WhT are common symptoms of thermostat failure? This morning I shut the truck down without paying attention to the coolant temp and started it right back up and it’s readimf around 145 155 degrees. Usually it’s right at 180 after I’ve driven it for 25 miles like I just did. 

180F thermostat can get as low as 172F. As for a 190F thermostat it could be as low as 182F. 

 

I'd say replace the thermostat. 190F is optimal for you rig.

20 minutes ago, Mopar1973Man said:

180F thermostat can get as low as 172F. As for a 190F thermostat it could be as low as 182F. 

 

I'd say replace the thermostat. 190F is optimal for you rig.

Cool thank you! 

Be aware the Cummins thermostat looks like NAPA's previous version thermostat. I've have had excellent results so far with NAPA thermostat's. I can say Gates is rather flakey and poor quality. 

 

Cummins Thermostat and NAPA old version on the left. The NAPA current version on the right. Much larger mouth. Also the current NAPA 190F is actually a 192F. On my truck I swapped back in the 190F and it runs about 197F to 201F with the winter fronts on the truck and outside temps running 20-30F with freezing fog. 

I just noticed some water at the thermostat housing and will be by Napa Tuesday and will get a thermostat. what is the part number of the one that is recommended. thanks 

375-190 is the 190F thermostat

375-180 is the 180F thermostat

This last year mine was opening at higher temps then it did the year before. I decided to change it and NAPA gave me the one on the right in MPM post . Mine is a 98 and it was listed as that one. I started getting wild swings in temp ( late opening and late closing) of over 10 degrees swings. My mistake was not taking mine out and comparing them. Being that 98s had both 12v and 24v they gave me a 12v which is the one on the right. I went back with my original and asked for one for a 99 and I got the one on the left and it has worked perfectly since.