Overheating due to fan issue

I have 2004 Civic.

Here is the problem:

(1) When the AC is on, the fans are on and the car runs fine.

(2) Even when the AC is off, the fans kicks in with regular intervals and the car runs fine.

(3) If I tun on the heat, the fans never kicks in and it gets overheated.

Someone told me it could be the relay. What relay would that be? Or could there be some other issue?


Honestly, that doesn’t make too much sense to me.

The air conditioning system that activates the radiator fan is on a different circuit that runs the radiator fan when the coolant temp switch indicates that the coolant is above a certain temperature.

The coolant temp switch is not associated with the heating or cooling settings on the HVAC controls in any way, shape or form.

The only things that I can guess at, would be the coolant temp switch, air in the system that only becomes apparent when the system is in heat mode, but if the cooling fan is turning on in both the a/c mode, and in normal mode, then I can’t see a reason why it shouldn’t work in heat mode.

So, my suggestions:

Bleed cooling system in heat mode for any trapped air.
Check the shut off valve in the coolant hose that leads to the heater core for proper functionality/
Coolant temp switch may be bad, but shouldn’t be since it functions properly in normal HVAC mode.


My only thought of the association would lead me to see how the fan is actuated. It’s obviously done through a relay, and probably a common one between them. The PCM/ECM probably grounds the relay depending on input. I doubt (although I could surely be wrong) that the AC fan actuation is done with the simplicity of a relay tagged to the compressor clutch. That would simplify things since it would then be strictly isolated to the coolant temp sensor and the PCM/ECM grounding it.

…or so I would reason with lack of any intimate knowledge of the system.

This is a pretty strange problem alright. By turning on the heat the engine coolant usually drops in temperature. I could see where that would drop the temperature enough so the thermostatic switch would not turn on the coolant fan. It might help to check and see if turning on the heater somehow changes the power going to the coolant fan switch and also see if the switch is turning on at the proper temperature. Since the fan seems to work normally with the heat off I would guess the temp-switch is ok. Also make sure 12 volts is getting to the proper areas with the heat and blower on and the grounding to the engine and chassis is ok.

I doubt the fan relay is a problem.

Question. When you refer to heat do mean HEAT (as in floor only) or do you mean one of the following; BI-LEVEL or DEFROST?

No, I mean the car engine is overheated – the thermostat goes all the way up to high. I switched the fan relay with another one. I will drive later today and see what happens. Maybe it’s totally something else … a head gasket or something?

I do not understand Number 3 of your original comments. You state that you “turn on the heat, the fans never kicks in and it gets overheated”.
I read this as you turn on the cabin heat and the car runs hot because the fans never cycle on.

The reason I’m trying to clarify this is because with many/most vehicles the air compressor operates whenever the mode control is in Bi-Level or Defrost positions. This means the cooling fans should be running in every position other than floor heat.

You’re also making (made actually) a big mistake by operating the car with the temperature gauge pegged out.

Has the thermostat and radiator cap ever been changed? These are two very cheap parts that can cause a wide variety of strange cooling issues. I’d change them and make sure the coolant is at the proper level and that there’s no air in the system before you start getting into overly exotic diagnoses.

Yes, the thermostat and radiator were changed a couple of month ago.

Ok, I drove it today for about 100 miles and it ran fine, but I am not sure that had anything to do with relay. I put a quart of coolant before I drove today. The fans kicked in fine with regular intervals, regardles if the heat was on or not. I don’t see any visible leaks anywhere. The radiator seems fine (and was replaced recently). I will check a few days later if the coolant level drops again.

Now, is it possible that the coolant is leaking into engine? I don’t see any white smoke coming out of exhaust but I do smell soomething (burning coolant?).

Now here is a question, if the coolant is indeed leaking into engine, how many months can I drive like that if I keep putting a quart coolant every week (or 3 days?) I drive almost 100 miles a day. Any negative consequences? Could it get worse if I drive like that (but check the coolant level every few days and fill it if it needs it)?

You need to check EVERYTHING, Like radiator, radiator hoses, heater hoses, bypass hoses, and heater core for leaks. Leaks internal to the engine either show up as white smoke or in the oil. Checking the spark plugs for a ‘steamed clean’ appearance is a tell-tale sign. Oil with a milky like appearance has water mixed in. External coolant leaks could quickly evaporate, but leave traces behind to show they were there. Heater core leaks tend to fog up and leave a residue on the windshield and wet floors.

Hopefully you can locate a small external leak at a hose connection. Check for leaks while the engine is hot and under pressure with the engine running. Have a block done to see if exhaust gases are getting into the coolant if you can’t find a external leak.

What has been going on in the coolant reservoir? Does the coolant rise in the tank with repeated drive-cooldown cycles?

The key is you are adding coolant and it has to be going somewhere. Have you seen any signs of distress on the plugs (white electrodes)?

My '04 Civic (1.7l SOHC) was transferring coolant into the reservoir which required manually putting the coolant back into the radiator at 1-2 day intervals. To make a long story short, in my case, cylinders #1 & #2 were leaking coolant into the bores due to a failed head gasket.

A cylinder leakage test confirmed the diagnosis.