Wandering Coolant Temperature

[duramax170]

Hello

For the past couple of months I have been watching my temperature gauge move around while driving. It goes from running temp of ~190 all the way down to 160 and everywhere in between. This became more noticable today while on the highway driving home for Christmas. As the temperature gauge would fluctuate the truck would surge as if it is switiching from cold to hot mode if that make any sense.

The temperature sending unit was changed last year when I Broke the plastic housing for the plug while changing the alternator.

I believe the problem is related to the sending unit somehow. Last week after driving the truck for awhile, I shut it off to go into the store. When I came out I had had a very long glow plug light as if the engine was cold. The gauge was all the way at the bottom. With the engine running I went and pulled on the sending unit wires and I could hear the engine change tones, and presto gauge was accurate.

So onto the question, Is it likely the sending unit, it seems like a simple piece of metal to me, not sure how possible it is for one to fail, is there a way to check it or do I start with swapping it and then start tracing wires.

Thanks
Sam

2001 LB7 190,000

[Kennedy]

If you used an aftermarket sebder I'd start there by changing to a GM part. Adding the cold front will help stabilize, and if your thermostats are original it would be a good idea to change them plus the coolant.

[More Power]

A scan tool or a programmer could be used to verify what the computer is seeing for temp. Sounds like a sender problem, but if a new one doesn't help, I'd look deeper with a scan tool to help narrow it down. Also, check the electrical connector and wiring for damage - since it was recently unplugged/plugged-in.

Jim

[Silvermac25]

Duramax170, the EXACT same thing happened to me. It’s funny how identical the circumstances are. What ended up fixing your problem?

[Scooby]

My LBZ was doing that occasionally after I got it, roughly a year ago. I found the pigtail that plugs into the sending unit was not making a good connection where the wires enter the plug. I could make the temp wander around by moving the wires around. I changed the pigtail and have no more trouble.

[More Power]

Low coolant due to a head gasket leak or compression pressure blowing the coolant out through the coolant surge tank overflow can also cause big swings in coolant temperature.

The situation mentioned in the opening post in this thread doesn't indicate low coolant, but we're hearing of more incidents like this as these trucks age.

So, I would check the coolant level whenever there's a big swing in engine coolant temperature (usually felt in the heater output).