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An engine overheats when the cooling system cannot pick up the heat, move it, or give it to the air. The same parts do that work on a petrol engine and a diesel: coolant, a pump, a thermostat, a radiator, hoses, a pressure cap and, on engines that need it, a fan. The failures that forums group under “coolant problems” and under “it overheated” are this system, plus two internal leaks that masquerade as it. One is the head gasket, or a cracked head. The other, on many diesels and on some direct-injection petrol engines, is an EGR cooler. This page is the shared diagnosis. Common diesel engine problems and common petrol engine problems both start here when the complaint is temperature or coolant loss.
What the temperature is actually saying
The gauge, or the coolant-temperature reading on a scan tool, is one sensor in one place. It can read high because the coolant there is high, because the sensor is wrong, or because the coolant has boiled locally and the sensor is in steam. It can read normal while a cylinder head is already in trouble if the coolant is not flowing past the sensor. A scan-tool temperature that swings as the engine runs is a classic sign of air at the sensor: the element sees coolant, then a pocket of air, then coolant again. That pattern is an air lock or a low level until proved otherwise.
There is no single opening temperature to quote. The thermostat rating and the fan-on temperature are calibration data for that engine. A map-controlled thermostat can also be heated by the controller so that it opens before the coolant alone would make it open. Gates notes that this sort of thermostat can fail like any other, stuck open or stuck closed, and can also fail because the electrical command never arrives.
Thermostat, pump, radiator, cap
Gates’ description of the two thermostat failures is the split to keep:
Stuck closed. Coolant does not reach the radiator. The engine overheats. The hose on the radiator side of the thermostat stays cool while the engine side is hot. A radiator blocked with scale, or a hose kinked shut, imitates this. Feel and temperature on both sides of the restriction before the thermostat is convicted.
Stuck open. Coolant circulates through the radiator all the time. The engine runs cool, uses more fuel, produces more emissions, and the heater is weak. On a diesel this also keeps exhaust temperature down, so a particulate-filter regeneration may never be allowed to start. A cool engine with a filter warning is a reason to look at the thermostat, not only at the filter.
The pump has to move the coolant, not merely spin. A pulley that turns does not prove the impeller is attached or intact. Corrosion and a plastic impeller that has come off the shaft both leave a pump that looks installed and does not circulate. External leaks at the weep hole matter, but an overheating engine with a dry pump is not a healthy pump. Confirming flow, once the thermostat should be open, means watching movement in the header tank or measuring a temperature rise across the radiator. On some engines the impeller cannot be seen without removing the pump. Rule out the tests that do not need that first.
The radiator has to be able to give the heat up. Mats of debris on the outside, internal scaling, and a fan that does not run at the temperature the calibration demands all show as overheating in traffic or at idle, and less so on a fast road where ram air is doing the fan’s job. A fan that runs all the time is a different fault, often a sensor, a relay or a controller that has decided the engine is hot.
The pressure cap is part of the boiling point. The cooling-system write-up in Motor’s July 2008 issue states the failure directly: if the system cannot hold pressure, coolant boils in the cylinder head and against the cylinder walls, vapour pockets form, and the engine overheats quickly. A cap test and a system pressure test are the checks. They also find the hose, the radiator and the heater-matrix leaks that a visual inspection misses when the leak only opens once the system is hot and pressurised. Test at the pressure the cap is rated for. A higher pressure is a way to burst a sound radiator.
Coolant itself
Low level is both a cause and a clue. A system that is low because of an external leak will overheat, and the leak is the repair. A system that is low with a dry engine bay is losing coolant into the cylinders, the oil, the heater matrix inside the car, or, where fitted, the EGR cooler. Sweet smell in the exhaust, white vapour that continues after warm-up, a wet footwell, or a rising oil level with emulsion that is actually coolant are the branches. Emulsion under the oil cap after short trips is often condensation. It becomes evidence of mixing when it is in the oil on the dipstick, or when coolant is demonstrably leaving the system. The gasket tests are on the head gasket page.
Use the coolant the manufacturer specifies. Mixing incompatible chemistries can drop the inhibitors out of solution and leave a system that looks full and corrodes from the inside. A system filled with plain water, or with the wrong concentration, boils sooner and freezes sooner. Neither mistake shows up as a single fault code.
