I have a 2001 Chevy Express 2500 Cargo Van. At the beginning of spring I had a rusted out freeze plug that I had replaced at a local mechanic.

Before I had the freeze plug replaced I had used some coolant system stop leak to slow the leak in the plug until I could get it in to be worked on.

After the plug was replaced it ran fine and temperature readings were normal. After outside temperatures warmed up I noticed that the temp gauge would stay going up to about 210 which isn't exactly normal. It should if I am correct remain around 190. In the last week the temperature gauge has been fluctuating even higher. Between 210 and pegging out at 260.

Heres what I have checked so far and what I know.

1. Water pump seems to be working fine.

2. No water leaks anywhere that I have detected.

3. Running heat on high seemed to help the temp gauge stay lower but no lower than 210.

4. When temp gauge is in or near the red there is no steam coming from anywhere and the engine still runs nice and smooth and doesn't bog down like an engine does when it over heats.

This morning as I was on my way to work the temp gauge pegged out in the red at 260 and didnt want to move from there to much. Engine was still running smooth, not bogging down. No steam as would be usual from overheating. I had driven approximately 5 miles at this point. Temperature outside was 70 degrees. I pulled over and shut off the engine. Putting my hand on the hood the hood still felt cool. Raising the hood I did not feel an excessive amount of heat from the engine. I slowly released pressure from the radiator and tested the temperature of the coolant and the coolant still felt cool.

One last thing. When temps were cold out the temp gauge would always read barely above the point it is when the engine is completely cold which I always thought was a bit odd

So heres what I'm wondering. Is the temp gauge malfunctioning and reading wrong? Should I flush the coolant system? Is it stopped up somewhere yet still circulating in other areas? Any help would be appreciated.

 

I would flush the system and change the temperature sending unit and thermostat as a first step. Both are relatively cheap items and should probably be changed anyway.

 

If the fluid is full, then replacing it won't help with overheating.

Make sure the system is holding pressure or change the rad cap.

The temp sensor can be tested with a multimeter.
https://troubleshootmyvehicle.com/gm...-temp-sensor-1

 

if running the heater seamed to help then i would doubt it was a faulty gauge or sensor. with the heater on does the water temp seam to correspond with the heat coming out of the vent (heater air gets warmer as the temp gauge rises)?
do you have a code scanner that can read live data?
are you moving (at what speed) or sitting still with these issue?

 

Running the heater does help and the issue occurs when driving and sitting still. To answer your question about the air temperature from the heater corresponding, yes the air from the heater gets warmer as the temperature gauge goes up. I am sure it's not a faulty temp gauge. I am thinking that the thermostat could be partially clogged or not opening properly. This weekend i intend to both flush the coolant system to remove any sediment or particles and put a new thermostat in. Once done I'll post again with the results.

 

do you have a manual fan or electric? does it do it when youre at highway speeds (i.e.50+mph) ?