Overheating or bad temp gage?

I have a 92 Z34. I had a coolant leak for a while but it never overheated. I finally got a chance to change out the water pump. Changed it and let it idle for like a half hour and bleed the lines until there were no bubbles. Temperature worked just fine. I left my buddy's house got on the freeway and after about 10 min temp went up fast like it was the rpm gage. Then it went down to the middle then back up, back down ect. Just kept doing it. The next day I figured I’d change the thermostat. I thought that would do the trick. Let it sit and idle for like a half hour. Temp was perfect. Got back on the freeway or even on the streets, the temp went up all the way to red then back down, then back up, ect.. It's driving me crazy. Bleed the lines again. No air just coolant. I Let it sit for a couple days to let the frustration boil down. Yesterday I took it for a ride, long enough to get the thermostat to open. Got back home and opened up the bleeders and bleed them. They seem to be coming out pretty good, but still overheats when driving on highway or streets that have 45 miles an hour or faster. But then today at work I decided to give it another try. So I fired it up and waited until it was warm enough for the stat to open. I was under the hood looking around while it was warming and noticed a ticking. It was pretty loud. Looked like it was coming from the stat. I grab the pipe next to it and felt the ticking there, so it was definitely the stat. I felt the hose get warm and I opened up the bleeder and it came out pretty strong and then it just stopped. Waited a few minutes. It would start to dribble out slowly then came out strong. It just kept doing that, weak then strong. Then temp gage bounced from ¾ hot to right before the first part of the red. So doesn’t that mean the stat should be open all the time? Does this mean it's a faulty stat? Sure hope it is. Thanks

It could be the stat, but the first thing I would do is check the wires to the water pump. Maybe the water pump is kicking on and off pressurizing and depressurizing the system. I don't know which wires are which, but get to the hot wire and hook up a multimeter to it. Have someone crank up the RPM's (after you are at operating temp) to get the system flowing and see if the volts going in are consistent.

I would venture to say that your guage is fine. I threw a belt (causing the water pump to fail) about two miles from my house a few months back and the engine temp rocketed up when the "spice did not flow." I have never seen an engine get that hot, that fast... but that is not saying much :)

are you sure your fans are working?? and have you had it flushed yet?? but you can get a bad water pump also and stat.....

GOOD LUCK.....

Well I took my stat out, so I now am running without a stat at all. It dosen't overheat any more. And it barely gets off the first line. So I hope it means I had a bad stat. I'll try a new one and hopefully it will work. Thanks for the replys and the help.

Don't buy a cheapo no-name stat. Stick with AC-Delco or Robertshaw, avoid Stant. On all GM 60-degree V-6's, I drill a 1/8" steam relief hole in the t-stat flange before installing it to help with purging air out of the system. If anyone reading this has a newer 3.1/3.4 with the t-stat that mounts with the flange vertical (on edge), make sure you put the hole at the 12 O'clock position.

http://www.stant.com/brochure.cfm?brochure=205&location_id=168


We sold Stants at the parts store I worked at, and while I agree they aren't the best, the Superstat line are very high quality and built like a tank...you can feel the difference as soon as you pick one up and compare the weight to the standard part. I've used them and will continue to...

I should have mentioned that in my post Jeff, the Stant Superstats are nice, but are not available for all apps.