Hey all,
I have an 04 trail blazer. Yesterday a high coolant temperature warning message came on and the temp gauge had moved from 210(normal) to 260 (red zone). I shut off the engine and checked under the hood. Coolant reservoir was full. No apparent leaks anywhere. After cooling down I removed radiator cap, radiator full. I restarted and drove home, about 20 minutes, with no problem. Left engine idling for about 30 minutes. No problem, normal temperature, cooling fan running. Any thoughts?

 

Possibly an air bubble &/or your thermostat is on it's way out.

You can burp the system by jacking up the front PS of the blazer or drive that wheel on a ramp. Remove the rad cap & let it idle until it gets to normal temp. Then crank the floor heat up in the cab & rev it to 1000 or so for 10 seconds, let it idle a min & rev it again. Do that 5 or so times & check the coolant level. If it's gone down then you've burped the system.

 

Or the coolant temp sensor is going out.

 

The three thermostats I have open up around 195-205 in a pan , at this point these are the things I’m thinking:

1. The factory gauge is just off and not accurate ( I can try to wire up a aftermarket one)
2. The new radiator and new fan clutch are working to good. ( I can remove the fan and do a test run)
3. Some how water is getting around the thermostat ( I don’t know how to check for this the beveled o ring gasket Seem to seal good and makes a sandwich with the housing

it was 8 degrees this morning it stayed around 170 and the heat was hot.

 

The voltmeter in my gauge clustee is way off.

Because I tow and burned up a transmission while doing that, I got an OBD gauge reader to monitor the transfluid temp. I have it set up to also show the coolant temp since I find the analog gauge hard to read with its odd subdivisions.

Here is a link to what I got. Be sure to get the one programmed to read the transmission temp. https://ultra-gauge.com/ultragauge/index.htm

 

Being a 95 and OBD 1.5. I’m not sure I have that functionality through the OBD port

I think I might take out the sending unit and use a portable stove to heat water to a know temp and see if the gauge is off

 

Boiling water is always 212F at sea level. Good idea!

 

And 196F at 9100 feet elevation. Yes, I found the equation sometime a number of years back when we moved up to CO about 10 years ago. Good thing to know with cooking, too!