“Why is my car engine overheating? What causes an engine to overheat”
I get this question all the time on my websites and especially during the hotter summer months. I made this video to help you determine what might be the cause of your overheating problem and what you can rule out as not causing the issue before you go to your mechanic. This can save you some time, frustration and money by doing so.
“My engine overheats at freeway speeds”
When you are on the freeway you have lots of airflow across the radiator which helps remove the heat that the engine antifreeze/coolant has accumulated from the cooling system. Since the engine is running at a much higher RPM than that at idle, the water pump is spinning around and pumping coolant at a much higher rate as well.
If there is a restriction in the radiator, the coolant will not be allowed to circulate fast enough inside the engine. The coolant will basically be roadblocked inside the radiator due to the restriction. A radiator usually gets build up of rust, minerals and calcium type deposits at the BOTTOM of the radiator. This restriction really can not be removed by “flushing” with a garden hose. In most cases this restriction will require a new radiator.
Think of this type of engine overheating problem like this. You are trying to run a 10 mile marathon, but you have to do it with your mouth taped shut. You can walk with your mouth shut but running at full steam for a long distance requires more air than your nose can provide.
A restricted radiator is the biggest culprit in an engine overheating complaint on the freeway or at higher speeds. Although, if the radiator is low on coolant….that can also be the problem, so check coolant level first.
“I am constantly having to add coolant to my radiator, do I have a leak?”
Anytime I hear of a coolant leak or engine overheating complaint I ALWAYS start my diagnosis with a cooling system pressure test. This is a simple and cheap test that any mechanic or fast lube oil change place should be able to do. Basically, a small hand held air pump (like a bicycle tire pump) is attached to your radiator or coolant overflow bottle and air is pumped inside the cooling system.
This makes finding a coolant leak much easier to do when there is pressure on the system to push out the coolant from the leak. This will also help me determine if there is an internal coolant leak, like from a headgasket as well. There is a small air pressure gauge on the pump that will drop in pressure if there is a leak. If I see a drop in pressure but can not see any external leaks, then there could be an internal leak inside the engine.
“My auto mechanic said I have a head gasket leak in my car”
I get tons, literally tons of emails each week with this question. I would say that most of them are NOT having a headgasket problem but rather a lazy auto mechanic problem who failed to do a proper cooling system pressure test.
Here are a few common symptoms I would expect to see if you had a blown headgasket or any other internal coolant leak.
1. Constantly having to add coolant to the radiator, with no visible external leaks found
2. White steam/smoke coming out the tailpipe, and worse or more smoke at freeway speed
3. Failing a cooling system pressure test, meaning the air pressure gauge drops but there are no external leaks to be seen.
4. An engine miss fire, due to coolant leaking inside the cylinders and fouling out the spark plugs. Lack of overall engine power and performance.
5. Usually a yellow check engine light will be on the dash, since the computer sees the engine miss fire and stores that code inside the computer memory.
6. Lack of engine compression. A manual compression test should be done on each cylinder to prove that there is a compression problem with 1 or more cylinder. This is different from the PRESSURE test which I mentioned above.
7. White powdery residue on the inside tip of the spark plug. When coolant enters the cylinder on the inside of the engine (which It should not be doing) the engine is going to try and burn that coolant, which it will have a very hard time doing. This coolant in the cylinder is what causes the engine to miss fire and produce the steam white smoke out the tailpipe. A white powdery residue will some times form on the internal engine tip of the spark plug.
“My car engine overheats at idle and at stop signs, but not on the freeway”
This type of complaint is usually due to an electric cooling fan problem or a lack of coolant inside the radiator.
Most cars today have an electric cooling fan behind or in front of the radiator. Some also have a fan for the A/C system, so you might have 2 electric fan motors.
The ENGINE electric cooling fan should cycle on and off as the engine temperature heats up and the cooling fan sensor inside the bottom of the radiator (usually found there) tells the engine computer that the coolant is too hot and to turn on the electric fan.
If the engine is running hot or overheating at idle speed and the electric fan is not ON, that is a problem. You could have a bad coolant temperature sensor which is not telling the computer to turn on the fan, or you could have a bad electric fan motor.
I have been able to take a broom stick and gently tap on and push the fan blade to get the electric motor to start working. Do not use your fingers or hand to do this. Low coolant level inside the radiator can also be an issue here, so always start your diagnosis with checking the coolant level first.
Think of this issue this way. If you are out in the 102 degree sun, you will get hot and sweaty pretty quickly. If you sit under a ceiling fan it makes you feel cooler, although the temperature is basically the same. The engine needs airflow across the radiator while it is at idle and stops, or it will run hot. The electric fan motor is the main culprit in overheating at stops and slow speeds.
Do NOT use a radiator stop leak additive! I have to admit the temptation to use a radiator stop leak additive is very real, but I have yet to see one that really works well on cars of today. If you have an older car (before 1980) with a real metal radiator and there is a leak at the radiator, BARS radiator stop leak can work, and would be worth a try.
Some other things to consider, and their symptoms
- A bad thermostat – its pretty rare to have a bad thermostat, although possible but I would not jump to conclusions and replace it for an overheating issue without ruling out the other stuff first.
- A bad water pump that is not pumping – I have seen this ONE time, the cooling system was sooooooo rusty due to neglect, that the impeller on the water pump had literally rusted away so there was nothing left to pump the coolant through the engine
- Bad radiator cap – the cap on the radiator is vented, and will allow any excess pressure inside the system to vent to the atmosphere. I cap can leak, which will not allow the system to reach it’s desired pressure which can cause overheating. A cap can also close shut and not allow the system to vent. Usually the upper radiator hose will swell greatly in size if this is happening. I like to replace the radiator cap during any cooling system repair just as maintenance.
- An air pocket inside the cooling system – if you have done some kind of cooling system repair (replace the water pump or radiator for example) there is a possibility that an air pocket has been trapped inside. This can cause a low coolant issue and a cavitation problem with the water pump. Some cars have a bleeder screw near the thermostat to help you remove the air pocket from the system.
I hope this helps you determine what is the cause of your car engine overheating.
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