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Engine Overheating Mystery

Old 05-30-2009, 02:46 PM
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Default Engine Overheating Mystery


2000 Honda Civic EX, 1.6L 127 hp I4, 4-Speed Automatic Overdrive


My 2000 civic is overheating. I have had this problem for around 2.5 years. The car tends to overheat after about 15 minutes of city driving, but has overheated on the freeway, as well (this occurs less frequently, however, because the constant air flow when traveling seems to keep the engine cooler). When the car overheats, the cooling fans will not turn on. To cool the car down when it overheats, I will turn the heater on high. Upon doing this, the temperature gauge will drop fairly quickly; however, I must then keep the heater on to prevent the car from overheating any further.

When the car overheats (while the car is parked with the engine running), the temperature gauge will rise about 8/10 of the way up, then the fan(s?) will kick on. The temp gauge will then fall to about the midway point. The fans will then shut off, and the temp will rise again. This cycle lasts around 3-5 minutes and will repeat itself for however long the car is running.


I have taken my car to several mechanics, yet NONE has been able to diagnose or reproduce the problem. Every time the car is in the shop, the fans will turn on properly. The last mechanic I took my car to even drove the car around town a few times, but never witnessed the car overheat until I drove the car directly to the shop myself after it had already overheated.

The first set of repairs I had done to address this issue were: coolant/radiator flush, new thermostat, new timing belt, new waterpump, new alt. drive belt, new p.s. drive belt, and new a.c. drive belt. Also, the mechanic inspected to make sure that there were no blockages in the hoses. I’m not sure if each of these was done to address the issue, specifically, or if they were done simply as some sort of maintenance. Either way, these repairs did not solve the problem. My car would still overheat.

The second set of repairs was done 9 months later. This time I had the coolant fan switch replaced. This seemed to fix the problem. My car didn’t overheat for nearly a year. Well, a few weeks ago, it began overheating again, and so I took it to a different mechanic.

The third, and most recent, set of repairs involved replacing the entire fan motor unit, the fan switch, and the thermostat (again). These mechanics said that although they weren’t sure what the problem was or how to reproduce it, the repairs they made should fix the problem. They didn’t. The car overheated on the way back.

Other Notes:

The coolant level has always remained stable throughout this ordeal.

The radiator has never appeared to malfunction (after speaking with the mechanics about overheating, some suggested feeling if the radiator had any cold spots between the fins. Mine did not. Nor has any mechanic felt that this problem lay with my radiator or engine…granted, they had no idea what the problem was, I just felt that I should point this out, for what it’s worth).

The oil has always remained the same color (the color of oil – not milky or grimy); however, after the car overheats, the oil level tends to drop slightly. The car is not leaking oil, however – or at least that I can tell. There have been no oil stains, residue, or drops underneath my car noticed by me or any of the mechanics I have used.

The car can produce a smell when overheating. Usually it smells sort of metallic. The mechanics I’ve used said this smell was boiling coolant.

After taking the car home from the mechanic, it overheated again. I pulled over to look at the engine. I noticed neither fan was on. I turned the heater on, then both fans switched on. I turned the heater off, then both fans switched off. I turned the air conditioner on, then the driver’s-side fan switched on. I turned the air conditioner off, then the driver’s side fan switched off. The same happened when I turned the fan on and off (the coolest setting without using the air conditioner). I then turned the heater back on, but this time only the driver’s side fan would turn on. I’m not sure if this is what happens each time my car overheats, but I did notice it today and have noticed that my fans weren’t on upon prior occurrences of overheating.

Last edited by ts87; 09-12-2010 at 09:32 PM. Reason: update/more detailed description of problem
Old 06-03-2009, 11:49 AM
jwmm04's Avatar
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Did you ever put in a new water pump? I know they have around a 100-150k mile life; I always replace the water pump when I do a timing belt.
Old 06-03-2009, 12:03 PM
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If the cooling system has been thoroughly bled, then the radiator may be clogged and therefore not cooling the engine properly. Replacing the radiator is a relatively simple job that you can do yourself.
Old 06-30-2009, 05:41 PM
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I have been having pretty much the same problem. I also have a 2000 Honda Civic EX, automatic, with 93K miles on it.

