel when the radiator is cold. As the radiator warms to about 150°F (65°C), the air hitting the thermostatic spring will cause the spring to change its shape. The new shape of the spring opens a valve that allows the drive to operate like the silicone coupling drive. When the engine is very cold, the fan may operate at high speeds for a short time until the drive fluid warms slightly. The silicone fluid will then flow into a reservoir to let the fan speed drop to idle.

A typical engine-driven thermostatic spring cooling fins

The fan is designed to move enough air at the lowest fan speed to cool the engine when it is at its highest coolant temperature. The fan shroud is used to increase the cooling system efficiency.

Tech Tip: Be Sure to Always Use a Fan Shroud

A fan shroud forces the fan to draw air through the radiator. If a fan shroud is not used, then air is drawn from around the fan and will reduce the airflow through the radiator. Many overheating problems are a result of not replacing the factory shroud after engine work or body repair work to the front of the vehicle.

Next Steps towards ASE Certification

Now that you’re familiar with Coolant Flow in the Engine, try out our free Automotive Service Excellence Tests to see how much you know!

Operation

The water pump (also called a coolant pump) is driven by one of two methods.

• Crankshaft belt
• Camshaft

Coolant recirculates from the radiator to the engine and back to the radiator. Low-temperature coolant leaves the radiator by the bottom outlet. It is pumped into the warm engine block, where it picks up some heat. From the block, the warm coolant flows to the hot cylinder head, where it picks up more heat.

• NOTE: Some engines use reverse cooling. This means that the coolant flows from the radiator to the cylinder head(s) before flowing to the engine block.

A demonstration engine running on a stand showing the amount of coolant flow that actually occurs through the cooling system.

Frequently Asked Quesiton: How much Coolant Can a Water Pump Move?

A typical water pump can move a maximum of about 7,500 gallons (28,000 liters) of coolant per hour, or recirculate the coolant in the engine over 20 times per minute. This means that a water pump could be used to empty a typical private swimming pool in an hour! The slower the engine speed, the less power is consumed by the water pump. However, even at 35 mph (56 km/h), the typical water pump still moves about 2,000 gallons (7,500 liters) per hour or 0.5 gallon (2 liters) per second!

Water pumps are not positive displacement pumps. The water pump is a centrifugal pump that can move a large volume of coolant without increasing the pressure of the coolant. The pump pulls coolant in at the center of the impeller. Centrifugal force throws the coolant outward so that it is discharged at the impeller tips.

As engine speeds increase, more heat is produced by the engine and more cooling capacity is required. The pump impeller speed increases as the engine speed increases to provide extra coolant flow at the very time it is needed.

Coolant flow through the impeller and scroll of a coolant pump for a V-type engine

Coolant leaving the pump impeller is fed through a scroll. The scroll is a smoothly curved passage that changes the fluid flow direction with minimum loss in velocity. The scroll is connected to the front of the engine so as to direct the coolant into the engine block. On V-type engines, two outlets are often used, one for each cylinder bank. Occasionally, diverters are necessary in the water pump scroll to equalize coolant flow between the cylinder banks of a V-type engine in order to equalize the cooling.

Water Pump Service

A worn impeller on a water pump can reduce the amount of coolant flow through the engine.

If the seal of the water pump fails, coolant will leak out of the weep hole. The hole allows coolant to escape without getting trapped and forced into the water pump bearing assembly.

The hole allows coolant to escape without getting trapped and forced into the water pump bearing assembly.

If the bearing is defective, the pump will usually be noisy and will have to be replaced. Before replacing a water pump that has failed because of a loose or noisy bearing, check all of the following:

1. Drive belt tension
2. Bent fan
3. Fan for balance

If the water pump drive belt is too tight, excessive force may be exerted against the pump bearing. If the cooling fan is bent or out of balance, the resulting vibration can damage the water pump bearing.

Tech Tip: Release the Belt Tension before Checking a Water Pump

The technician should release water pump belt tension before checking for water pump bearing looseness. To test a water pump bearing, it is normal to check the fan for movement; however, if the drive belt is tight, any looseness in the bearing will not be felt.

