Temp gauge issues

[bridgeman]

Hello to everyone

I am new here and could use some help, about a month a go I bought a 1999 Tahoe LS 4x4 with 125,000 miles 5.7L all stock. When I first got the tahoe the temp gauge would only get up to about 160deg. So after reading some post here I changed the thermostat (195deg) & and also put on new hoses a new belt a new water pump and new coolant. I changed the hoses because they looked orginal. I also had the alternator rebuilt as it was not working very good. After all this work the temp on the dash gauge still read about 160 degs. So then I changed both coolant sensors the one on the drivers side of the block by the #1 cyclinder and the one in the top of the engine at the upper radiator hose. Now when I drive the tahoe the temp gauge goes up to about 230 deg and will fall back around 195deg then runs back up to 230 deg and down again 195. I had a 1996 Chevy PU 5 years ago and the temp gauge never went over 195 deg in it. I have tried 2 different themostats does the same thing. Have I missed a sensor or is the gauge bad? I am now lost as to what to do. I don't want to overheat the engine.

Thank you for any help.

Bridgeman
1999 Tahoe LS 4x4

 

[branndon_b]

bridgeman, check your coolant level. i know it seems silly, but if your coolant level gets low, the coolant has to really get hot to boil up enough to push through the thermostat (or heat the wax pellets enough to open it). when my yukon's low (slow mystery leak, probably intake or water pump, not enough to pool or show under the truck) my temp gauge will climb up and then fall like a rock when the thermostat opens. usually only takes a couple times and then it reaches steady state. depends on how low it is too. this may not be your problem at all, but it's worth a look. keep us posted. branndon

 

[cam3439]

+1 on that. You could have air in your coolant system that you need to work out.

 

[bridgeman]

I will try to get any air out of the system Saturday.

Would a bad radiator cap cause this? How would you test a radiator cap.

Bridgeman

 

[branndon_b]

i would think that if you had a bad rad cap you would keep having a full antifreeze expansion tank due to the antifreeze leaking past the cap and into the overflow tube to the exp. tank. but as cheap as the caps are, if you even suspected it, you could just replace with new for a couple bucks.

 

[bridgeman]

I put on an new rad. cap and tried to get the air out last nite. I still have the same gauge issues. So got a temp gun, got the truck hot and used the temp gun to get a reading on the coolant sensor that goes to the dash and it was reading about 215 deg the same as the dash gauge. I also took a reading at the top radiator hose inlet on the block and it was at 165 degs. the top radiator hose was about 120 deg and the bottom radiator hose was reading 55 deg. Everything seems to be working I am starting to think I have a plugged coolant area in the engine itself. I guess time for a flush of the cooling system.

The truck runs fine but the temp gauge going up to 230 and back 195 all the time just bugs me when driving.

Bridgeman

 

[fyremanpat]

if you did get all the air out its possible the t-stat took a dump already..I know its new and all, but it can happen...still sounds like air in the line though or like what you said a clog somewhere

 

[TheBeast99]

hmmm, this makes me wonder if i should check out my system completely. the only difference with me is that once my temp drops to 160F it doesn't fluctuate like the OP's does.

 

[bridgeman]

I got a new thermostat from Chevy and it looks some what different than the ones I got from NAPA. I will put it in once I Flush the system. Is there anything I could add to a flush other than using the Prestone rad flush to get rid of any deposits?

Bridgeman

 

[blueflamed03]

run a seperate gauge, you may have a faulty gauge on your dash, and you are chasing a rabbit trail under the hood. Verify you are running 160 first...is the heat HOT air?