On my 1999 Tahoe all of a sudden my temp gauge has started going haywire. At a stoplight it will rise up near the red, and when I accellerate the gauge drops down to a cooler temp than it should be. When the temp gauge is way up my heat blows out cold air, and when it goes down the heat blows hot air. My first guess would be to replace the thermostat, but I'm not sure if thats the problem and I don't want to do anything that is unnecessary. What the heck is going on here? Thanks

 

I would say the thermostat is sticking so when stuck shut it gets real hot but also keeps the hot fluid out of the heater core then when it opens it gets to the heater core and alows it to flow and get cool.

 

well I guess I will go ahead and replace the thermostat and see if that fixes the problem. thanks

 

Did the thermastat repair the issue with the Gauge?

 

Not sure about that.

Typically the plumbing for the heater core is taken before the thermostat .... as a bypass.

When it's cold and the engine temperature isn't up the thermostat would be closed .... meanwhile the water pump is circulatingcoolant through the heater core andon to the rad ........ even though the thermostat is closed .

If you have a thermostat that is stuck in the closed position ... you can offset the over heating by turning on the heat setting inside the cab. since the heater core is a small second rad.

It would seem strange the thermostat would close shut when the vehicle comes to a stop and suddenly open when it accelerates.

In this case I would be inclined to check the flow through the water pump. At a stop the flow seems to be reduced and at acceleration it seems to increase.

Just a thought.

 

I am having a problem with my 1999 chevy tahoe, it was -4 degrees outside when I went out to start my truck. I drove approx 20miles and the gauge for temp went all the way to the right past redline. I got off on the next exit and the truck died. I turned the car off and opened the hood to see if there were any problems I could see. The overflow tank was bubbling and then would stop and do it again a few mins later. I did not have much steam coming from the radiator and there was no pressure when I opened the cap. (has a red lever to release pressure). I opened up the cap and added some more antifreeze. A "mechanic" stopped to help and said that since the engine was not "hot" I should try and start it. I had been sitting on the side of the road about 30mins. The truck did not want to start like the battery didn't have enough of a charge. The man attempted to jump my truck but put the cables in the wrong position on his vehicle. It sparked when he attached them to my truck but he said that was because of the snow. I told him to stop and had the truck towed to my house. I bought a new battery and started the truck. I let it run and noticed that the check gauges light came on and the needle again dropped to below 260 then dropped to cold. It ran for a total of 35min. I went and bought a new thermostat and put it in. The truck started up and after about 5 mins the needle did the same thing. I turned off the truck and came here to ask what next. I am out of ideas and do not have a clue. Any help would be appreciated, thank you.