weird overheating problem

[california3rdgen]

Hi again, I've been quiet for a few months. My 89 2.0Si has a weird overheating issue. The gauge will come up to normal (below half) in a mile or two and sometimes sit there just fine for miles and miles. The next time I get in the car I can drive 2 miles and the gauge will suddenly rise quickly to about 3/4 then go down to normal, then rise, and again fall. It can then go back to normal and sit there for the rest of the trip (mostly town driving) as if all is well. Things I've done: new timing belt and water pump 3K miles ago. new radiator, new radiator cap, new thermostat. Checked fans - o.k. - and definitely on when the a/c switch is on (which I leave on to make sure is on). I've tested compression - all 155-160 lbs. No white smoke, no evidence of leakage. Bled the system multiple times with the bleeder screw near the thermostat. What am I missing?

[spiffyguido]

When you bled the system did you have the heat setting set to full hot?

[california3rdgen]

I didn't think of the heater hoses. I'll try again tonight with full heat. I am thinking there is somehow still an air bubble in the system as I often get the bouncing idle when it is warming up. Since I had bled the system I figured that my idle control valve was actually bad, but perhaps not... hmmm

[spiffyguido]

Yeah, if your idle is also bouncing you may well have a bubble in the line.

[california3rdgen]

I think I found the problem. I removed the thermostat and put the housing back on with the rubber gasket in place to simply see what was going on. I've driven it now for 3 days and no overheating issues and no bouncy idle. Of course there are lack of heating issues since the water is never stopped from circulating. But I think I remember testing the thermostat a few months ago on the stove and, thinking it was good because I saw it open up, assumed I could save $10 and just put it back instead of actually putting in a new one. I'm pretty sure that was a stupid way to save money. I'll put a new thermostat in and see what happens. It could have been a combination of low water and bad thermostat...

[spiffyguido]

Yeah, that was likely the problem. I'm guessing your thermostat wasn't opening as far as required.