Air left in the system after a repair parks in the high points, stops circulation through the heater or through one bank, and produces exactly the swinging temperature reading above. Some engines have bleed screws. Some need a vacuum fill. The procedure is the vehicle’s, because the high point is not in the same place on every head.
Internal leaks that look like a cooling fault
Combustion gas in the coolant heats it from the inside and pushes it out of the header tank, often within a few minutes of a cold start. The thermostat, the pump and the radiator can all be doing their jobs. Engine Australia’s bulletin SB041 lists the places a leak-down test sends the air: the intake, the exhaust, the oil filler, a neighbouring cylinder, or bubbles in the radiator. Bubbles in the radiator mean a path from the combustion chamber into the coolant, which may be the gasket, a cracked head or a cracked block. An EGR cooler is a further path on engines that have one, because its core is the wall between exhaust gas and coolant. Isolate it before the head comes off. The method is on the EGR page and the gasket page.
Overheating is also how gaskets and heads are damaged in the first place. A warped head will not seal a new gasket. Flatness and a crack test belong to the repair once the head is off, not to a guess from the gauge. Burnt exhaust valves are part of the same thermal story: a valve that cannot shed heat into its seat runs hot, and a cooling system that is already marginal is one of the reasons.
A usable order of tests
Level, visible leaks, the colour and the oil interface. Note whether the heater works. A dead heater with a hot engine is often air, a blocked heater matrix, or a blend door, and those are not the same repair.
Pressure-test the system and the cap cold, at the specified pressure. Watch for a drop and for the leak that appears only under pressure.
From a cold start, compare engine temperature with the radiator-side hose. It should stay relatively cool and then heat once the thermostat opens. No heat to the radiator with a rising engine temperature is a closed thermostat, a blocked radiator or no pump flow. Heat to the radiator immediately is a thermostat that is already open.
Confirm the fan runs when the calibration says it should, and that the radiator core is not matted on the outside.
If the level falls with no external leak, or the header tank pressurises early, run a combustion-gas test and a leak-down test, and include the EGR cooler where one is fitted.
After any repair that drained the system, bleed it by the vehicle procedure and watch the temperature reading for air.
Sources
Gates, engine cooling troubleshooting manual. Thermostat stuck closed and stuck open, the effect of a cool-running engine on fuel use and emissions, and electrically assisted thermostats.
Motor, July 2008, cooling-system diagnosis. Loss of system pressure allowing coolant to boil in the head, air pockets, and pump impellers that cannot be judged from the pulley.
Engine Australia, service bulletin SB041. Compression and leak-down paths, including bubbles in the radiator.
HELLA, cooling system check. Thermostat temperature drop across the housing, and confirming the pump impeller is secure on the shaft.
RAC, engine smoke. Coolant entering the combustion chamber after overheating, seen as white exhaust, and emulsion where oil and coolant mix.
Open each version to inspect formatting, tables and photographs.
Revision 1: Engine overheating and coolant faults
An engine overheats when the cooling system cannot pick up the heat, move it, or give it to the air. The same parts do that work on a petrol engine and a diesel: coolant, a pump, a thermostat, a radiator, hoses, a pressure cap and, on engines that need it, a fan. The failures that forums group under “coolant problems” and under “it overheated” are this system, plus two internal leaks that masquerade as it. One is the head gasket, or a cracked head. The other, on many diesels and on some direct-injection petrol engines, is an EGR cooler. This page is the shared diagnosis. Common diesel engine problems and common petrol engine problems both start here when the complaint is temperature or coolant loss.
What the temperature is actually saying
The gauge, or the coolant-temperature reading on a scan tool, is one sensor in one place. It can read high because the coolant there is high, because the sensor is wrong, or because the coolant has boiled locally and the sensor is in steam. It can read normal while a cylinder head is already in trouble if the coolant is not flowing past the sensor. A scan-tool temperature that swings as the engine runs is a classic sign of air at the sensor: the element sees coolant, then a pocket of air, then coolant again. That pattern is an air lock or a low level until proved otherwise.
There is no single opening temperature to quote. The thermostat rating and the fan-on temperature are calibration data for that engine. A map-controlled thermostat can also be heated by the controller so that it opens before the coolant alone would make it open. Gates notes that this sort of thermostat can fail like any other, stuck open or stuck closed, and can also fail because the electrical command never arrives.
