Strange overheating problem

[Cephas]

Thanks for any help - this one has me stumped.

When the ambient temperature is above 70f, my car will operate at normal temps 100% of the time. When it's colder out, the car will operate within the 1/2 to 3/4 range.

If I'm at a stop in cooler weather, the temps will creep up until I start moving again. The overheating is not dependent on vehicle speed, but engine RPM. If I simply rev the engine, the car cools down. If I'm maintaining speed in 5th on the highway, however, the car will creep to 3/4 again until I reduce my speed and drop a gear.

I have replaced the fan clutch already and the PO gutted the electric fan.

The crazy thing is that this problem is non-existent when it's warmer outside, which makes no sense. My next guess is that the accessory belt might not be tight enough, or potentially that the reservoir cap is not holding pressure. Anyone heard of anything like this before?

[jlevie]

Try bleeding the system using the procedure below. A moderate sized air bubble in the system could cause this.

1) Using ramps or a jack get the front of the car 1' or more higher than
the rear. That will make the radiator and bleed screw the highest point
on the engine and facilitate removal of air.

2) With the bleed screw open, add coolant until no more air comes out of
the bleed.

3) Leave the filler cap off, or at least loose, and set the heat for max
temp and fan speed. Leaving the cap loose will prevent air that's still
in the system from causing a "coolant fountain" once the engine heats
up. Start the engine and allow it to warm up to operating temp. As it
warms up occasionally crack the bleed screw to release any air and top
up the coolant as necessary.

4) Once the engine is at temp bring it up to 2000-2500rpm for a few seconds
several times. Then crack the bleed until no more air is released. At
this point the heater should be throwing lots of hot air, which
indicates that the heater core is filled with coolant. You may have to
repeat this a few times to get all the air out.

5) Drive the car a bit, allow it to cool back down, and recheck the
bleed for air. Over the next few days you may get very small amounts (a
few bubbles) of air out of the bleed screw.

[dnguyen1963]

In addition to bleeding the system, I would remove the SI board and examine it for cracks. Also check the batteries for proper voltage.

[Cephas]

Thanks for the replies. I bled the system following the directions in jlevies post. No air ever came out, just coolant and the heater was warm/hot the whole time, so I highly doubt there's any air in there.

I wikl be checking the SI board tomorrow. In the meantime, I know that the PO replaced the t-stat a few thousand miles ago when he did the timing belt, but is it possible that it's sticking? Maybe when it's colder out the water coming out of the rad is at just the right temp to not let the defective t-stat open all the way. That would make sense, but I want to make sure I'm not just grasping at straws here.

[UserError]

While you're messing with the tach, pull it out and tighten the grounding nut on the back of the temp gauge. If that nut's loose, it'll make the gauge read high/erratic.

[Cephas]

Pulled the SI board. Looks like it was replaced with a unit manufactured August of 1993. The batteries are Lithium and are showing 3.11 volts across their terminals. The board has no corrosion anywhere. I don't think that it's the problem. The nut behind the temp gauge was tight as well. I'm going to loosen it and re-tighten for shits and grins.

The hunt continues!

[Ceeker]

I would also check the temp sensor on the thermostate housing. if this is defective it could give you false readings. make sure you didn't get a cheaper stat put in and may also need to check if it was installed correctly, arrow facing up and pin side facing out.

[Cephas]

Update: Took the car out for a spin after re-installing the cluster. Before putting the thing back in, I loosened and re-tightened the nut behind the temp gauge a few times.

I don't want to jinx it, but temps stayed very normal. A few times while sitting at long reds, it moved to just over 1/2, but most of the time it was staying just below that like an e30 should. I'll update again if this still isn't fixed, but I was pretty pleased to not see temps creeping up!

[tcvd]

Maybe fan clutch?

[Herr Faust Schinken]

I would bleed the system a few times over a coupe days to get all the air out , just to be sure , and you could check your fan clutch while your at it just to make sure