Next Steps towards ASE Certification

Now that you’re familiar with Water Pumps in Automotive Engine Coolant Systems, try out our free Automotive Service Excellence Tests to see how much you know!

Purpose and Function

Excess pressure usually forces some coolant from the system through an overflow. Most cooling systems connect the overflow to a plastic reservoir to hold excess coolant while the system is hot.

When the system cools, the pressure in the cooling system is reduced and a partial vacuum forms. This vacuum pulls the coolant from the plastic container back into the cooling system, keeping the system full. Because of this action, the system is called a coolant recovery system. A vacuum valve allows coolant to reenter the system as the system cools so that the radiator parts will not collapse under the partial vacuum.

The level in the coolant recovery system raises and lowers with engine temperature.

Surge Tank

Some vehicles use a surge tank, which is located at the highest level of the cooling system and holds about 1 quart (1 liter) of coolant. A hose attaches to the bottom of the surge tank to the inlet side of the water pump. A smaller bleed hose attaches to the side of the surge tank to the highest point of the radiator. The bleed line allows some coolant circulation through the surge tank, and air in the system will rise below the radiator cap and be forced from the system if the pressure in the system exceeds the rating of the radiator cap.

Some vehicles use a surge tank, which is located at the highest level of the cooling system, with a radiator cap.

Real World Fix: The Collapsed Radiator Hose Story

An automotive student asked the automotive instructor what brand of radiator hose is the best. Not knowing exactly what to say, the instructor asked if there was a problem with the brand hose used. The student had tried three brands and all of them collapsed when the engine cooled. The instructor then explained that the vehicle needed a new pressure cap and not a new upper radiator hose. The student thought that because the lower hose did not collapse that the problem had to be a fault with the hose. The instructor then explained that the lower radiator hose has a spring inside to keep the lower hose from collapsing due to the lower pressure created at the inlet to the water pump. The radiator cap was replaced and the upper radiator hose did not collapse when the engine cooled.

Next Steps towards ASE Certification

Now that you’re familiar with Coolant Recovery Systems and Surge Tanks, try out our free Automotive Service Excellence Tests to see how much you know!

Types of Radiator Cores

The two types of radiator cores in common use in most vehicles are:

• Serpentine fin core
• Plate fin core

In each of these types, the coolant flows through oval-shaped core tubes. Heat is transferred through the tube wall and soldered joint to cooling fins. The fins are exposed to the air that flows through the radiator, which removes heat from the radiator and carries it away.

The tubes and fins of the radiator core.

Since the 1980s, most radiators have been made from aluminum with nylon-reinforced plastic side tanks. These materials are corrosion resistant, have good heat transferability, and are easily formed.

Core tubes are made from 0.0045 to 0.012 in. (0.1 to 0.3 mm) sheet brass or aluminum, using the thinnest possible materials for each application. The metal is rolled into round tubes and the joints are sealed with a locking seam.

The two basic designs of radiators include:

1. Down-flow radiators. This design was used mostly in older vehicles, where the coolant entered the radiator at the top and flowed downward, exiting the radiator at the bottom.
2. Cross-flow radiators. Most radiators use a cross-flow design, where the coolant flows from one side of the radiator to the opposite side.

A radiator may be either a down-flow or a cross-flow type.

How Radiators Work

The main limitation of heat transfer in a cooling system is in the transfer from the radiator to the air. Heat transfers from the water to the fins as much as seven times faster than heat transfers from the fins to the air, assuming equal surface exposure. The radiator must be capable of removing an amount of heat energy approximately equal to the heat energy of the power produced by the engine. Each horsepower is equivalent to 42 BTUs (10,800 calories) per minute. As the engine power is increased, the heat-removing requirement of the cooling system is also increased.

With a given frontal area, radiator capacity may be increased by increasing the core thickness, packing more material into the same volume, or both. The radiator capacity may also be increased by placing a shroud around the fan so that more air will be pulled through the radiator.

