Strange problem
 

My son has a '04 GMC with the 6.0 engine. The truck has 185k miles and has been a great ride. Recently it has developed a problem that has stumped me and the mechanics he has taken it to. We have a lot of experience here and maybe one of you can give us a lead.

The truck runs great around town. Never a problem at low speeds or in stop and go situations.
When you get on the highway and start running steady at 65-70 mph, the temperature gauge will drop back to 160* and the truck feels like it's running out of gas. If you drop back to about 60mph, the temp gauge moves back up to where it belongs and the truck runs smooth as you would want.
I thought maybe the water pump or fuel filter but I don't see how the water pump would affect the engine running or the fuel filter affect the temperature gauge. He has had it into two different shops and they can't find a thing. Any ideas are greatly appreciated.
Slim

Try replacing the Temp Coolant sensor. Its on top of the number 1 cylinder. Its a brass fitting. Very easy cost's bout $10. When these go out, they give faulty temp readings, and the computer relies on temp readings to adjust for air/fuel ratio, spark timing, etc.

Thanks, Ruffneck. That makes sense. I guess the higher speed may cause an increase in coolant system pressure which causes the sensor to fail. I'll try that and let you know how it works. Thanks again.
Slim

Hey no problem friend. Hope it fixes it!

Hey, Ruffneck, I just wanted to get back and let y'all know what happened on this one. My son has been out of town but got back in this weekend. He looked at the sensor and it looked good but the cable attached to it was very loose. He shook it around and the temp gauge acted just like it did when the truck was messing up. He just replaced the cable and the trouble is fixed. Thanks for steering us in the right direction.
Slim

Sweet!! Thats good info for someone who might come across this same problem.