Temp gauge issue

[skora]

On my 97 X SOHC, I have a temp gauge that is rather erratic. It reads high even with the engine off and cold and rises when the engine is running to the point the "Check gauges" light illuminates. Then bounces to normal then back near red line. This cycle continues.

To evaluate this, I bought a scan tool and the ECT reads as expected, around ambient to start, gradual climb till the thermostat opens, then hovers there while the radiator does its thing. Heater works as expected. So I'm fairly confident that my X doesn't have a cooling issue.

What my questions are:
Can I disconnect the temp sender so I'm not watching the gauge bounce around and then the Check Gauges light is available for other issues that arise?

Does the check gauge light also send a signal back to the PCM?

Can I do any damage to the truck or will the PCM throttle the power back and flash the Check Engine light if a real cooling issue materializes?

I'm a lurker, but appreciate the help very much.

[aleekat]

My guess. Trapped air in the system or head gasket....

[shorod]

This does sound like it could be air trapped in the cooling system. When you say it will move around even when the engine is off, I assume are referring to having the key in the Run position, but the engine off, correct? If it's bouncing around even with the key off, there is something else going on.

A decent check to confirm the issue is not with the gauge would be, like you questioned, disconnecting the sending unit and making sure the gauge no longer bounces. The PCM uses the Engine Coolant Temperature (ECT) sensor to adjust timing and injector pulse width, but that's independent of the gauge. The gauge takes its reading from the Coolant Temperature Sender. Disconnecting the sender will not put you at risk for damaging your engine as long as it's not actually overheating.

-Rod

[skora]

The gauge didn't move with KOnEOff, but started at the mid point. Then with KOnERunning, it started to climb to high (sometimes enough to illuminate the "Check Gauge" light and would then bounce to what the ECT reading was showing and then back to the top. I took it to work (12 miles one way) and the scan tool showed exactly what would be expected. So unless there's a little pocket that the sender can have an air bubble in but the sensor stays in the coolant, I'm guessing a bad sender.

The truck has a DNR order as it is, so I'm thinking I will pull the sender harness and be done with it. Even if it is the gauge and not the sender, I don't plan on tearing apart the dash for this. She's driven under 2k miles per year now. Just want to make sure I'm not doing damage unless I do have a real overheating issue.

Again, thanks for all the help. If there's something else I should be looking at, I'll definitely take a look.

[Majoroutlaw]

in your situation, i would want to know the exact running temp of coolant at operating temp. so i would buy an infrared thermometer to double check that your temp sensor isnt giving your scan tool a false reading. because your scanner is only going to see temp that sensor recognizes, not actual temp. while theres a possibility of air in the system, you could cancel this idea by checking your actual temp. if air exists it should of gradually worked its way out granted theres no faulty head gasket. you could also have a bad instrument cluster or faulty ground. as i said these options could be eliminated if you check the temp with an external thermometer at the cts.