I have a 2005 Civic EX Coupe. It has 205,000 miles on it. Lately it has been running hot but only when I get off of the interstate and in "stop & go traffic". I took it to an auto shop. They tested it for combustion and leaks & such and nothing so a blown head gasket was not the problem. They drove it and it would not run hot for them. I have replaced the radiator cap. I cannot figure out what is making this car run hot like that. It runs fine on interstate going 75+ but when I slow down is when it gets hot. Fans are running also. Anyone have any suggestions?

 

The car doesn't get hot at highway speeds because air is forced into the engine compartment, cooling the radiator and engine. The fan doesn't even come on at highway speeds (or at least, it shouldn't). I had a similar problem with a Tacoma I used to have and it turned out to be a bad fan clutch, causing the fan to not spin as fast. I'm on my 4th Civic now and I know they use electric fans, so maybe your radiator cooling fan isn't spinning as fast as it should be. If the temperature was rising at highway speeds as well, I would say you have a bad radiator, but since it's not, I'd say check the fan.

 

Any case of intermittent overheating you should start by replacing the thermostat. Don't bother trying to test it, it is cheap enough to just replace. Make sure the cooling system is completely full. When there is a bubble of air in the top of the engine, the water can't flow at idle rpm but it may when driving. This can be diagnosed by checking that hot air comes from the heater at idle. The heater usually won't work if there is much air in the top of the engine.

The fans are definitely running while the engine is too hot? Note that you can force the fans on by turning on the A/C. It seems counterintuitive to run the A/C when the engine is too hot but if everything else is normal it will cool off because the fans are on. The fans are I think controlled by a sensor on the bottom of the radiator.

 

check fan by jumping it from fan switch on thermostat housing, if it turns on the fan good. Then just replace the thermostat and fan switch and make sure you properly bleed your system of air.

and then update on status of car.

 

hey i know its a little late but did the shop do a leak down test when the engine was cold or hot? i have my 05 civic in the shop right now due to a faulty head gasket and it too was hard to detect.....the bill is $$$$$

 

After I left the shop with it (when I made this post), I needed a new battery. The Honda Dealer would not give me a new battery (warranty) unless it read bad on their test machine. I had to charge the battery every morning yet it was reading good on their meters. Whatever. After a few days, I brought it back and it eventually read that it was a bad battery.

For some reason, after that, my car quit running hot...for a month or so.

Then I broke down and yes, it was a blown head gasket. Apparently a blown head gasket is hard to detect. I wish they would have found it the first time I brought it to the shop. I was confused because I had one place saying it was a blown head gasket yet they did no tests to be sure. The other shop ran leak tests and all kind of tests and said it was not blown.

The head gasket has been replaced and it runs like a champ again. Still gets 33 miles a gallon and with my long daily commute, I will run this car until it is no longer able to go.

 

What happened when it "broke down"?

 

well the shop had the head off did they do any valve work, replace valve guides,or anything of that sort?