Thermostat, pump, radiator, cap
Gates’ description of the two thermostat failures is the split to keep:
- Stuck closed. Coolant does not reach the radiator. The engine overheats. The hose on the radiator side of the thermostat stays cool while the engine side is hot. A radiator blocked with scale, or a hose kinked shut, imitates this. Feel and temperature on both sides of the restriction before the thermostat is convicted.
- Stuck open. Coolant circulates through the radiator all the time. The engine runs cool, uses more fuel, produces more emissions, and the heater is weak. On a diesel this also keeps exhaust temperature down, so a particulate-filter regeneration may never be allowed to start. A cool engine with a filter warning is a reason to look at the thermostat, not only at the filter.
The pump has to move the coolant, not merely spin. A pulley that turns does not prove the impeller is attached or intact. Corrosion and a plastic impeller that has come off the shaft both leave a pump that looks installed and does not circulate. External leaks at the weep hole matter, but an overheating engine with a dry pump is not a healthy pump. Confirming flow, once the thermostat should be open, means watching movement in the header tank or measuring a temperature rise across the radiator. On some engines the impeller cannot be seen without removing the pump. Rule out the tests that do not need that first.
The radiator has to be able to give the heat up. Mats of debris on the outside, internal scaling, and a fan that does not run at the temperature the calibration demands all show as overheating in traffic or at idle, and less so on a fast road where ram air is doing the fan’s job. A fan that runs all the time is a different fault, often a sensor, a relay or a controller that has decided the engine is hot.
The pressure cap is part of the boiling point. The cooling-system write-up in Motor’s July 2008 issue states the failure directly: if the system cannot hold pressure, coolant boils in the cylinder head and against the cylinder walls, vapour pockets form, and the engine overheats quickly. A cap test and a system pressure test are the checks. They also find the hose, the radiator and the heater-matrix leaks that a visual inspection misses when the leak only opens once the system is hot and pressurised. Test at the pressure the cap is rated for. A higher pressure is a way to burst a sound radiator.
Coolant itself
Low level is both a cause and a clue. A system that is low because of an external leak will overheat, and the leak is the repair. A system that is low with a dry engine bay is losing coolant into the cylinders, the oil, the heater matrix inside the car, or, where fitted, the EGR cooler. Sweet smell in the exhaust, white vapour that continues after warm-up, a wet footwell, or a rising oil level with emulsion that is actually coolant are the branches. Emulsion under the oil cap after short trips is often condensation. It becomes evidence of mixing when it is in the oil on the dipstick, or when coolant is demonstrably leaving the system. The gasket tests are on the head gasket page.
Use the coolant the manufacturer specifies. Mixing incompatible chemistries can drop the inhibitors out of solution and leave a system that looks full and corrodes from the inside. A system filled with plain water, or with the wrong concentration, boils sooner and freezes sooner. Neither mistake shows up as a single fault code.
Air left in the system after a repair parks in the high points, stops circulation through the heater or through one bank, and produces exactly the swinging temperature reading above. Some engines have bleed screws. Some need a vacuum fill. The procedure is the vehicle’s, because the high point is not in the same place on every head.
Internal leaks that look like a cooling fault
Combustion gas in the coolant heats it from the inside and pushes it out of the header tank, often within a few minutes of a cold start. The thermostat, the pump and the radiator can all be doing their jobs. Engine Australia’s bulletin SB041 lists the places a leak-down test sends the air: the intake, the exhaust, the oil filler, a neighbouring cylinder, or bubbles in the radiator. Bubbles in the radiator mean a path from the combustion chamber into the coolant, which may be the gasket, a cracked head or a cracked block. An EGR cooler is a further path on engines that have one, because its core is the wall between exhaust gas and coolant. Isolate it before the head comes off. The method is on the EGR page and the gasket page.
Overheating is also how gaskets and heads are damaged in the first place. A warped head will not seal a new gasket. Flatness and a crack test belong to the repair once the head is off, not to a guess from the gauge. Burnt exhaust valves are part of the same thermal story: a valve that cannot shed heat into its seat runs hot, and a cooling system that is already marginal is one of the reasons.
A usable order of tests
- Level, visible leaks, the colour and the oil interface. Note whether the heater works. A dead heater with a hot engine is often air, a blocked heater matrix, or a blend door, and those are not the same repair.
