ok i have since put the truck all back to stock but the tranny and exhaust, and now when driving down the highway the truck will peg out the temp gauge and the check gauges light comes on then it go back to normal and start doing it again. i have since done a BG coolant flush and replaced with new coolant and water-wetter but it's still doing it. But you can let the truck set and ideal with the a/c on all day and it runs ice cold on the temp gauge. Do you think the thermostat could just all of a sudden gone bad and why would it only heat up going down the road?

 

i would suggest checking the actual engine temp rather than relying on the guage... i use a "raytek mini temp".

 

I see two possibilities. First have you ever cleaned the radiator where the vent bottle splatters oil? Also, there is a possibility of a failed water pump. The thermostats are designed to fail open, not closed.

 

well last year about Sept i did both... the water pump was leaking and at the same time i pulled the rad and degreased it along with the rest of the motor

 

I know stupid question.....but did you remove the cardboard from between the rad and intercooler?
JM.02c

 

what cardboard? if your asking if i put anything behind the intercooler to help it heat up for the heater... is no i've never done that b/c its never had a problem blowing hot air even when it's in the 0's

 

So now that we have taken that out of the equation. Let's look somewhere else.
check the wiring to the sending unit and look for chaffed wires bad ground, could be a bad sending unit from the goofy way the gauge is acting.
Good luck let us know what you find.

 

I would pull the thermostat and check it or just replace it with a cummins unit. they are notorious for going bad. On pull is when they act up. I cannot tell you the problems I have had with thermostats in the dodge.

 

Thermostats don't always fail open. I would check/ replace yours as well as cleaning the radiator as posted above.

 

I would change the T-stat. They're relatively cheap. Then you can eliminate that as a cause.

If it pegs the temp gauge, does it boil over? Is it really getting that hot?

 

I will give you a warning, I lost an engine because the bottom of a new thermostat came off and blocked the bypass port on the engine. there may be pictures on the board, there were originally.

 

i'm going to replace the t-stat tomorrow and give that a try... when it pegs the gauge i can hear it making noise in the heater... which one would think there is a air pocket but i've checked it a ton of time and it's always full and i've let it run with the cap off and still nothing