Thermostat gauge bouncing

[Obsjaxon]

99 Tahoe: Thermostat gauge rapidly rises as soon as i get over 90kmh.I let off and can watch it go back to where it normally sits. It doesn’t seem to have a coolant leak and is topped up and im not sure if it was actually hot when the gauge rose but im thinking its the gauge itself that's the problem. I was only driving for a few minutes before i noticed it and my heat was blowing cold air still, even cranked to the max like it usually does when its not fully up to temps. Lots of talking sorry just looking for other opinions before i start tearing things apart and if i should be worried about overheating it

 

[Schurkey]

Connect a scan tool, compare the gauge reading to the scan tool "coolant temperature" reading. If they agree, it's not your gauge. You've almost certainly got real overheating problems.

If they DON'T agree, look for reasons the temp sending unit near the #1 spark plug is getting overheated at heavy throttle--maybe an exhaust leak blowing hot exhaust gas on the sensor. Maybe restricted coolant flow in that cylinder bank. Or maybe the wire on the sensor has a bare spot in the insulation that's grounding on some piece of metal.

 

[Obsjaxon]

I will connect a scanner first and move from there thank you!

 

[L31MaxExpress]

Start with changing the thermostat, put a 2 x 1/8" holes in the thermostat. Then top off the coolant.

 

[454cid]

The temperature gauge reading hot, with cold "heat" and suddenly dropping it a classic sign of low coolant. You say it's "topped up", so I suppose you could have an air pocket. Do you hear any gurgling in the dash? If so, that's air in the heater core.

First thing I'd do is use a pressure tester to pressurize the cooling system. You'll want to see if it holds pressure, and if not, find where the leak is.

You said it doesn't seem to be leaking, but they're not always easily visible, especially with a hot engine. The coolant will evaporate too quickly from engine heat.

It's possible you have several things going on.

 

[Obsjaxon]

The temperature gauge reading hot, with cold "heat" and suddenly dropping it a classic sign of low coolant. You say it's "topped up", so I suppose you could have an air pocket. Do you hear any gurgling in the dash? If so, that's air in the heater core.

First thing I'd do is use a pressure tester to pressurize the cooling system. You'll want to see if it holds pressure, and if not, find where the leak is.

You said it doesn't seem to be leaking, but they're not always easily visible, especially with a hot engine. The coolant will evaporate too quickly from engine heat.

It's possible you have several things going on.

Thank you! The rad was full of coolant on the day of the post but the overflow was a little below the line so I topped it off today not a noticeable amount considering i havent had to add coolant ever since i got it almost a year ago and when i shut it off i heard gurgling/bubbling from around the glovebox not sure if that’s because i topped it up while it wasn’t running or what you said heater core. If it is the heater core what am i in for? Pulling the dash? Thank you again!

 

[454cid]

Thank you! The rad was full of coolant on the day of the post but the overflow was a little below the line so I topped it off today not a noticeable amount considering i havent had to add coolant ever since i got it almost a year ago and when i shut it off i heard gurgling/bubbling from around the glovebox not sure if that’s because i topped it up while it wasn’t running or what you said heater core. If it is the heater core what am i in for? Pulling the dash? Thank you again!

The heater core is an easy change, but the leak could be anywhere. Get a pressure tester from the parts store. The major chains have a loaner program.

 

[Obsjaxon]

The temperature gauge reading hot, with cold "heat" and suddenly dropping it a classic sign of low coolant. You say it's "topped up", so I suppose you could have an air pocket. Do you hear any gurgling in the dash? If so, that's air in the heater core.

First thing I'd do is use a pressure tester to pressurize the cooling system. You'll want to see if it holds pressure, and if not, find where the leak is.

You said it doesn't seem to be leaking, but they're not always easily visible, especially with a hot engine. The coolant will evaporate too quickly from engine heat.

It's possible you have several things going

The heater core is an easy change, but the leak could be anywhere. Get a pressure tester from the parts store. The major chains have a loaner program.

Sweet, i work at a body/mechanic shop so hopefully someone there has one and i can check this right away if not on to parts store