I have a 2004 Honda Civic thats new to me and just started overheating. I first noticed a month or two after I bought it...I was idling in park for 45min or so when I noticed steam coming from the engine bay and the temperature gauge reading all the way hot. It overheats the most when I slow to a stop or am idling and is less likely to overheat while I am driving faster than 40MPH or so but at times it will overheat and then remain at HOT for an extended period of time while driving at speed (I try to avoid driving in this condition obviously but I had to at the time).

I took it in and they performed a pressure test on the coolant system: came back fine; also have not observed white smoke from tailpipe and oil is not milky (suggests NOT headgasket problem I concluded). They didn't do anything to it and it ran fine for a day and then went back to overheating.

Fans: Both fans run when the AC is switched to on in the car. However, neither run (even when car reads as overheating on the gauge) unless AC is on (in which case both fans run but it still is overheated 5 min later). Since the car still does not cool even with fans running via the AC switch, I concluded there must be some issue aside from the fans not turning when the car starts overheating.

Water Pump: Not sure how to check this but after the car has been running from a cold start for 10 minutes the upper radiator hose is too hot to touch suggesting the coolant is being cycled out of the engine into the radiator. The lower hose is warm at this point but not hot. Earlier I was able to get hot air out of the vents with the AC turned to hot but I just tried again later today and it was still blowing cold at max heat on the AC dial, not sure if that means anything.

Hoses: In addition to what I observed above, there appears to be white splattered dried material (potentially evaporated coolant) on both the upper and lower hoses. Also when it does steam, it comes partly from near the upper radiator hose and partly from behind the engine (see vid).

Thermostat: If the upper radiator hose is too hot to touch at operating temperature, this must be opening so that cannot be the problem I would think.

Coolant Reservoir: At operating temperature when the fans should be on, I held a flashlight to the coolant resorvoir and it did not appear that any of the coolant had been sucked into the engine at all even though I believe it should have at that point. This was the case even though the upper radiator hose was too hot to touch. I don't understand this.

Gauge Temp Sensor: Seems to be working; slowly increases from cold start and steam comes out of engine after gauge has been reading all the way hot for a while suggesting it is accurate.

Question: Does anyone have experience with coolant systems to conclude something from all this information? The fan itself is clearly fine along with the relays and fuses for it since it is able to run, there is coolant in the car, and the one temperature gauge sensor seems fine. Do these symptoms suggest anything to anyone? I am at a loss and I am way to broke to let the mechanic figure it out by trial and error.

Pictures of oil, Coolant still in reservoir while engine at operating temperature, and video of steam while engine is overheating here (sorry it makes you download to view): https://ufile.io/f/1dx8l

  • What kind of coolant is in the vehicle? The head gaskets in the D17 engine which is in your Civic are known to go bad if anything but Honda Blue coolant is used. This held true for both D17A1 (non-VTEC) & D17A2 (VTEC) engines. My 2003 Civic had this problem and ended up putting in a JDM replacement engine. The engine would be fine as long as I kept up with the coolant levels. If I let it go for more than two days, it would start having coolant temp spikes once warmed up. Commented Mar 21, 2022 at 11:54
  • @Pᴀᴜʟsᴛᴇʀ2 uh oh, yeah I use some generic 50/50 mix from autozone. Man I hate coolant leaks there so elusive
    – NULL
    Commented Mar 21, 2022 at 14:39

3 Answers 3


You are not getting hot coolant through the heater core and it's overheating. The 3 most common causes of this are

  1. Low coolant which would indicate a leak. But you've had it tested and I assume it's full by your post.

  2. Related to low coolant you may have air in the system. You mention a coolant reservoir so these are most of the time bleed easily, but I'd still check the internet for special burping instructions. Some vehicles have a bleed screw.

  3. Restricted coolant flow. Most likely places are from plugged radiator. A good flushing would thake care of that. And of course a bad water pump would prevent coolant flow.

It would bother me that the fan(s) don't turn on when the temperature demands it. Your temperature gage works which would indicate a temperature sensor is working, but many cars have two temp sensors. So you might check that out. Your symptoms point to the fans not turning on when hot so that is the first thing to look at. Just because the fans turn on , doesn't mean everything. Some systems have low and high speeds. Maybe the A.C. requires low speed and the temp sensor is supposed to demand high speed. Sorry this isn't as vehicle specific as it ought to be, but it may give you something to start with.

  • Re coolant yes it is full they topped it off when I took it in and it was not empty before. I'll look into getting potential air out of the system. Re temp sensor you are correct there are two. The second one is near the thermostat and I think is referred to as an ECT. I guess I might try replacing that. I'll have to circle back thanks!
    – NULL
    Commented Mar 21, 2022 at 2:08

Sounds like this vehicle grossly low on coolant. My suspicion is bad vacuum seal on the two way flow system on your radiator cap. That means coolant in the overflow tank is never getting sucked into the coolant system after shutdown.

Closely inspect the radiator cap. Replace it as required.

Here’s a link with complete description and image.

enter image description here

  • Just checked and the cap looks fine to me (and is wet on the underside suggesting coolant has been passing through) but I don't really have much experience with the coolant system on cars so I will definitely try this and let you know if it works. Thanks
    – NULL
    Commented Mar 21, 2022 at 2:09
  • The key is that photo note. It should have a gentle spring force pulling it to the rubber seal. If that small disc flops around freely, the cap is bad.
    – zipzit
    Commented Mar 21, 2022 at 22:13
  • ok thanks I just checked, it is right up against the rubber seal. I can pull it up away and it pops back against the rubber when I let go. My wallet is very sad this was not the solution.
    – NULL
    Commented Mar 21, 2022 at 22:30
  • So that cap is good. After you run the car, then shut it down for the night, that cap enables coolant from the overflow bottle to enter any voids in the engine’s coolant system. This takes 8 to 10 hours after shutdown. Verify overflow bottle has coolant, verify no cracks in overflow hose. After two or three days of this, before you start engine the coolant will be at the top of the radiator.
    – zipzit
    Commented Mar 21, 2022 at 22:38

Wanted to update because I'm very appreciative of the help. I ended up taking it to the mechanic and it turned out all it was, was a disconnected hose between the overflow reservoir and the radiator. So there was no fluid in the radiator because it couldn't get there. I guess the engine was cycling water or something, not sure. They hooked it up and filled it back with fluid and it has been staying cool so far! So basically y'all were both right to some extent in terms of the underlying problem.

I have absolutely no clue how that hose could have become disconnected I want to hope it was not intentionally disconnected by a previous shop. Thank you every one very much, hopefully this solved the problem for a while.

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