The mysterious overheating Honda

Hello, big fan of the show and I hope someone here can help me with this difficult car problem…

This is a tale that goes back about a year when I found my very first car. I needed it to commute back and fourth to school. When I bought the car it worked great, everything I could have wanted. However, that soon changed when it began overheating, and to be specific it is a 2003 Honda civic ex, automatic. I took it back to them after having to pay towing expenses. They said they didn’t know it had issues (bull) they said it was due to a radiator, so they installed a new one. When I got it back it drove fine then it began exhibiting the same symptoms soon after. I decided since I loved the car I wouldn’t even bother with the people I bought it from, big mistake and I should have just returned it…I didn’t because I was sick of looking for a car and school was about to start. This began the routine of having to fill it with antifreeze everyday before and after class, if I didn’t it would most certainly overheat and then I’d have to blow hot air all day. So I took it to another mechanic who figured it must be something wrong with the cooling system and said the fan was acting slow, so he replaced the fan-switch and thermostat and said that it wasn’t the water pump. I was hoping this would do it, but it did not. He then tested the head-gasket twice and it came back negative. Then they tested the catalytic converter, came back negative. This mechanic, then proceeds to tell me the people who sold it to me may have put a sealant in which gummed up the engine and is clogging things so it can’t function correctly, or put on the head-gasket wrong or any number of things to the engine. Although, wouldn’t this still make the head-gasket test fail? He tells me I may need a new engine because to figure out if it truly is an engine problem that would cost as much as a new engine. I don’t know what to do or where to go I am at a loss here. Spending my money just to have mechanics tell me there is some mysterious “undiagnosable” problem with my car is the most frustrating thing ever. No one can fix this car it seems. PLEASE I beg anyone who knows how to figure this mysterious problem out!

Any help would be greatly appreciated.

Thank you

Some more info might help in formulating at least a wild guess. :slight_smile:

Overheat at idle and in-town driving, on the highway, or both?

Overheat with the A/C off or A/C on, or both?

Boiling any anti-freeze out, steaming, etc?

I wouldn’t be to quick to condemn the dealer as knowing about the problem. Dealers buy cars at auctions, take them in trade, etc. and quite often don’t know that much about the car. They send them to the detail shop and place them for sale while quite often driving them very little if at all.

There is no such thing as an undiagnosable cooling system problem. Cooling systems are basic, operate in a common manner, and there are a number of standard tests that can be performed. You simply need a better diagnostician (a better shop).

How was the headgasket checked? There are simple ways, and there are definitive ways. A simple way is to run the engine (while cool) with the radiator cap off. If you see bubbles coming out the fill hole, you probably have a blown headgasket.

A bit more definitive way is to perform what I call a “lab test” on the coolant in the radiator (take it directly from the fill hole, not from the reservoir). You can buy a kit to do this, or even test strips. Both are cheap and easy to use, available at any parts store. They’ll come with instructions. A headgasket breech will allow combustion gasses into the cooling system, and the tests will detect that.

A slightly more definitive way is to do a cylinder leakdown test. Again, the kit is available at any parts store. It’s easy to do and the kit comes with instructions. Basically, you pump some air into each cylinder with the valves closed and monitor it with a gage to see if it leaks.

All three of these aforementioned tests can b e performed by you at home very inexpensively and with no danger to yourself or your vehicle.

Another test when you’re losing coolant is a system pressure test. You basically put some air pressure in the radiator fill hole and see if the system leaks. Again, see the parts store. Follow the instructions and you’ll pose no risk to yourself or your car. You can even add some UV sensitive dye to the coolant while you’re at it. It’ll light up the leakpath under a blacklight, making the leak easy to see. The only thing to be aware of is that it may not catch a water pump leak. The most common way pumps leak is around the impellar shaft seal, and that sometimes only leaks when the shaft is spinning.

Another possibility is a collapsed radiator hose liner, but IMHO that’s unlikely on a hose this age, albeit possible since it’s been overheated.

A leaking heater core is another possibility. Has your passenger floor been getting wet?

I think that’s enough for you to chew on for now. Stop by the parts store, pick up some test stuff (I’d start with the coolant test for hydrocarbons), and let us know how you make out. All of the above tests are safe to both you and the car, and all are far more affordable than having a shop throw parts at the problem.

Post your results. We’ll go from there.

