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2004 Honda Civic

Hello everyone, needing some guidance here. I have a 2004 Honda Civic. I just drove the car to Texas this weekend, 1800+ miles. On the way home it started to overheat if I was going slow, 45MPH or less. I pulled over, shut it off for about 2 minutes, started it up, the guage read about normal. I drove for a little while and it worked fine. Awhile later it started doing it again. I turned off the A/c and let the engine build up temp. At about the normal reading, both the fans on the radiator started cycling (I think one is the A/c cooling fan, and the other is the radiator cooling fan). So, both fans are at least working. I checked the fuses, they all looked good, and the fan relay worked as well ( I switched it with the horn relay, and they both seemed to work the same). What should I check next? Does it sound like a thermostat issue, or a temp sensor, or the fan switch? And how could I troubleshoot this further? Thanks for any help you can provide.

If the fans are both operating, it’s not the fan relay or fuse.

Replace the thermostat, it may fix this, and it’s an easy and cheap fix.

Did you check the antifreeze level (when the engine is cold) ?

How many miles on this car?

Yes, antifreeze is full. The car has 155k miles

If your Civic has yet to have the head gasket replaced, they often fail on Civics the same age as yours and at about 150K miles. I had similar initial symptoms with my 03 Civic EX. The coolant isn’t being pulled back into the radiator from the overflow tank when the motor cools. The tank looks to have an OK level, but when you take off the radiator cap you will see you need to add coolant. Take off the radiator cap in the morning when the motor is cold. You will need to add about a quart of coolant. You will be fine for a month or so, then things just repeat. The coolant never returns from the overflow tank, and eventually the radiator empties out and the overheating returns.

These head breaches on that motor are so small, they can fool you for awhile. I also changed thermostats and did all kinds of things. The symptoms come back and it took a new head gasket to get it back to normal. 30K miles since the job and all remains well.

If the head gasket is blown, wouldn’t I see foam on the oil cap?

Maybe…maybe not.

Absolutely true, but they will all introduce hydrocarbons into the coolant that can be detected with a lab kit. The water jacket surrounds the tops of the cylinders, and if there’s a breech in the gasket the explosions will drive gasses into the coolant.

Your symptoms sound just like my daughter inlaws Civic, same vintage, same problems. Replaced the thermostat, radiator, flushed the system. Turned out to be the head gasket slowly failing. These engines are very robust and will last a long time, but the gasket is a weak area.

The civic head gasket breaches are so small that a pressure test isn’t conclusive. You have to wait at least overnight to see if you lose any pressure.

Sounds like a thermostat issue to me.

As Bill said, it’s a very cheap and easy replacement. I would just drain the coolant before hand and then refill after replacement.

If that does not fix the issue, head gaskets going bad are a very common problem in Civics.

Your issue could be a result of the head gasket slowly deteriorating if the thermostat replacement does nothing.

Replacing the head gasket yourself saves you a lot of money, but requires all of the equipment and the know how to do it plus a decent amount of time. So if don’t really have access to the equipment and/or the know how, I would suggest taking it to a shop and letting them replace it.

Best of luck to you!

Usually the radiator fans don’t come on when driving down the road b/c there’s already enough air flow. Unless the AC is turned on, then I think at least one of them comes on.

It could be a head gasket, and a chemical test would tell you for sure. Before I’d do that – assuming there’s no reason to suspect a head gasket problem, like the engine severely overheated in the past, here’s what I do

  • replace coolant and thermostat. test old and new thermostat in a pot of hot water first.
  • replace radiator cap
  • make sure cooling system is properly bled of air
  • test both of the radiator fan’s speed

If above are ok and it still overheats,

  • cooling system pressure test, if that tests ok, and chemical test is ok,
  • replace or have a radiator shop clean the radiator.

Important note: If the engine starts to overheat, turn off the AC and turn on the heater to max and the heater blower to max. That will provide some cooling for the engine, albeit at making you inside the passenger compartment a might un-comfy on a hot day.