Temperature Gauge fluctuation

[totalpackage]

Last couple weeks i have noticed on my 96 2 door Tahoe that the temperature gauge will fluctuate back and forth from about 140 to 250. Sometimes split second fast and other times slowly. Its cold here now and noticed when its around 140 the heat from the AC is warm as it should be but when the guage is up in the 250 range the ac is putting out cooler air. BRRRR ,i need my heat running and so whats up with this guage? Fan clutch? Thermostat?

 

[totalpackage]

Last couple weeks i have noticed on my 96 2 door Tahoe that the temperature gauge will fluctuate back and forth from about 140 to 250. Sometimes split second fast and other times slowly. Its cold here now and noticed when its around 140 the heat from the AC is warm as it should be but when the guage is up in the 250 range the heater is putting out cooler air. its taking longer for my heater to start working, it appears once the gauge gets into the 140 range it works fine. BRRRR ,i need my heat running and so whats up with this guage? Fan clutch? Thermostat?

 

[ISU-152]

I'd start with just changing the tstat. It's cheap and easy. While you're there get a test strip to make sure your coolant is up to *****

 

[YukonandtheHOE]

I'd start with just changing the tstat. It's cheap and easy. While you're there get a test strip to make sure your coolant is up to *****

He should swap out the gauge sending unit too.

 

[totalpackage]

Coolant is less than a year old. I replaced the thermostat over the weekend and it is still continuing to do it. and again it now slowely moves back and forth from 140 to 250 and as it is in the higher range, my heater will not blow warm air. Once in the 140 range, the heater then starts blowing warm air...
After getting home today, after an hour i checked the coolant. I was able to add about half a gallon and not knowing how good the ratio was i added about 2 cups of water.
...next question, how much will the radiator completely hold and why does it not pull from the reservoir? thats what i thought the reservoir was for.

 

[YukonandtheHOE]

Because thats a surge tank for over flow,

...next question, how much will the radiator completely hold and why does it not pull from the reservoir? thats what i thought the reservoir was for.

Its a overflow surge tank, not a fill reservoir. I wonder if your impeller in the water pump has worn out and is kind of free wheeling on the pump snout so its not pushing water as hard as it could be, seen that before.. or you have a lower intake leak sucking air into the coolant system pushing it to the high spot in the system (heater core), pull the rad cap off when its cold and run it till the thermostat opens to see if theirs flow

 

[2door2wheel]

in my truck these symptoms ended up being cracked heads. but every truck is different.

 

[99Vortec]

Have you tried pressure testing your cooling system?

 

[SunlitComet]

it will push coolant into the reservoir when it gets hot and draws it back when it gets cold. so if there is air space int the cooling circuit it will eventually bleed itself out and put coolant in its place.

 

[rockola1971]

Engines dont consume coolant. You had to add a half gallon of coolant....you have a leak. Could be water pump (Check the weep hole) but most likely its the seal of one of your plastic tanks on your radiator that is leaking. They are pretty notorious for this. The reason you are seeing fluctuation on your temp gauge AND your heater fluctuates is because there is air in the system because your coolant is really low. When it gets low enough an air pocket ends up where the temp sensor is in the head and thats what causes the temp gauge to go goofy. The air pocket temp is substantially lower than what the coolant would be on a warmed up engine. When the air pocket ends up in the heater core then that causes the heat to be low or nonexistent out of the vents.
2 other very common issues was heater hose fitting front passenger side of intake cracked and leaking and the intake manifold gasket leaking coolant into a cylinder(s)...Fel Pro makes a problem solver gasket set for this.