• NOTE: The lower air dam in the front of the vehicle is used to help direct the air through the radiator. If this air dam is broken or missing, the engine may overheat, especially during highway driving due to the reduced airflow through the radiator.

When a transmission oil cooler is used in the radiator, it is placed in the outlet tank, where the coolant has the lowest temperature.

Many vehicles equipped with an automatic transmission use a transmission fluid cooler installed in one of the radiator tanks.

Pressure Caps

Operation

On most radiators the filler neck is fitted with a pressure cap. The cap has a spring-loaded valve that closes the cooling system vent. This causes cooling pressure to build up to the pressure setting of the cap. At this point, the valve will release the excess pressure to prevent system damage. Engine cooling systems are pressurized to raise the boiling temperature of the coolant.

• The boiling temperature will increase by approximately 3°F (1.6°C) for each pound of increase in pressure.
• At sea level, water will boil at 212°F (100°C). With a 15 PSI (100 kPa) pressure cap, water will boil at 257°F (125°C), which is a maximum operating temperature for an engine.

Functions

The specified coolant system temperature serves two functions.

1. It allows the engine to run at an efficient temperature, close to 200°F (93°C), with no danger of boiling the coolant.
2. The higher the coolant temperature, the more heat the cooling system can transfer. The heat transferred by the cooling system is proportional to the temperature difference between the coolant and the outside air. This characteristic has led to the design of small, high-pressure radiators that are capable of handling large quantities of heat. For proper cooling, the system must have the right pressure cap correctly installed.

A vacuum valve is part of the pressure cap and is used to allow coolant to flow back into the radiator when the coolant cools down and contracts.

• NOTE: The proper operation of the pressure cap is especially important at high altitudes. The boiling point of water is lowered by about 1°F for every 550 ft increase in altitude. Therefore, in Denver, Colorado (altitude 5,280 ft), the boiling point of water is about 202°F, and at the top of Pike’s Peak in Colorado (14,110 ft) water boils at 186°F.

Metric Radiator Caps

According to the SAE Handbook, all radiator caps must indicate their nominal (normal) pressure rating. Most original equipment radiator caps are rated at about 14 to 16 PSI (97 to 110 kPa).

The pressure valve maintains the system pressure and allows excess pressure to vent. The vacuum valve allows coolant to return to the system from the recovery tank.

However, many vehicles manufactured in Japan or Europe use radiator pressure indicated in a unit called a bar. One bar is the pressure of the atmosphere at sea level, or about 14.7 PSI. The conversions can be used when replacing a radiator cap, to make certain it matches the pressure rating of the original.

• NOTE: Many radiator repair shops use a 7 PSI (0.5 bar) radiator cap on a repaired radiator. A 7 PSI cap can still provide boil protection of 21°F (3°F x 7 PSI = 21°F) above the boiling point of the coolant. For example, if the boiling point of the antifreeze coolant is 223°F, then 21°F is added for the pressure cap, and boilover will not occur until about 244°F (223°F + 21°F = 244°F). Even though this lower pressure radiator cap provides some protection and will also help protect the radiator repair, the coolant can still boil before the “hot” dash warning light comes on and, therefore, should not be used. In addition, the lower pressure in the cooling system could cause cavitation to occur and damage the water pump. For best results, always follow the vehicle manufacturer’s recommended radiator cap.

Working Better Under Pressure

A problem that sometimes occurs with a high-pressure cooling system involves the water pump. For the pump to function, the inlet side of the pump must have a lower pressure than its outlet side. If inlet pressure is lowered too much, the coolant at the pump inlet can boil, producing vapor. The pump will then spin the coolant vapors and not pump coolant. This condition is called pump cavitation. Therefore, a radiator cap could be the cause of an overheating problem. A pump will not pump enough coolant if not kept under the proper pressure for preventing vaporization of the coolant.

