Alright here it goes. When I first bought the truck the temp guage was fluctuating up and down, and when I replaced the t-stat and rad cap the problem pretty much went away, it would fluctuate a little but nothing that concerned me. So lately because it's been getting colder in the morning I've been using the heater first thing around 7ish when I'm heading to work. Now I don't know if it was coincidence or not but I noticed that the stock guage would shoot up to the top end of normal then fall down, and do this repeatedly.

When I first got the truck I replaced everything to do with the coolant system including sensors with the exception of the rad and water pump.

Fast forward to two days ago when I figured that maybe it's the guage just acting screwey, so I go and pick up a sunpro temp guage, nothing crazy expensive just a way to get a second opinion so to speak, it did the same thing. Alright so it's not the guage that's at fault, so I replace the thermostat thinking maybe I got a faulty one. After I do this on my lunch break I drive the truck to work, the temp spiked all the way too 230deg and fell back down to 195, then did it again and by that time I was at work so I shut the truck off and someone told me that it sounded like air in the system, I for some reason never though of that so I uncapped it, I started adding fluid slowely with the truck off, I could see bubbles comming up and kept adding till it was full and no more bubbles were present. Then I drove home from work (about a 5 min drive to my place.) it did the same damned thing on the way home, got home and uncapped it again (carefully) added more coolant while the truck was running and got a few more bubbles. Put the rad cap back on and the damn temp spiked higher than i've ever seen,shut the truck off and uncapped it again.

So I filled up what spilled out because of uncapping it while still hot and re-filled it a little more again with the truck running and just watched the coolant with the truck running and the cap off, it was moving so the pump is operational. I just let it run probably for about 10 minutes with the rad cap off it never overflowed and the temp guage sat at 160 and didn't move a millimeter. So I took it out for a road test on the highway by antrim and drove about 10km and came home....still didn't move more than a cm or two the whole trip.

So now it's not overheating, but it's overcooling. I'm going to go freakin crazy I swear...

 

did ya install one of them newer type fail safe t-stats? might be stuck open,or if someone used silicone while doin a water pump at one time there might be chunks of it running around in your cooling system. i didnt believe silicone could make such a mess outa cooling systems till i cleaned a pickup load of radiater's about 3/4 of them had pluged core's full of the blue silicone

 

yeah I put in a failsafe, that's what I'm thinking too. I'm gonna try one more t-stat today at lunch and see what effect that has.

 

Did the same thing here. put in one of those fail safe t stats when i was trying to figure out why my truck was over heating. when it over heated again I replaced the rad and then I could not get the truck hot. The guy at A*** Z*** told me that they were designed to stick open if the t stat goes bad. but the truth is they stick open anytime the truck over heats. I dont care for disposable parts my self. went to napa and got a good one and it runs great now.

 

K, well it did stick open so I got one from napa but now it's doing the bouncing thing again. in between 195 and just over the 210 mark. It's not irratic but it will climb within about 5 seconds and then fall within 5 seconds. So far on my short trip it didn't climb above 210 but it still shouldn't be doing that. I've about had it with this truck, I'm seriously fed up. So basically with no thermostat it stays at 160 but then as soon as I put one in it starts jumping around. This is the 3rd that I've put in not including the one that was in it when I got it.

 

look at your hoses are they colapsing when on the gas? theres spose to be a spring in the bottom one for shure some time even the top does too

 

Nope,the hoses are fine.Guess the only other thing it could be is the rad, but wouldn't that be an issue even if the t-stat was stuck open?

 

the guage you got did it actally have a sensor that screwed into a coolant passageway or just hook up to the stock sensor via a wire? if it had a wire then it wouldnt trust it becuase the sensor could be going out, when you open the hood does it seem hotter when it get up to 210 compared to 195? dont ever let the truck run when the coolant temp is over 225 dont risk warping a head, my stock guage goes up to "O" on normal and stays there doesnt matter how hot it is it will go up to "R" when towing up a big hill, you can also try a colder thermostat 180* and a radiator flush and 60%water-40% coolant mix

 

It came with a new sensor, so that's not the issue. I've tried a colder thermostat and all it does it jump in between 180 and 210 instead of 195 and 210. I've flushed the coolant already so that's not it.

Basically if I take out the t-stat it doesn't overheat and stays at 160, as soon as I put one in it will overheat and there's no way they can ALL be bad.

 

Well I replaced the rad today. SAME ****ING PROBLEM!!!!

 

If your tstat dont have a small hole in it to let the air to pass up through it will give the trouble that your going through GM cars are real bad at this.

I use a 1/16" drillbit to pop a hole in the tstat then make SHUR YOUR HOLE IS TO THE TOP just about an 1/8" in from the edge of the gasket

Wayne

 

yup, all the thermostats I've put have a little hole in them with a piece of brass or copper loosely put through I would assume for this reason. They are keyed so they only go in one way too.

 

I replaced both the Tstat and the sensor and the problem went away.

 

so did I, problems still there.

 

How's your radiator cap?