The car generally overheats when I am at slower speeds or idle but mostly when I am at idle. It overheats when I don’t have hot air blowing out, but even then it’s just a matter of time before it gets so hot that the extra pesudo-radiator doesn’t make a difference. When it overheats and I have to stop to check it I’ll slowly release the radiator cap then it will spit out streaming hot antifreeze from the reservoir.

Thank you

@the same mountainbike. I had the chemical head-gasket test done twice and it came back negative. Before I went to the mechanic I was certain it was the head-gasket because it fit all the signs and this Honda is numerously known for them. But the dipstick is not milky and no white smoke from the exhaust. I also had a pressure test done for leaks and nothing seemed to be out of the ordinary. I am leading toward the water pump being the culprit, which is the only part of the cooling system which hasn’t been checked. I don’t want to jump to conclusions I would appreciate all advice on this issue before I have to spend what little funds I have left ;’(

Try the UV sensitive leak detecting additive. If the pump shaft seal is leaking when the engine is at full temp and running, that’ll show it to you.

A pump can fail by its impellars eroding also. A flow test can be performed, again using a test kit readily available at parts stores.

Keep in touch. I’d like to try to help you through this, as I’m sure others here would.

By the way, you commented that the mechanic said “the fan was acting slow. He replaced the fan switch and thermostat.”. A cooling fan is a DC motor driven fan that’s either on or off. The only possible causes of it acting slow are something resistive in the circuit, lowering the voltage at the fan, or a binding fan motor. The voltage at the fan can be tested. The motor can be bench tested to see if it’s running free. You can actually spin it on the bench using a simple 9VDC transistor radio battery or two 6VDC lantern batteries in series. If it’s binding, it’ll be easy to detect.

It crossed my mind that the impellers or the bearing had gone bad.
Curious, how will the UV additive show me that pump shaft seal is leaking?
As for the flow test, do you think this would suffice?

As for the fans, the mechanic told me they were operating correctly. He had the car for the entire day while checking the temperature of the catalytic converter and everything was in correct order.

Based on the overheating at idle and slow speed I tend to think there might be a fan issue.
With the A/C on both fans should be running.
With the hood up and the engine idling you might watch the fans (A/C off) and make sure that one of them cycles on and off when the engine temperature reaches normal.

With the engine off (AND cold) you might also take a finger and try to rotate the fan blades. They should turn easily. If not they could be dragging due to wear and that will slow down the fan blade RPMs which will translate into fewer cubic feet per minute of air being pulled through the radiator.

If the pump shaft is leaking when the system is hot and the impellar shaft is spinning, it’ll leave a UV sensitive trail that will light up under a blacklight.

I would suspect the water pump vanes are eroded. This is a commonly overlooked cause of overheating at low speed/idle. You said one mechanic told you “it was not the water pump.” How did he determine the pump was good?

How did he determine the pump was good?

Probably just didn’t see coolant leaking from it.

I’ve had the head off of my '04 Civic after it exhibited symptoms similar to what you have described. I did the “add coolant, drive the car”, refill" for too long and finally had the work done. The gasket leak was confirmed by pressurizing each cylinder in turn, and two adjacent cylinders passed air into the coolant.

I have the W/P from the car when the timing belt was changed, and it is perfect. No corrosion, and all vanes (stamped steel, not cast) are intact.

Another question…when the car is overheating, does the heater work, particularly at idle?

I believe this car uses a timing belt that drives the water pump so I wonder if the timing belt service, which would be way overdue by now, has ever been done. You can go to any Honda dealer and have them run the VIN in their system to see if it was ever done at a Honda dealership. That wont tell you if it has been done by an independent mechanic though.

The fact that you have to add coolant before your start the car every time means that coolant is leaking out somewhere, unless you mean that the coolant loss only occurs when you open the radiator cap on a hot engine. Small leaks can be hard to find so a fluorescent dye is a good idea or just use the orange dexcool compatable Prestone antifreeze. It leaves a pretty visible trail, unlike the green or yellow stuff.

If the water pump is leaking, this is critical as it will eventually cause the timing belt to fail. The water pump is hidden under the belt cover so the leak will be at the front bottom of the engine. If you can’t find the leak, then I’m afraid it is the head gasket, no matter what the tests may have said.

One last thing, and this is cheap, replace the radiator cap. You would be surprised how often these are the culprit and how often they are overlooked, but usually they cause overheating when going up hill, not at idle.