Next Steps towards ASE Certification

Now that you’re familiar with Radiators in Automotive Engines, try out our free Automotive Service Excellence Tests to see how much you know!

Two important things about a thermostat include :

1. An overheating engine may result from a faulty thermostat.
2. An engine that does not get warm enough always indicates a faulty thermostat.

To replace the thermostat, coolant will have to be drained from the radiator drain petcock to lower the coolant level below the thermostat. It is not necessary to completely drain the system. The hose should be removed from the thermostat housing neck and then the housing removed to expose the thermostat.

Failure to set the thermostat into the recessed groove will cause the housing to become tilted when tightened.

The gasket flanges of the engine and thermostat housing should be cleaned, and the gasket surface of the housing must be flat. The thermostat should be placed in the engine with the sensing pellet toward the engine. Make sure that the thermostat position is correct, and install the thermostat housing with a new gasket or O-ring.

CAUTION: Failure to set the thermostat into the recessed groove will cause the housing to become tilted when tightened. If this happens and the housing bolts are tightened, the housing will usually crack, creating a leak.
The upper hose should then be installed and the system refilled. Install the correct size of radiator hose clamp.

Next Steps towards ASE Certification

Now that you’re familiar with Thermostat Replacement, try out our free Automotive Service Excellence Tests to see how much you know!

Cooling System

Purpose and Function

Satisfactory cooling system operation depends on the design and operating conditions of the system. The design is based on heat output of the engine, radiator size, type of coolant, size of water pump (coolant pump), type of fan, thermostat, and system pressure. The cooling system must allow the engine to warm up to the required operating temperature as rapidly as possible and then maintain that temperature.

Peak combustion temperatures in the engine run from 4,000°F to 6,000°F (2,200°C to 3,300°C). The combustion temperatures will average between 1,200°F and 1,700°F (650°C and 925°C). Continued temperatures as high as this would weaken engine parts, so heat must be removed from the engine. The cooling system keeps the head and cylinder walls at a temperature that is within the range for maximum efficiency. The cooling system removes about one-third of the heat created in the engine. Another third escapes to the exhaust system.

Typical Combustion and Exhaust Temperatures inside an engine

Low Temperature Engine Problems

Engine operating temperatures must be above a minimum temperature for proper engine operation. If the coolant temperature does not reach the specified temperature as determined by the thermostat, then the following engine-related faults can occur.

• A P0128 diagnostic trouble code (DTC) can be set. This code indicates “coolant temperature below thermostat regulating temperature,” which is usually caused by a defective thermostat staying open or partially open.
• Moisture created during the combustion process can condense and flow into the oil. For each gallon of fuel used, moisture equal to a gallon of water is produced. The condensed moisture combines with unburned hydrocarbons and additives to form carbonic acid, sulfuric acid, nitric acid, hydrobromic acid, and hydrochloric acid.

To reduce cold engine problems and to help start engines in cold climates, most manufacturers offer block heaters as an option. These block heaters are plugged into household current (110 volts AC) and the heating element warms the coolant.

High Temperature Engine Problems

Maximum temperature limits are required to protect the engine. Higher than normal temperatures can cause the following engine-related issues.

• High temperatures will oxidize the engine oil producing hard carbon and varnish. The varnish will cause the hydraulic valve lifter plungers to stick. Higher than normal temperatures will also cause the oil to become thinner (lower viscosity than normal). Thinned oil will also get into the combustion chamber by going past the piston rings and through valve guides to cause excessive oil consumption.
• The combustion process is very sensitive to temperature. High coolant temperatures raise the combustion temperatures to a point that may cause detonation (also called spark knock or ping) to occur.

Cooling System Operation

Purpose and Function

Coolant flows through the engine, where it picks up heat. It then flows to the radiator, where the heat is given up to the outside air. The coolant continually recirculates through the cooling system, as illustrated in

Cooling System Operation

The temperature of the coolant rises as much as 15°F (8°C) as it goes through the engine and cools as it goes through the radiator. The coolant flow rate may be as high as 1 gallon (4 liters) per minute for each horsepower the engine produces.

