So I have a 97 12 Valve, and my coolant temp gauge has been fluctuating bad. Idling it’ll sit at 180-190 and then when you get on the throttle it shoots to 230+ and let off the throttle it goes back to 180-190. I’ve replaced the thermostat, thinking maybe it’s the coolant temp sensor? How often do those go bad? Thinking about just running a new gauge to get a more accurate reading and by passing the factory wiring. If I was to do so could I install the probe into the sensor hole on the back of the block?? Or could I use the plug on the top of the thermostat housing??

I think it takes a 1" socket.... maybe bigger, that will be your best bet, hard to get a swing on a wrench. It will need to be a deep well, and keep in mind that the plastic on it is fragile. It is very simple to test with a multimeter. Hook to it and put it in a pot of water, preferably with the plastic out of the water so the water isn't being measured. Set it on high and you should see a fairly linear change in resistance from cold water to boiling. I dont think thats the problem though. It isn't something that typically fails, particularly a factory cummins unit. Engine temp also doesn't spike that quickly. The gauge can respond pretty quick (140-190 in maybe 2 seconds), but if it is jumping then that's not the probe.

 

You could plumb a in another gauge in either location, but the back of the head is typically chosen because that is where the hottest temps are.

 

It seems like you may have a ground issue/wiring issue. 

Is it purely RPM dependant? What does the gauge do when in neutral while idling/free revving? Do any other gauges seem affected when you rev the engine (speedo/fuel/voltage)? What is the resistance from the negative battery post to other grounded metal things (engine/body/metal part of dash/frame)? Shouldn't be higher than 1 ohm once scratch around on clean metal.

Edited July 29, 20204 yr by That Guy

Doesn’t fluctuate at idle, none of the other gauges have an issue besides fuel doesn’t work at all because of the sending unit. But I just unplugged and plugged back in the single wire going into the sensor and it seems to work now? Lol weird 

Sounds like a bit of corrosion on the connector. As you throttle up, the top of the engine torques over to the passenger side, that could cause the wire to move a bit and affect continuity of the connector. By unplugging and plugging it back in, you made better contact in the connector.