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Symptoms and most likely causes




(there may be more than one present!)

Master index
back to Cooling Problems

Overheats when in heavy traffic or after extended idling
Overheats when driving at speed, or on repeated, heavy acceleration
Overheats immediately after coming off highway
Overheats any time, or erratically
Overheats seconds after the car is started
Seems to run slightly too hot all the time, gauge sometimes nears the red zone
Bubbles in coolant reservoir
Air in radiator, expansion reservoir is full
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Even more information (optional reading for those who like lots of text and detail)...


Overheats when in heavy traffic or after extended idling
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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
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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
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Radiator fan not coming on
Bad thermostat

Overheats anytime, or erratically
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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
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Electrical problem with gauge temperature sender or associated wiring

Seems to run slightly too hot all the time, gauge sometimes nears the red zone
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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
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Bad radiator cap
Failed head gasket
Failed water pump seal admitting air from suction side


Air in radiator, but expansion reservoir full
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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,
 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:
 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.
From Valvoline's Web site comes this PDF file on that subject.
If that PDF doesn't work, try here for a local copy.

More on coolant and coolant compatibility from Car and Driver magazine's Patrick Bedard:
http://www.caranddriver.com/article.asp?section_id=27&article_id=2334&page_number=1
http://www.caranddriver.com/article.asp?section_id=27&article_id=2264&page_number=1

It's always best to be safe and use Honda's own Type 2 premix.

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
 As you may have gathered from the above section, the radiator cap has three seals, any of which may fail independently of the others:
  1. 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.
  2. 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.
  3. 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 Hondas, 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 Hondas 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.

A new genuine Honda thermostat is about $30 and is guaranteed to work properly in your Honda. In my mind, it's foolish to trust such an important job to an aftermarket part. Changing it every 5 years is excellent preventative maintenance.

A good link with more info. "Jack" is not the best writer, but he's got lots to say:
http://www.users.bigpond.com/jack_stands/automotive_advice/cooling_system/engine_cooling.html#TSTAT

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 Hondas use two

Wiring and connectors on older cars begin to corrode and break down, which cause their own headaches.

Older Hondas (my '91 Integra for instance), 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.)
Here is some troubleshooting information on this setup.

Newer Hondas 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.
Thanks to poster Remco, here is an extensive troubleshooting guide for this type of installation. switches: One for the fan, one for the temperature gauge. This means that a gauge reading will not necessarily coincide with what's actually happening inside the engine. Bottom line: Check your coolant level in the rad AND the expansion tank, just in case.

(Note to users of Netscape, Mozilla and Firefox: Remco's page was designed for Internet Explorer. If you have trouble with the text being jumbled and difficult to read, reset your Fonts down one size as shown below, view the page, then set it back up again).

Setting font size down in Netscape or Mozilla          Setting font size down in Firefox

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.

More on coolant and how corrosive coolant can cause problems, from Car and Driver magazine's Patrick Bedard:
http://www.caranddriver.com/article.asp?section_id=27&article_id=2334&page_number=1
http://www.caranddriver.com/article.asp?section_id=27&article_id=2264&page_number=1

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 Honda 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
 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.

The one on the right is actually from a batch of Honda OEM pumps that had a quality problem years ago. Needless to say, Honda fixed that and it hasn't happened again. (Photo by Mista B0ne)
good pump, bad pump