Coolant circulates through the water jackets in the engine block and cylinder head

Hot coolant comes out of the thermostat housing on the top of the engine on most engines. The engine coolant outlet is connected to the radiator by the upper radiator hose and clamps. The coolant in the radiator is cooled by air flowing through the radiator. As the coolant moves through the radiator, it cools. The cooler coolant leaves the radiator through an outlet and the lower radiator hose, and then flows to the inlet side of the water pump, where it is recirculated through the engine.

• NOTE: Some newer engine designs such as Chrysler’s 4.7 liter V-8 and General Motor’s 4.8, 5.3, 5.7, and 6.0 liter V-8s place the thermostat on the inlet side of the water pump. As the cooled coolant hits the thermostat, the thermostat closes until the coolant temperature again causes it to open. Placing the thermostat in the inlet side of the water pump therefore reduces the rapid temperature changes that could cause stress in the engine, especially if aluminum heads are used with a cast iron block.

Radiators are designed for the maximum rate of heat transfer using minimum space. Cooling airflow through the radiator is aided by a belt- or electric motor–driven cooling fan.

Next Steps towards ASE Certification

Now that you’re familiar with Cooling System Operation and Diagnosis, try out our free Automotive Service Excellence Tests to see how much you know!

Diesel engines can be diagnosed using a scan tool in most cases, because most of the pressure sensors values can be displayed. Common faults include:

• Hard starting
• No start
• Extended cranking before starting

Compression Testing

Compression Gauge that is designed for the higher compression rate of a diesel engine

A compression test is fundamental for determining the mechanical condition of a diesel engine. Worn piston rings can cause low power and excessive exhaust smoke. To test the compression on a diesel engine, the following will have to be done.

1. Remove the glow plug (if equipped) or the injector.
2. Use a diesel compression gauge, as the compression is too high to use a gasoline engine compression gauge.

A diesel engine should produce at least 300 PSI (2,068 kPa) of compression pressure and all cylinders should be within 50 PSI (345 kPa) of each other.

Glow Plug Resistance Balance Test

Glow plugs increase in resistance as their temperature increases. All glow plugs should have about the same resistance when checked with an ohmmeter. A similar test of the resistance of the glow plugs can be used to detect a weak cylinder. This test is particularly helpful on a diesel engine that is not computer controlled. To test for even cylinder balance using glow plug resistance, perform the following on a warm engine.

1. Unplug, measure, and record the resistance of all glow plugs.
2. With the wires still removed from the glow plugs, start the engine.
3. Allow the engine to run for several minutes to allow the combustion inside the cylinder to warm the glow plugs.
4. Measure the plugs and record the resistance of all glow plugs.
5. The resistance of all glow plugs should be higher than at the beginning of the test. A glow plug that is in a cylinder that is not firing correctly will not increase in resistance as much as the others.
6. Another test is to measure exhaust manifold temperature at each exhaust port using an infrared thermometer or a pyrometer. Misfiring cylinders will run cold.

Injector Pop Testing

Typical Pop Tester Used to Check the Spray Pattern of a Diesel Injector

A pop tester is a device used for checking a diesel injector nozzle for proper spray pattern. The handle is depressed and pop-off pressure is displayed on the gauge.

The spray pattern should be a hollow cone, but will vary depending on design. The nozzle should also be tested for leakage (dripping of the nozzle) while under pressure. If the spray pattern is not correct, then cleaning, repairing, or replacing the injector nozzle may be necessary.

Next Steps towards ASE Certification

Now that you’re familiar with Compression Testing, Glow Plug Resistance Balance, and Injector Pop Testing, try out our free Automotive Service Excellence Tests to see how much you know!

Purpose and Function

There is a normal operating temperature range between low-temperature and high-temperature extremes. The thermostat controls the minimum normal temperature. The thermostat is a temperature-controlled valve placed at the engine coolant outlet on most engines.

