My 99 m/c 3.1 (170k) is overheating -- my symptoms are a little different then the other overheating posts I've seen on the forum. Disclaimer -- I'm not much of a mechanic -- my uncle's been helping me.

Some background: car had misfire codes and difficulty starting. Replaced the plugs and plug-wires and the coolant temperature sensor. Misfire codes went away and the car is now starting with no problems. However, after the fix the car started to overheat (according to the gauge) and we can't figure out why.

Symptoms: After starting car, the temp starts to rise fairly quickly (in 20 degree weather). If only left to idle, the needle will climb to about 3/4 of normal range. If the car is driven or the engine revs, the needle will climb all the way to 260. If left to idle again, the needle does not come back down on its own. I have good, consistent heat the whole time. The fans DON'T come on at all, even with the temp needle buried.

Things tried: Originally thought there was air in the system, but we've bled the crap out of it several times while revving - squeezing hoses, etc. Replaced the thermostat. No difference. Even replaced the driver-side wiring harness that was corroded and bleeding voltage (thinking maybe it was preventing the fans from kicking in.) Tested the fans directly and they work, but don't kick on on their own.

At this point we're not sure what to do. We have good heat -- so the pump seems to be OK. Is the sensor bad? Why aren't the fans kicking-in?

Any help or ideas would be greatly appreciated. Thanks.

EDITED: We also swapped-out the new coolant temp sensor with another new one in case it was bad. No change.

 

Each time you bled the car, was there air coming out?

Or was it just instant coolant each time??

 

the fan relay could be bad. i think my car had a similar problem which was bypassed by the PO with a manuel toggle swich to the left fan. it could also be something like a fuse has gone bad. as for the idle, you may also have a bad TPS ( throttle position sensor), although that should throw a code.

are there any check engine or service engine soon lights on?

 

Seems like we're getting only coolant when we bleed.

 

... We tried swapping relays with no effect. Testing the lead from the PCM shows hot -- I think it needs to be ground to close the relay.

... All the fuses test OK.

... No check engine lights / no codes in system

Thanks

 

Can you do a pressure test to see if there is a leak?

 

Don't seem to be losing any coolant -- but that's probably a next step. Thanks!

 

Thanks for the replies. We replaced the ECT sensor (for the third time) and everything is working fine now.

Seems odd that we would get two bad sensors in a row... Not sure if I did something wrong?

 

Hmmm...That does seem odd that you got 2 bad sensors in a row. But i'm glad you got it fixed. Maybe the sensor was not making a good connection or something.

 

When was the last time you flushed your coolant completely? Could be gumming up the sensors.