All Info and Q&A about Engine Overheating

Overheats when in heavy traffic or after extended idling

Low coolant level
Bad radiator cap
Bad thermostat
Radiator fan not coming on
Failed head gasket
Water pump impeller corroded 

Overheats when driving at speed, or on repeated heavy acceleration

Radiator and/or block internally clogged with rust, scale, silt, gel, or externally blocked with road debris
Bad radiator cap
Bad thermostat
Radiator fins corroded and falling off
Water pump impeller corroded
Lower radiator hose collapsing 

Overheats immediately after coming off highway

Radiator fan not coming on 
Bad thermostat

Overheats anytime, or erratically

Low coolant level
Bad radiator cap
Bad thermostat
Electrical problem with gauge temperature sender or associated wiring
Radiator fan not coming on 

Overheats seconds after the car is started

Electrical problem with gauge temperature sender or associated wiring

Seems to run slightly too hot all the time, gauge sometimes nears the red zone

Radiator and/or block internally clogged with rust, scale, silt, gel, or externally blocked with road debris
Bad radiator cap
Bad thermostat
Radiator fins corroded and falling off
Lower radiator hose collapsing
Radiator fan not coming on 

Bubbles in coolant expansion reservoir

Bad radiator cap
Failed head gasket
Failed water pump seal admitting air from suction side 

Air in radiator, but expansion reservoir full

Coolant leak
Too much air in system after fluid change
Bad radiator cap
Bad seal between radiator cap and expansion reservoir
Failed head gasket 

Low coolant level

Can be caused by many, many things. Among them,
Failed head gasket
Worn water pump seals
Lack of maintenance
Hoses leaking
Radiator core leaking
Radiator upper tank cracked


If the coolant is allowed to get low, the water pump can't circulate it through the radiator at idle, so heat cannot be removed. Normally the pump can circulate even low coolant if the pump spins fast enough, so a low coolant level generally will not cause overheating at speed.


If the low coolant level is accompanied by an expansion reservoir level that's much higher than normal, your rad cap or head gasket is suspect.


Age, or the use of inexpensive silicated coolants will wear the water pump seals rapidly, causing premature leakage.


The water pump has a small "weep hole" in it, and a noticeable drop in coolant level over several months is normal. You need to keep an eye on the level and top it up once in a while.


Air in the system is very bad. Your cooling system is meant to operate with all parts and surfaces completely immersed in coolant, as that's how the anti-corrosion properties of the coolant work. Air equals corrosion. Also, with air in it, the system cannot be pressurized, which is one way boiling is controlled. Boiling (or vaporization of the coolant) is just as bad as a low level.


If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is probably low. Best to check.

Radiator or block internally clogged with rust, scale, silt, gel, or externally blocked with road debris

The radiator is the engine's main heat exchanger. Unless coolant can pass freely through it at the speed the water pump and thermostat want to push it, it can't get rid of the heat it needs to get rid of, and deposits prevent fluid movement.


Any deposits at all in the radiator are bad and are caused by one of more of the following:
Incorrect coolant
Mixing incompatible coolants
Old, corrosive coolant
Use of tap water to mix with aftermarket coolants
When you pull the rad cap off and shine a flashlight inside, the coolant should be transparent (plus whatever color it was 


when put in), and the fins should be clearly visible.


If the fluid is murky, brown, smells like rubber, goopy, or if the fins have white, crusty deposits on them, you've got circulation problems, and likely corrosion problems inside the engine.


Old coolant gets acidic and corrosive, and will eat all sorts of internal parts, from your head gasket to the water pump impeller. It also cannot carry heat as effectively as fresh coolant.


Inexpensive silicated coolants (the cloudy stuff) can cause silt to build up in the engine's water jacket as well as the rad, providing an impediment to free flow of coolant. Honda emphatically recommends that you use their coolant and no other in their cars.


Coolant should be changed every two years with a non-silicate, non-borate type. Long-Life coolant is supposed to be good for five years, but I've never personally been comfortable with that. I change mine every two years or less.


Mixing coolants of different formulations can destroy the corrosion-inhibitors, leaidng to rapid corrosion, water pump, thermostat and radiator failure, as well as head gasket problems.