- Pressure-test the system and the cap cold, at the specified pressure. Watch for a drop and for the leak that appears only under pressure.
- From a cold start, compare engine temperature with the radiator-side hose. It should stay relatively cool and then heat once the thermostat opens. No heat to the radiator with a rising engine temperature is a closed thermostat, a blocked radiator or no pump flow. Heat to the radiator immediately is a thermostat that is already open.
- Confirm the fan runs when the calibration says it should, and that the radiator core is not matted on the outside.
- If the level falls with no external leak, or the header tank pressurises early, run a combustion-gas test and a leak-down test, and include the EGR cooler where one is fitted.
- After any repair that drained the system, bleed it by the vehicle procedure and watch the temperature reading for air.
Sources
- Gates, engine cooling troubleshooting manual. Thermostat stuck closed and stuck open, the effect of a cool-running engine on fuel use and emissions, and electrically assisted thermostats.
- Motor, July 2008, cooling-system diagnosis. Loss of system pressure allowing coolant to boil in the head, air pockets, and pump impellers that cannot be judged from the pulley.
- Engine Australia, service bulletin SB041. Compression and leak-down paths, including bubbles in the radiator.
- HELLA, cooling system check. Thermostat temperature drop across the housing, and confirming the pump impeller is secure on the shaft.
- RAC, engine smoke. Coolant entering the combustion chamber after overheating, seen as white exhaust, and emulsion where oil and coolant mix.
Revision 1: Engine overheating and coolant faults
An engine overheats when the cooling system cannot pick up the heat, move it, or give it to the air. The same parts do that work on a petrol engine and a diesel: coolant, a pump, a thermostat, a radiator, hoses, a pressure cap and, on engines that need it, a fan. The failures that forums group under “coolant problems” and under “it overheated” are this system, plus two internal leaks that masquerade as it. One is the head gasket, or a cracked head. The other, on many diesels and on some direct-injection petrol engines, is an EGR cooler. This page is the shared diagnosis. Common diesel engine problems and common petrol engine problems both start here when the complaint is temperature or coolant loss.
What the temperature is actually saying
The gauge, or the coolant-temperature reading on a scan tool, is one sensor in one place. It can read high because the coolant there is high, because the sensor is wrong, or because the coolant has boiled locally and the sensor is in steam. It can read normal while a cylinder head is already in trouble if the coolant is not flowing past the sensor. A scan-tool temperature that swings as the engine runs is a classic sign of air at the sensor: the element sees coolant, then a pocket of air, then coolant again. That pattern is an air lock or a low level until proved otherwise.
There is no single opening temperature to quote. The thermostat rating and the fan-on temperature are calibration data for that engine. A map-controlled thermostat can also be heated by the controller so that it opens before the coolant alone would make it open. Gates notes that this sort of thermostat can fail like any other, stuck open or stuck closed, and can also fail because the electrical command never arrives.
Thermostat, pump, radiator, cap
Gates’ description of the two thermostat failures is the split to keep:
- Stuck closed. Coolant does not reach the radiator. The engine overheats. The hose on the radiator side of the thermostat stays cool while the engine side is hot. A radiator blocked with scale, or a hose kinked shut, imitates this. Feel and temperature on both sides of the restriction before the thermostat is convicted.
- Stuck open. Coolant circulates through the radiator all the time. The engine runs cool, uses more fuel, produces more emissions, and the heater is weak. On a diesel this also keeps exhaust temperature down, so a particulate-filter regeneration may never be allowed to start. A cool engine with a filter warning is a reason to look at the thermostat, not only at the filter.
The pump has to move the coolant, not merely spin. A pulley that turns does not prove the impeller is attached or intact. Corrosion and a plastic impeller that has come off the shaft both leave a pump that looks installed and does not circulate. External leaks at the weep hole matter, but an overheating engine with a dry pump is not a healthy pump. Confirming flow, once the thermostat should be open, means watching movement in the header tank or measuring a temperature rise across the radiator. On some engines the impeller cannot be seen without removing the pump. Rule out the tests that do not need that first.
The radiator has to be able to give the heat up. Mats of debris on the outside, internal scaling, and a fan that does not run at the temperature the calibration demands all show as overheating in traffic or at idle, and less so on a fast road where ram air is doing the fan’s job. A fan that runs all the time is a different fault, often a sensor, a relay or a controller that has decided the engine is hot.
