Car Overheating Causes Explained

Hello Austin, thanks for putting this site together, it has been a huge help for me the past few years. You are my go-to guy when it comes to my car problems.

My question is, with my daughter’s Honda Accord. The engine seems to overheat, but only sometimes. I was told it might be a head gasket issue because they can not seem to find any external leaks. What should I do next, this is very frustrating for me and her and she is back home for the summer until school starts again. Would love to fix this for her.

Thanks in advance for your time.

Hi there Don,

I am glad my website has been a help to you, thank you for your support over the years. Car overheating can really be simplified in 4 separate scenarios, which you will need to help me identify which one seems to be happening.

1. A coolant leak somewhere. This can be an external coolant leak, like from a radiator hose, the radiator itself or water pump etc. etc. In ANY engine overheating complaint I always start off with a cooling system pressure test to rule out a possible leak as the culprit.

The pressure test is simple and cheap to do. It is basically a small hand operated air pump similar to a bicycle tire pump. It screws onto the radiator where the cap goes, then the mechanic pumps air into the radiator which causes the coolant to leak out of the hole where ever it may be.

Not only will the test help locate an external leak, it will also tell you if you have an internal coolant leak inside the engine like a head gasket. If the tester slowly starts to drop in pressure and no external leaks are visible, I would assume there is an internal leak inside the engine….which is not good. :(

So in this case, get a cooling system pressure test done first and rule out a possible coolant leak. If they already did this and see no leaks, then it would tell me there are probably no internal leaks either if the tester held pressure.

2. Low coolant flow inside the engine – if the radiator is low on coolant the engine will overheat. This can be due to a leak mentioned above and not enough coolant is available inside the radiator to remove the engine heat.

Low coolant flow can also be a cause of air pockets inside the engine and cooling system. It is not uncommon for a major repair to be done, say a water pump replacement and air getting trapped inside the cooling system when the mechanic refills the radiator with coolant.

Air pockets in the system will take up space that should be filled with coolant, and it can also cause the water pump to not be able to properly circulate what ever amount of coolant there is inside the engine properly. On some cars there is a coolant “bleeder” screw located around the thermostat housing which the upper radiator attaches to on the engine. Loosen the screw slightly with the engine running to bleed out any air that is trapped inside.

3. Low coolant flow inside the engine, but due to a restriction. The most common types of coolant flow restrictions are due to a radiator that is stopped up at the bottom with rust, water deposits, calcium deposits etc. that can not be flushed out. This sediment at the bottom of the radiator will restrict the flow of coolant that is being circulated inside the engine. A new radiator is the cure for this.

4. Lack of air flow across the radiator – most cars and trucks these days will have an electric cooling fan located on or near the front of the radiator that will help push air across the radiator to remove heat. Most larger trucks will have a plastic fan attached to a belt on the engine that will suck air across the radiator. That fan is regulated by a fan clutch, that will speed up and slow down the fan speed as needed. Your vehicle has only an electric fan.

If air flow across the radiator is impeded in some way (electric fan not working, fan clutch is bad and not turning fan fast enough) the engine will overheat. I had a customer once with a large passenger van and had a spare tire mounted to the front of the vehicle in front of the radiator and grill.

This caused a reduction in air flow and would over heat the engine on long trips in the mountains when he was on vacation. Removing the tire was the cure in his case.

A large amount of dead bugs, or a plastic bag or load debris that has attached itself to the front of the radiator can also cause loss of air flow, and overheating.

Now, let’s talk about what the symptoms are with each scenario I have given you above so you can narrow things down and solve the problem.

We want to know WHEN the engine overheats.

If the engine runs hotter or overheats at idle speed or at stops but not much at freeway speeds. I would assume we have a low air flow condition or a low coolant flow condition. Checking the electric cooling fan, and the level of coolant in the radiator (not just in the plastic overflow tank) would be my first step. I would also want to rule out an air pocket situation as well, but if no recent repairs to the cooling system had been previously made…its probably safe to rule it out.

If the engine runs hotter or overheats at freeway speeds I would assume there to be an airflow issue with the radiator or a lack of coolant flow inside the radiator. A radiator restriction is usually the culprit in these situations. Consider a restriction like this. You are trying to run a 1 mile marathon, but you have to do it with your hand over your mouth. You probably can walk 1 mile with your hand covering your mouth but running requires more air flow.

A radiator restriction usually only shows up when the engine is at freeway speeds or under severe loads like towing a trailer or going up steep grades where the maximum amount of coolant flow is needed. I should also note, that running the air conditioner will also add stress to the cooling system and the engine needs more coolant flow to avoid overheating.

A simple test you can do when the engine is overheating is turn on the heater inside the vehicle. If the heater blows hot air, then there is enough coolant in the system, but might be lacking proper flow to cool the engine. The heater will act as a secondary radiator and might be able to reduce the engine temperature enough for you to continue to drive safely until you can get repaired.

If the heater is not hot when the engine is overheating, I would assume there is not enough coolant in the engine and a coolant leak has probably occurred somewhere.