I had the water pump, timing belt, thermostat and seal replaced last year. I just now replaced the radiator and my mechanic put new hose clamps on (they were leaking after my radiator jobbie). I drove it many many highway and backroad miles when bringing it back home and it seemed fine.

The day after I brought it back from the mechanic, the temp gauge went up about 3/4 of the way while driving uphill on a highway. Once I got off on my exit (about a mile down the road), the temp gauge went down as my speed decreased.

I don't know what level the coolant reservoir was at when I brought it back from the mechanic, and it was low so I topped it off tonight to see if I eat coolant tomorrow.

Maybe we can keep each other updated on our issue, and help each other out. K?

Very frustrating isn't it?

Old 07-01-2009, 10:45 AM
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Oh I forgot to mention.....my mechanic did a leak down test and a pressure check (when he found the hoses leaking......new clamps solved that)..........BUT he did say that the leak down is not 100%.

So.......I might be replacing the thermostat and seeing what happens from there. Right now, at least for today, I am not losing coolant. <crossing fingers>
Old 07-01-2009, 02:57 PM
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Okay, no gauge fluctuations today, no coolant loss......for now my story is that the hose clamps were loose (and possibly needed new radiator). Will update this thread if anything else happens.
Old 07-06-2009, 07:01 PM
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Went on a long trip this past weekend, and gauge would rise on backroads doing 45mph up hills. On the highway with AC it was fine (sometimes). No coolant loss. When I got back I replaced the thermostat, and the old one seemed to be sticking a bit. Did a test run, bled the system, gauge was all over the place. I parked it overnight, went to work today, drove it all over the place and now it seems fine......for now. <crossing fingers again>
Old 08-05-2009, 08:59 PM
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Default Ok here we go on this again,is this a sender problem?

Mine is doing this too, but acting very strange, sometimes the water temp will just shoot up,but will drop back down within a few seconds, the needle was bouncing all over the place, I don't think it's physically possible for an engine to heat up and cool off that quick in that short of a time span, i bought the gauge sending unit from autozone, of course it was the wrong range unit, so i had to put the original one back in, one time it overheated in minutes, i stopped the car opened the hood, and the engine didn't seem anywhere near as hot as what the gauge indicated. I replaced the thermostat, and it seemed to stop doing this for a while, could it be when the gauge reaches the temp that causes the fans to turn on, that the sender goes out of range? they are resistive, and could be failing. anybody got an extra oem sender on a junk engine they want to send me? we are dependent on this car and I have to go to work. it's a 98 lx 4 door, not sure on the engine,it's the smallest one i know that,and a dam automatic. I'm not used to these civics, last civics i really was into were 81-83 ,I'm more into vintage accords now. Tim
Old 08-05-2009, 09:11 PM
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next time make your own post .....
and it sounds like a bad temp sending unit your engine is a d16y7
try advance autoparts and buy one i wouldn't suggest buying a used one because you never know if its about to go out and would be a waste of money..
Old 08-05-2009, 10:23 PM
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Originally Posted by 94civichatchback View Post
next time make your own post .....
and it sounds like a bad temp sending unit your engine is a d16y7
try advance autoparts and buy one i wouldn't suggest buying a used one because you never know if its about to go out and would be a waste of money..
I wasn't trying to interrupt your post, every other forum i belong to says use an existing post if it's the same question, so i searched and i was having the same problems as listed earlier in this thread . also does this engine have a seperate fan switch for temp or does the computer read the temp off it's sensor and send a signal to switch on the fans, I'm more into carbs and things like that then these modern FI engines, It's a good car, only 163000 miles,just this strange problem.

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