Thermostat Operation

An encapsulated wax-based plastic pellet heat sensor is located on the engine side of the thermostatic valve. As the engine warms, heat swells the heat sensor.

A cross section of a typical wax-actuated thermostat showing the position of the wax pellet and spring

A mechanical link, connected to the heat sensor, opens the thermostat valve. As the thermostat begins to open, it allows some coolant to flow to the radiator, where it is cooled. The remaining part of the coolant continues to flow through the bypass, thereby bypassing the thermostat and flowing back through the engine.

(a) When the engine is cold, the coolant flows through the bypass. (b) When the thermostat opens, the coolant can flow to the radiator

The rated temperature of the thermostat indicates the temperature at which the thermostat starts to open. The thermostat is fully open at about 20°F higher than its opening temperature.
If the radiator, water pump, and coolant passages are functioning correctly, the engine should always be operating within the opening and fully open temperature range of the thermostat.

Thermostat Testing

Thermostat Testing

There are three basic methods used to check the operation of the thermostat.

1.  Hot water method. If the thermostat is removed from the vehicle and is closed, insert a 0.015 in. (0.4 mm) feeler gauge in the opening so that the thermostat will hang on the feeler gauge. The thermostat should then be suspended by the feeler gauge in a container of water or coolant along with a thermometer. The container should be heated until the thermostat opens enough to release and fall from the feeler gauge. The temperature at which the thermostat falls is the opening temperature of the thermostat. If it is within 5°F (4°C) of the temperature stamped on the thermostat, the thermostat is satisfactory for use. If the temperature difference is greater, the thermostat should be replaced.

Checking the opening temperature of a thermostat

2. Infrared thermometer method. An infrared thermometer (also called a pyrometer) can be used to measure the temperature of the coolant near the thermostat. The area on the engine side of the thermostat should be at the highest temperature that exists in the engine. A properly operating cooling system should cause the pyrometer to read as follows:

• As the engine warms, the temperature reaches near thermostat opening temperature.
• As the thermostat opens, the temperature drops just as the thermostat opens, sending coolant to the radiator.
• As the thermostat cycles, the temperature should range between the opening temperature of the thermostat and 20°F (11°C) above the opening temperature.

NOTE: If the temperature rises higher than 20°F (11°C) above the opening temperature of the thermostat, inspect the cooling system for a restriction or low coolant flow. A clogged radiator could also cause the excessive temperature rise.

3. Scan tool method. A scan tool can be used on many vehicles to read the actual temperature of the coolant as detected by the engine coolant temperature (ECT) sensor. Although the sensor or the wiring to and from the sensor may be defective, at least the scan tool can indicate what the computer “thinks” is the engine coolant temperature.

Next Steps towards ASE Certification

Now that you’re familiar with Diesel Engine Thermostats, try out our free Automotive Service Excellence Tests to see how much you know!

Although some exhaust smoke is considered normal operation for many diesel engines, especially older units, the cause of excessive exhaust smoke should be diagnosed and repaired.

Black Smoke

Black exhaust smoke is caused by incomplete combustion because of a lack of air or a fault in the injection system that could cause an excessive amount of fuel in the cylinders. Items that should be checked include the following:

• Fuel specific gravity (API gravity)
• Injector balance test to locate faulty injectors using a scan tool
• Proper operation of the engine coolant temperature (ECT) sensor
• Proper operation of the fuel rail pressure (FRP) sensor
• Restrictions in the intake or turbocharger
• Engine oil usage

White Smoke

White exhaust smoke occurs most often during cold engine starts because the smoke is usually condensed fuel droplets. White exhaust smoke is also an indication of cylinder misfire on a warm engine. The most common causes of white exhaust smoke include:

• Inoperative glow plugs
• Low engine compression
• Incorrect injector spray pattern
• Coolant leak into the combustion chamber

Gray or Blue Smoke

Blue exhaust smoke is usually due to oil consumption caused by worn piston rings, scored cylinder walls, or defective valve stem seals. Gray or blue smoke can also be caused by a defective injector(s).