A radiator problem peculiar to Northern regions is external corrosion of the fins. This is caused by winter and road salt, and reduces the fins to a white powder. The fins then crumble and fall off, and once they do, that section of the radiator cannot exchange heat with the outside air. Run your hand gently over the fins at the middle of the rad's core, at the very bottom, under the bumper. If they're crumbly, you'll know.


Warmer areas may suffer from sand, pine needles and other debris embedded in the lower half of the rad, which prevent air movement through the rad and thus impair heat exchanging. 

Bad radiator cap

The radiator cap does several things. It seals the system against the outside world (main seal function) keeps the system pressurized when needed, so as to raise the boiling point of the coolant allows excess pressure and coolant expansion to vent to the expansion reservoir (pressure seal function) allows coolant to return to the radiator when the engine cools down (return seal function)


As you may have gathered from the above section, the radiator cap has three seals, any of which may fail independently of the others:


The main seal is the one that seals the cap against the top of the filler neck. Just a rubber gasket that operates just like one on the lid of a pickle jar. Simple and reliable.


A failed pressure seal will allow the coolant to boil at a lower temperature, and coolant will be able to travel freely and foamily to the expansion reservoir. This will cause localized hot-spots inside the engine, which can lead to premature head warpage, and may hasten head gasket failure. It will also cause the rad coolant level to be low, just like a failed head gasket.


A failed return seal will prevent the coolant from returning to the radiator as the rad cools off, causing a vacuum that can collapse the radiator's hoses. This will prevent the coolant from circulating if the hoses don't re-expand as the engine warms up.


A bad rad cap can cause similar symptoms to a failed head gasket, so it's a cheap first step to try before bringing it in. 


If you replace the rad cap and you still have bubbles in the coolant (or foam in the reservoir), then suspect the head gasket.


If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is probably low.


Moreover, a neglected cooling system can load up the cap with crud and corrosion, preventing proper coolant flow in and out through it. Peel the seals back with your fingernail to check for goop. If you find any, a blast with a garden hose and probing with a toothpick should clear most of it out.


But anyway, a new rad cap is less than $20. Make a habit to change it every 5 years, just in case. It's pretty important.

Bad thermostat

This part is the traffic cop that controls when the coolant is allowed to circulate and when it isn't. It's the device that's meant to quickly allow the engine to warm up to its design temperature, but no hotter than that.


Thermostats can stick shut or open, get lazy, or fail to open at the correct temperature. Depending on how and when they fail, they will cause either overheating or underheating. Overheating usually happens when the thermostat fails to open, or fails to open enough. If it fails to close, the engine will run too cool, causing all sorts of other problems.


A cooling system full of rust, scale, silt, or gel will interfere with the thermostat's operation, causing even more cooling problems. Gunk can plug up the thermostat, causing overheating, or make it stick open, causing underheating. 


Incorrect installation of the thermostat (can be done, even by professionals!) will also interfere with thermostat operation.


Normally mounted in the top of the lower rad hose in modern cars, the thermostat senses engine heat in the coolant. It is supposed to open up when the coolant in the block has warmed up enough, allow the cooled coolant in the rad to flow into the block, pushing the hot coolant from the block into the rad.


When the thermostat is closed, a small bypass hose allows coolant to circulate through the engine block, around the business end of the thermostat, through the water pump, and back around again. This keeps block temperature even, and helps the thermostat warm up as well. Once the thermostat opens, the bypass is closed off by means of a special extension on the bottom of the thermostat.


Aftermarket thermostats are highly associated with overheating and underheating. Most car require 78C (172F) thermostats (that's the opening temperature). Too many aftermarket thermostats are wrongly rated for your car and are poorly made. 


Your emissions system may not work correctly with a different rating installed. In addition, some aftermarket units lack a bleed hole, the absence of which can trap air and lead to overheating.


Changing it every 5 years is excellent preventative maintenance.

Electric fan not coming on

The engine's heat is removed from the coolant through the radiator. When you drive, the motion of your car is enough to push sufficient air through the radiator to effect proper cooling, but when you are stopped, or moving slowly in heavy traffic, your radiator needs help. This is what the fan does: It pulls air through the rad when the rad isn't moving.


The problem is that, in order to work, it needs to be turned on. There are various fuses, sensors, switches, relays, and several yards of wiring and connections concerned with turning the fan on, and they do go bad. The fan motor itself is very robust, and rarely goes bad.