So to recap. In your situation I would do the following

1. Pressure test the cooling system for leaks.
2. Check to make sure the electric cooling fan is working
3. Check to see if there is an air pocket in the system
4. Check to see if a radiator restriction is present. This can be hard to diagnose, so using the example of overheating at freeway speeds above you might have to just guess and replace the radiator once you have ruled everything else out.

On a side note, many people replace the thermostat as a guess first right off the bat. A stuck thermostat can cause persistent overheating both on freeway and idle speed scenarios. Rarely do we see defective thermostats much anymore, not unheard of just not very common these days.

I should also mention, the radiator cap is there to seal and pressurize the cooling system. Most caps have a PSI number stamped on them, which is what they will hold the system at normal operating temperature.

A bad radiator cap will either leak coolant externally or stick in a closed position and not properly vent the system. This will cause the upper radiator hose to swell in size considerably and be quite obvious.

I hope this helps you, keep me posted as to what you find out.

Comments, Review, Facebook likes, Google +1 appreciated.

Austin Davis

Posted in: Over Heating

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
  • austin

    Thank you very much

  • Anonymous

    Thanks and good blog.

  • Colin Brisbane Australia

    Gday Austin, Thank you very much for sharing your time and knowledge with us all. l’m sure your father and family are very proud of you.
    My father was a mechanic, but has recently died, but l let him know of your site when he was alive and he was aware of your helpful insights.
    Again, thank you.
    Kind Regards, Colin, brisbane, Australia

  • Steven Stanley Bayes


    In response to “Engine Overheating”, what I would do is I would take all antifreeze out of the system and put water instead. This must not be done in the winter. I would then change the water every week until all gunk flushes out. I would do this throughout the whole summer. In states, where there is no winter, such as Texas, for example, this can be done whenever one wishes.

    However, be aware of the following:
    The water pump membrane does not like water but expects some kind of lubrication as well. One may add a bit of glycerin, for example. Glycerin is supposed to be inexpensive and rather safe. Even without the addition of glycerin, the membrane is supposed to take the pure water just as good.
    Pure water may lead to corrosion and corrosion particles may start to flow through the system and may increase the possibility of clogging the radiator, mainly with old iron blocks. Regular flush may prevent radiator clogging.
    Water has a lot of components, such as minerals which may be deposited in the radiator and the system. Distilled water is always preferable and may be inexpensive.
    I have used pure water from the tab a lot over the years. No problems.

    Be also aware: when there is a leakage in the water system, some people add different components to the antifreeze to seal the leak “temporarily” and do not flush thereafter. One of the easiest road fix is to add red pepper to the radiator. The particles of the red pepper do not get dissolved in boiling water, they are supposed to be tiny and accumulate around the oozing point thus sealing the leak. They may also dry into a stronger sealant. Adding too much of these may block the radiator and the water system.

    A cracked water pump membrane may also be a reason for lower water circulation.

    Pinched hoses too.

    Untightened water pump belt too. Some cars get the momentum straight from the shaft. Most would use belts in order to prevent engine damage with pump damage, as well as for an easy repair and replacement, mainly on the road.

    Clogged radiator is the most common problem.

    In case of a broken gasket from the inside, there would be loss of antifreeze without any visible leakage. One way to tell may be to inspect the oil. The oil would change in colour and would form foam. A mechanic should be able to tell.

    I would also put a transparent hose or a glass pipe temporarily somewhere on the water flow fixed with tights and hoses, to monitor the water flow. Mechanics may have flow meter.

    Electrical fan inspection should be easy as well as the inspection of the sensor and, most importantly, the thermostat. The thermostat may be taken out and put in a pot on the oven to check the spring movement with the increase of the temperature, mainly around boiling. The retraction and expansion of the spring must be such as to allow full opening and closure of the valve in the two positions: 90 degrees water and cold water.

    The cooling fan may have broken OR TWISTED “wings”. This will reduce the cooling ability.

    Some people put tricky covers of the radiators for a warmer run in the winter and forget to take them off.

    Cars may normally overheat when engines are kept running without moving of the car or when the engines rev at high revs at slow movement of the car at lower gear when hauling.

    When cars are given to children, they race the hell out of them which would also lead to overheating mainly in Nevada desert where the sand would prevent the car from reaching high speeds and the car has to run at lower gear and high revs.

    An open reserve tank due to, say, forgetting to put the cap on, may lead to sand, dust and other particles to block the radiator.

    Use of “alternative” fuels which may burn at higher temperature, use of high compression, problems with the mixture can all lead to overheating of the engine, mainly at hot weather.

    Cars, sold in the Northern States, may slightly differ from cars sold in the Southern States in terms of the cooling system design. Cars from the North brought to the South may tend to overheat more easily.

  • fred schilling

    Seldom? Broken down/overheated vehicles by the score on Baker Grade (I-15 Ca/Nv) in the summer time. People utterly disregard signs to turn off their A/C on the upgrade and are surprised when “old faithful” erupts.