O'k I'll try and make this short I put in a new radiator and figured while I was doing that I might as well replace the T-stat. The old T-stat had rubber around the end with the spring and it had on it this end toward the radiator. So that's what I did with the new one the new one didn't have this same rubber around the spring end and no it isn't the gaskets. The problem I'm having is sitting at idle the temp gauge goes past 190 t-stat is 180 and keeps climbing. As soon as I go down the road the temp drops to 140 and stays there. It doesn't cycle like it use to. So I sit at idle temp climbs again. So according to my haynes manual the T-stat is in backwards. Why would the old T-stat have written on it the opposite of the haynes instructions?

 

so ur temp goes up to 190+ then drops to 140 ish?
thats one thing that happens with these trucks. mine does it as do alot of the other guys on here. i've tried 3 tstats, ended up sayin screw it n live with it

 

it stays at 140 and doesn't move I've had this truck since new I'm well awhere of how the temp flactuates. I'm pretty sure the new one is in backwards. Maybe I just put the old one back in if it aint broke don't fix it might apply here.

 

I don't believe thermostats should fluctuate randomly but the engine temp should increase some when a load is put on like a hill... Mine (97 & 95) DOES NOT cycle constantly !!!! I have a STANT (gasser) # 29479 195* with a small (.060) hole drilled by the middle bevel... It operates @ 190 - 210* & maintains 195* on the flat with camper on.... The fan is not coming on... I have put in 3-4 or 5 before one works properly !!! Most of M R JUNK !!!
Who & why is the stock thermo over engineered like rocket science ??? $$$ !!! ???
Think of your hot h2o faucet & pipe & how you let it run to get hot h2o & then it is 2 hot & U open the cold 2 comfort level... U open 2 much & then would adjust ^>^<^ or U could leave the H cracked open so hot would B avail'bl instantly... The h2o is hot a distance into the line & is cooler further along the pipe... The small bleed hole allows the heated h2o to flow past the thermo so it doesn't need 2 heat the deadheaded h2o B4 the stat will open wide & then get a shot of colder h2o & closes 2 much & starts the cycling.......
I drive Freightliner rigs & Isuzu box trucks & they don't cycle constantly !!! The temp comes up with a heavy load (hill) & often the fan engages... They run steady on the flat !!! Hmmm... Bob. Puy, WA.

 

mine fluctuate real bad done it for years but is perfectly fine

 

That thermostat you pulled out is the latest/upgraded Cummins thermostat. Its rubber coated to insulate the bulb to slow down the "normal" fluctuation.

The metal disc deal on the back is also there to control the coolant flow for the heater. Some aftermarket thermostats dont have this or its made wrong too.

 

Some more "abnormal" thinking, eh... Many more forums with like thinking...

Cummins thermostat - Forums

 

im having a problem with the heat in my truck could it be the metal disk that that you all said that controls coolant flow maybe be my problem?