Rep Power: 0

2004 Civic LX sedan, AT. We have replaced virtually all of the parts of the cooling system (radiator, thermostat, fan switch, fan relay, temp sending unit) as well as the head gasket. We are not throwing parts at the car. All of these needed to be replaced. But after replacing the head gasket, our mechanic found the car overheating, apparently because the radiator fan does not kick on at the appropriate temperature. I have never found a blown fuse indicating a bad fan motor. Any suggestions as to how to diagnose further? As I said, the problem is intermittent. At times, the fan kicks on properly and the car runs fine without overheating. Then other times it won’t. As far as I know, both fans always run when the AC is on.

 
Rep Power: 518

Radiator is staying completely full? Low coolant level will make it overheat.

I know you said the fans work with the AC on, but....

When the fans do not come on by themselves, have you tried turning the AC on? The fans SHOULD run with the AC controls on (IF the PCM has not decided to kill the AC due to high ECT value).
(If the fans do not operate by coolant temperature but will run when the AC is on, that may be a clue. If fans will not run by either system commanding them on, that points to a different clue.)

As a test, I might remove the connector from the fan switch on the thermostat housing. Turn the key on, use a jumper (paper clip) to connect the two terminals in that connector, make sure both fans run.

Do this a hundred times in a row. Hopefully whatever is sticking (relay?) and causing it to not work will act up while you do this repeatedly.


Get the fans to run with the jumper in place as above, then wiggle wire harnesses in the engine compartment to see if they quit?

BTW there is a relay for each of the fans.



Verified the accuracy of the coolant temp sensor (the one for the computer)?