About 7 miles into my trip home from work the other day the temperature gauge was encroaching in on the "A" of "NORMAL" and climbing. It usually runs around the "R" so I was concerned. I pulled off the road and let the truck idle at which time the temperature gauge began to move back to the left towards the "R". I checked under the hood, under the truck and on the floorboard to see if there were any obvious leaks...nothing. Since I recently had to R/R the heater core and back flushed the system I paid particular attention to the HC connections. When temperature was in the range it normally runs at I continued and again the guage worked its way back up to between the "A" and the "L". I stopped at a place I could let the truck completely cool down and add water if needed. After it was cool enough for me to remove the radiator cap the radiator was full. Again, no obvious leaks either when I checked again. I then got back onto the highway and traveled the remaining 40 miles home with no problem at all. Temperature gauge stayed between the "R" and the "M". After getting home I checked the fluid level again and it was fine, I also checked the radiator fluid for effective range with hydrometer and it was good from -10 degrees F to 230 degrees. Any ideas or suggestions as to what might have caused the temperature to read so high???

 

It's hardly a scientific reply, but in 40 years of playing with engines, I have never come across an engine better than the 460 at making thermostats stick.

 

That was one of the things that had crossed my mind when I was pondering the situation. Drove into work this morning and guage held right at the "R" and "M". Thinking of maybe pulling the thermostate and testing it the next time I get the chance or just R/Ring it since I'll already have it out.

 

A bad tstat could sure do what you're describing, or a trapped air bubble could also do that. I've chased entrapped air before and it can be a pain to fix, open the highest point in the system and see if that helps. Or just turn the heater on and drive the thing, that's also worked for me.

 

One of the guys here at work was saying the same thing when we were talking about it the other day. He was calling it an "air pocket" and also suggested about running with the heater on. I'm thinking it is the probable cause since it hadn't happened before and it did fine on my way into work Monday morning. Plus I did just drain/flush the system when I R/R'd the heater core which would then introduce the possibility of entrapped air. Thanks for the input.