Also you should have two radiator fans. Turn on the AC and see if both fans are spinning. If one is not, the fan is probably bad. If both are not, that indicates a bad relay.

I also don’t see where the thermostat has been checked and are you aware that there is a bleed valve located in teh thermostat housing that must be opened when ever the coolant gets low in the system. If it is not bleed, it will overheat again.

That being said…you CANNOT SIMPLY JUST FILL THE RADIATOR and then go motoring around and expect things to work properly…NOT in a Honda. ALSO…Water Pumps can really only fail in a couple of ways…the impeller can snap off… OR the pump itself can leak out of the weep hole… They really dont fail in many other ways than this… So I wouldnt suspect your pump too quickly. I have only seen pumps fail by weeping fluid out the weep hole…ONLY ONE TIME in over 27yrs have I seen an impeller fail… This one happened to just Fall off… LOL SO the pump shouldnt be suspect very often or too quickly. People think the pump is bad when often the impeller is stuck in a pocket of AIR…and it cannot circulate coolant while in an Air pocket…it simply has no chance to do its job properly

Why no other people have even mentioned an Air pocket…ESPECIALLY when talking about a Honda is beyond me… Hondas and Cooling system Air pockets are EXTREMELY COMMON…and the cause of most if not all of the cooling system issues I have ever seen in them… Otherwise their systems are pretty damn good at what they were meant to do…

After or even WHILE you are filling your rad…you MUST open the Air screw located USUALLY near the thermostat housing…it looks just like a brake bleeder screw…

You must fill the rad till it accepts no further coolant… then find the air burp screw and OPEN IT to burp the cooling system…

This will purge the air out of your system and allow the water pump to do its job properly…if you do NOT do this the water pump impeller will be sitting in a pocket of AIR instead of liquid coolant…and will be unable to move the coolant thru the system.

If for any reason you cannot find the burp screw the ONLY other way to burp the system is to fill up the rad AND the overflow container…run the engine till it gets warm or hot…and then shut it down and wait until it completely cools off… Upon engine cooling, you SHOULD notice that the overflow resevoir has dropped in level… what is happening is that the coolant in the rad will expand and either push out some air…or overflow coolant a bit…filling up OR overflowing the overflow reservoir…then upon cooling…it will suck fluid back in…

You need to do this heat up and cool down about 3 times… each time having the overflow FULL…so that the system can overflow…and then upon cooling…suck in some coolant… In time…you will completely purge your system of any air pockets…and it will begin to work properly again

If you have a head gasket issue…the leaking head gasket CAN leak cylinder pressure into the cooling system and constantly create new air pockets… You can notice if this is the case by filling both up…the over flow container…and the rad…start and run the engine…and then look at the overflow container…IF you see bubbles being burped into the overflow on a constant basis…and they never seem to stop…then you are leaking cylinder pressure/Air…into the cooling system…and you will NEVER be able to properly burp your system… the head gasket needs to be repaired in this case

Hondas will NOT tolerate ANY air in their cooling systems… Look for that burp nipple and USE IT…


I don’t know how he determined this. I think he was under the impression that it must be the fan switch/thermostat…y’kno first things first. I know those are standard procedure for an overheating car. He was talking about it might be some blockage somewhere/resistance in the engine because he was trying trying to burp it and it was acting odd. He also stated that he wanted to look at the water pump, so he offered a fan belt replacement in order to see what’s what with the water pump.

@Oliver70 I keep thinking it is the head-gasket but it shows none of the signs of a blown gasket, no milky dipstick or white pummels of smoke from the exhaust. And had a gasket chemical test done twice. And I can run the heat at idle. As a side-note though how much did that head-gasket change cost? it if somehow does indeed turn out to be that I’d like to know that I am not getting ripped off.


It sure sounds like a bad head gasket?

Is this what the mechanic used to test for combustion gases?

It is not unheard of to have a head gasket leak enough not to smoke or leak into the oil. If you have access to a pressure tester and a bore scope pressurize the cooling system with the plugs out if the pressure drops use the bore scope to look inside the cylinders. You should be able to see coolant on top of the pistons on the affected cylinder(s).


@db4690 yes that is it.
@SteveC76‌ I’ve heard of that from a friend of mine. And I do not but my mechanic does ill mention that to him.