The pressure cap is part of the boiling point. The cooling-system write-up in Motor’s July 2008 issue states the failure directly: if the system cannot hold pressure, coolant boils in the cylinder head and against the cylinder walls, vapour pockets form, and the engine overheats quickly. A cap test and a system pressure test are the checks. They also find the hose, the radiator and the heater-matrix leaks that a visual inspection misses when the leak only opens once the system is hot and pressurised. Test at the pressure the cap is rated for. A higher pressure is a way to burst a sound radiator.
Coolant itself
Low level is both a cause and a clue. A system that is low because of an external leak will overheat, and the leak is the repair. A system that is low with a dry engine bay is losing coolant into the cylinders, the oil, the heater matrix inside the car, or, where fitted, the EGR cooler. Sweet smell in the exhaust, white vapour that continues after warm-up, a wet footwell, or a rising oil level with emulsion that is actually coolant are the branches. Emulsion under the oil cap after short trips is often condensation. It becomes evidence of mixing when it is in the oil on the dipstick, or when coolant is demonstrably leaving the system. The gasket tests are on the head gasket page.
Use the coolant the manufacturer specifies. Mixing incompatible chemistries can drop the inhibitors out of solution and leave a system that looks full and corrodes from the inside. A system filled with plain water, or with the wrong concentration, boils sooner and freezes sooner. Neither mistake shows up as a single fault code.
Air left in the system after a repair parks in the high points, stops circulation through the heater or through one bank, and produces exactly the swinging temperature reading above. Some engines have bleed screws. Some need a vacuum fill. The procedure is the vehicle’s, because the high point is not in the same place on every head.
Internal leaks that look like a cooling fault
Combustion gas in the coolant heats it from the inside and pushes it out of the header tank, often within a few minutes of a cold start. The thermostat, the pump and the radiator can all be doing their jobs. Engine Australia’s bulletin SB041 lists the places a leak-down test sends the air: the intake, the exhaust, the oil filler, a neighbouring cylinder, or bubbles in the radiator. Bubbles in the radiator mean a path from the combustion chamber into the coolant, which may be the gasket, a cracked head or a cracked block. An EGR cooler is a further path on engines that have one, because its core is the wall between exhaust gas and coolant. Isolate it before the head comes off. The method is on the EGR page and the gasket page.
Overheating is also how gaskets and heads are damaged in the first place. A warped head will not seal a new gasket. Flatness and a crack test belong to the repair once the head is off, not to a guess from the gauge. Burnt exhaust valves are part of the same thermal story: a valve that cannot shed heat into its seat runs hot, and a cooling system that is already marginal is one of the reasons.
A usable order of tests
- Level, visible leaks, the colour and the oil interface. Note whether the heater works. A dead heater with a hot engine is often air, a blocked heater matrix, or a blend door, and those are not the same repair.
- Pressure-test the system and the cap cold, at the specified pressure. Watch for a drop and for the leak that appears only under pressure.
- From a cold start, compare engine temperature with the radiator-side hose. It should stay relatively cool and then heat once the thermostat opens. No heat to the radiator with a rising engine temperature is a closed thermostat, a blocked radiator or no pump flow. Heat to the radiator immediately is a thermostat that is already open.
- Confirm the fan runs when the calibration says it should, and that the radiator core is not matted on the outside.
- If the level falls with no external leak, or the header tank pressurises early, run a combustion-gas test and a leak-down test, and include the EGR cooler where one is fitted.
- After any repair that drained the system, bleed it by the vehicle procedure and watch the temperature reading for air.
Sources
- Gates, engine cooling troubleshooting manual. Thermostat stuck closed and stuck open, the effect of a cool-running engine on fuel use and emissions, and electrically assisted thermostats.
- Motor, July 2008, cooling-system diagnosis. Loss of system pressure allowing coolant to boil in the head, air pockets, and pump impellers that cannot be judged from the pulley.
- Engine Australia, service bulletin SB041. Compression and leak-down paths, including bubbles in the radiator.
- HELLA, cooling system check. Thermostat temperature drop across the housing, and confirming the pump impeller is secure on the shaft.
- RAC, engine smoke. Coolant entering the combustion chamber after overheating, seen as white exhaust, and emulsion where oil and coolant mix.