Would that make it read hot? I thought it would read zero or too cool if the connection was bad.

 

Wiring issues can do all kinds of strange things (it may not get a reading, it may shoot straight up, those are the most common scenarios). The fact your hot idiot light came on I find concerning. As I understand, that is controlled separately from the gauge and only triggered when the computer is getting a reading from the temp sensor informing the temp is too hot.

I'm still finding it odd all this began after a water pump change. But if your mechanic used a scan tool while the car was running to do a comparison check of what the car computer states the temp is reading vs what the gauge says the temp was reading and found a large difference between the two reading, I trust what the scan tool is stating.

Perhaps it's a bad stepper motor in the cluster not letting the gauge go back down....

 

The question is which computer module triggers the HOT light? Is there any way to test a BCM? The gauge goes all the way back down after sitting for a while and seems to warm up normally but then keeps going up. If I pull over and let it idle it gets back down to normal range. It only seems to rise while I'm cruising for a stretch of a mile or more over 35 MPH.

 

OK, now I'm getting mixed info, you say the gauge climbs while driving but if you stop and idle, it goes down? Did this information get compared with a scan tool reading the computer to see what temp the computer has? Basically, when you see it constantly climbing, has this data been compared LIVE with a scan tool? That description goes back to a possible air pocket in the system.

Another thing to keep in mind, the gauge on your dash is not a 100% accurate tool, there is a margin for error as far as what it reads. The GM mechanic I know see that calibration issue more commonly in modern Pontiacs.

I am not sure if it's the BCM or the PCM that triggers the hot light, but I know it's NOT the instrument cluster. To have the BCM tested, you may be stuck taking it to a dealership. I know a normal scantool can't hit that and I'm not sure if a GM Tech II will do it or if you need something beyond that (that question is out of my league).

 

I had the water pump changed at the dealership about 8 years ago and took the car back a couple days later after the temp gauge pegged out, back and forth (hot then cold). Of course...they couldnt find anything wrong and the mechanic, who was standing right there never said a thing, even though he had to know the pump he took off my car was NOT the same as the wrong pump he installed. A lot of pumps will fit but are NOT the correct pump.

I was hearing a noise when I started the car cold, but after a while it would go away. I thought it was a pulley bearing going bad. My mechanic thought it was the belt and changed it( Jan 2013). The noise came back in a few days, and I thought, okay I'll just drive this baby until the problem shows itself, and when it got worse and I started losing antifreeze it was looking like the water pump was bad.

Well...the water pump finally went out this month (April 2014) and when I took the old pump off I immediately knew they had installed the wrong pump on my car. The pump they installed had only 6 impellers, and the correct GM pump had 9 impellers.

I installed the new pump (it was not easy) and the temp gauge is back to normal operation. Its a wonder I didnt destroy my engine with that pump not properly providing cooling to the engine, I chalk that up to the fact that I dont drive very much or far (currently 69000 miles). I'll never trust that dealership again. Hope this helps someone else.

 

I would double check the coolant level. As mentioned, it does sound like air trapped in the system. Is the heater in the on position while filling the radiator so the heater core is being filled also?

The temp sensor sends a voltage signal to and is grounded through the PCM. Did your mechanic replace the temp sensor (on engine) or the coolant level sensor (in radiator)? If the level sensor is malfunctioning it could be sending a false reading to the PCM causing it to trigger the warning lights. I would also double check the connector at the sensor(s). An intermittent ground could cause the gauge to dance around like that.