I have worked on vehicles off and on since the late 60s, gas  and diesel , air cooled and water cooled and haven't had this happen before. I did a coolant flush as it was due and changed the thermostat because it was running about 5 degrees hotter the last year. The first new thermostat that I installed was listed as 190 degree and it did not open until 207 and not close until 183. I filled the radiator through the thermostat hole as i normally do and topped off in the cap , let it run through open 2 cycles. All good . Took it out for a drive of 20 miles with short but steep hills and flats. The first thermostat was very consistently opening at 207 to 208 and closing 182 to 183. Any motor that I have worked on before never varied more than about 3 degrees after maybe 4 open and close cycles on flat road with it being 75 degrees out. I installed a second one that was a 190 by a different parts house and it is doing the same thing but opens at 197 to 198 and closes at any where from 177 to 182. The second one was driven about 135 miles with no change. I did install the old thermostat and all is well but it is a 190 that was installed 4 years ago and after I brought it out of winter storage this year it has been about 5 degrees over normal but very consistent. I have changed more thermostats then I want to remember and cannot remember a bad one out of the box more or less two in a row. The first one was from advance auto and the second one was a napa premium. Is this the new standard or am I the luckiest person in the world this week. My next move will be to go to cummins and just bite the bullet and pay the price unless someone can come up with a better way to go.

Ah, so they gave you a 12V one. 

 

That explains the proper temps, but easy to get there. 

I took a pic. and you can see the difference. I thought that maybe a up date or a different manufacturer so I didn't question it. The flow has got to be twice the stock unit. The one on the right is the old one the one on the left is what 2 different parts houses gave me.

The one on the left was the old school NAPA I use to get all the time. Now the one on the right is what the new NAPA thermostats look like now. 

 

 

Both NAPA and Advance gave me the one on right but it was the lady put in 99 and I got the one on the left. The one on the left was the same as the one that was put in it 4 years ago from NAPA in Jackson Ca.

Maybe it’s the photo angle, but the one on the right doesn’t look like it will work with the bypass in the motor. It doesn’t look like it has the correct shape to seal it. 

The bypass valve part is a bit bigger and tapered slightly for a good seal. They really did change the design quite a bit. 

1 hour ago, Mopar1973Man said:

The bypass valve part is a bit bigger and tapered slightly for a good seal. 

 

Any pics of that part of it?

 

1 hour ago, Mopar1973Man said:

 They really did change the design quite a bit. 

 

Who is they? Cummins or aftermarket?

 

I've got a 4 year old Cummins one in mine that is like the one on the left.