The fan switch is immersed in coolant. When the coolant gets hot enough, the switch closes, and grounds either the fan itself, or a relay, which then provides power to the fan.


In some cars, if the coolant in the rad is too low, the fan switch may not be fully immersed in coolant and would thus sense a false temperature and not switch the fan on. Some cars have the fan switch deep at the bottom of the cooling jacket, others have it higher up. Higher up is more prone to this issue. Keep in mind that all but the very newest caruse two Wiring and connectors on older cars begin to corrode and break down, which cause their own headaches.


Older car, have no relay for the fan switch. The switch simply grounds the fan, allowing current to flow, and the fan to come on. (The fan itself still has a relay though.)


Newer car use a relay to help insulate the switch from having to carry the full current that the fan's drawing. The idea is to help the switch last longer, but the downside is a bit more complexity and more parts to fail.

Failed head gasket

When the gasket goes, typically the first thing that happens is that combustion chamber gases are pumped into the water jacket. You will see this as bubbles in the expansion reservoir. This will quickly lead to low coolant level in the engine, and overheating at idle, even if the fan comes on, since the water pump can't move the coolant around properly any more.


If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is low.


The symptoms are similar to a failed rad cap. If you top up the rad, replace the rad cap and the problem persists, the head gasket is definitely questionable. To confirm this, a garage can apply a pressure test (NOT a "compression test"), where air is blown into each cylinder in turn until the technician sees bubbles in the radiator. If no bubbles are seen, the problem is elsewhere, such as an external leak.


This can be accompanied by an expansion reservoir level that is much higher than normal, and which does not go down once the engine cools off.


If you choose to simply add fluid and ignore the issue, eventually oil and coolant will begin mixing together, and coolant will get sucked into the combustion chamber. Ignoring this is a good way to toast a perfectly good engine. Get it fixed early and there will be no further issues.






Bad seal between radiator cap and expansion reservoir, or too much air in system after fluid change


When coolant expands, it needs to go someplace temporarily so it doesn't burst hoses or blow the rad. The expansion reservoir is where it goes.


The fluid pushes out of the pressure seal in the rad cap, and travels down the skinny rubber hose at the filler neck to the expansion reservoir.


As the engine cools off after you shut it down, the return seal in the cap opens up, and the coolant is sucked back in through that same rubber hose.


If the reservoir has run dry, the amount pushed out may not be sufficient to cover the bottom of the reservoir's intake pipe, leaving the system unable to pull the coolant back in, so you'll end up with air in the rad, and resulting loss of pressure. If the rubber hose is disconnected or split, the expanded coolant may end up on the road instead of the in the reservoir, leading to the same situation. And air in your rad is a bad thing.


If you've just changed the coolant, there will be some air trapped that will work its way loose back to the upper radiator tank. If there is a lot, there will be too much for the expanded coolant to push all the way into the reservoir. What this means is you'll have a situation similar to that immediately above, where the engine will be unable to suck in coolant from the reservoir. When you change the coolant, take the car for a drive after. Let it cool down, then check the rad for air. Top it off, do the same thing the morning after the next drive, and you should be OK from then on.


Regular checks of your coolant level, both in the expansuion tank and the rad, are important in any case. 

Lower radiator hose collapsed

If you're using genuine hoses, you'll never see this problem unless your rad cap has gone bad and won't allow coolant back into the engine.

When the water pump is turning, it's sucking water through the thermostat and the lower rad hose. That suction can cause the same sort of hose collapse as you get in a drinking straw when you try to suck a thick milkshake through it. If the hose is insufficiently reinforced on the inside, it can get sucked flat and coolant flow will stop. When you let off the pedal or shut the car off, the hose pops back to normal. If you use aftermarket hoses, not all of them will be of high quality, and may be prone to this phenomenon.


To check for this, rev the car hard with your hand on the throttle in the engine compartment, and watch the lower hose. If it doesn't collapse, it's probably fine. 


This can also happen as the car cools off, if the rad cap return seal has gone bad. 

Water pump impeller corroded

The only time you'll ever see this is if


you're using a low-quality aftermarket water pump, or
you've mixed incompatible coolants, or
the coolant has never, ever been changed.
If the impeller is gone or much reduced in size, coolant can't be pumped into and out of the rad, so it will boil and overheat.




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