My issue: Engine and transmission running cold. Less than 130 on the transmission, and less than 150 on the engine. The transmission is less of a concern than the engine. I changed over to synthetic in both, and immediately noticed the drop. The engine got to 150 while towing our 3 horse goose neck, otherwise it stays at a needle width above 140. The performance (mpg) is down slightly, maybe 1-2 mpg. The heater is lacking a lot, so I suspect the reading is accurate. I checked codes and only get a 1688. ( had that one for 18 months now) I did the cluster check and it appears fine. I've read mixed reviews regarding temp. drops with synthetics. There is a new belt, new filters etc. I'm suspecting the sender but don't believe in coincidences. I'd prefer the engine run at 170-180, and have a 3000 mile trip coming up. What am I missing? Thanks

 

I would start by replaeing the thermostat. Its got to be stuck but i cannot for the life remember wich way a cold motor would be i would think its stuck open.

 

Thanks Dieselcowboy. Sometimes I get too close to see the problem, then I over think. Now that you mention it, that sounds logical. I'll get a t-stat with the other items next on the list. Have a good weekend

 

Apparently it is important to use a Cummins t-stat not a Stant aftermarket one. Lots written about that on this board.

 

Mine did the same thing last winter. A new Cummins 190F stat fixed it

 

I'll go to cummins in the morning or monday and get the proper thermostat. I researched the forum and found answers and pictures. Thanks Dodgecowboy ( I mistyped previously) and sixslug.

 

the p1688 code bothers me....my code list says " internal fuel injection pump controller failure"..........better check it out!

 

I'll start a new thread.