I am planning the do a cooling system maintenance on my 1998.5 and have not been able to find the correct thermostat for it . The ones that I have been handed are the large one for the 12 valve. I did try one about 7 years ago and had such wild swings in temp that I thought I had done something wrong. After much thought and trouble shooting I found that the thermostat did not look like what I removed ( my fault I did not compare them when I removed the old one). I went back with the old one and finally found one that matched. Here I am 7 years later and the only one that I can find is for the 12 valve and in their data base it said that that is the only one fits from 89 to 09 which I know is wrong.The stant pn 14289 or 46289 for the heavyduty but I could not contact anyone at stant but I did contact a gates tech support and he said that stant makes theirs but he has not got back to me yet. The picture shows the difference with the one on the left is the correct one. I have been to every place I can think to find one including cummins dealers  within a 100 mile raduis and they all pull the 12 valve one. How are the rest of you finding yours. I even ordered one from Rockauto that had the correct picture but when it showed up it was the 12 valve.

Have you tried DAP or Genos? 

Below is a comment I made a while back. Before this thermostat I ran a 4th gen 200 ⁰F stat and had no issues, it's in the parts box should I want to use it again.

 

"I run a Stant Super-Stat, 190F ($8 at Rock Auto).  Part number 46289.  Watching the ECT on the Adrenaline (not the dashboard gauge), it holds temps very steady, slow swing of just a few degrees when cruising down the interstate, regardless of winter or summer.

 

Edited December 9, 2020 by LorenS"

Just today I had Advance auto order in both stant and motorad and they both were 12 valve style with the 24 valve number on the box. The one from rockauto was the correct number but still not the 24 valve style. I guess the next step is to start calling genos or dap.

 

3 hours ago, Nekkedbob said:

The picture shows the difference with the one on the left is the correct one. I have been to every place I can think to find one including cummins dealers  within a 100 mile raduis and they all pull the 12 valve one.

The one on the right sure looks like it could flow more, and resembles the Motorad heavy duty unit on RockAuto.

Designs do change over time, sometimes for the better.

If you put in 98 as your year on Rockauto then you are going to get a right pile of incorrect results coming up, just put 99 problem solved, right now I have had nothing incorrect for my 98.5 using 99 as the year, I ordered a new thermostat last year on 99 and it was correct, problem is Rockauto doesn't see 1/2 years

 

BUT on 1998 year they do list 2 different diesels, can't remember the actual words used but there is 2, 99 year is better as just one

2 hours ago, wil440 said:

Rockauto doesn't see 1/2 years

Indeed, that's why when ordering rear brake parts for my 2001.5 I just search under the 2002 model year. I have rear disc brakes.

At all of the parts houses that I went to I had them go from 99 to 08 and got the same units. I did install one 7 years ago and got continuous fluctuations from 215 before it would open and a drop to 165 where it would close while driving down the road with minimal  load. I too discovered that I have to ask for a 99 because of the midyear change from the 12 valve. I also had the dodge dealer tell me that my truck was built as a 12 valve and that motor was installed by someone but on the build sheet it said 24 valve and was built in may and there paperwork said that the 12 valve was stopped in dec. of 97. I did still have my old one in its replacement box under my seat with my spare parts and the parts house data base show it as the correct number as the one from rockauto. The one from rockauto did have a picture of the correct one but that was not what was in it. After some thought I was wondering if anybody has run the 12 valve unit and did it work correctly and the one that I got 7 years ago was sticking untill it got hotter then it should and then popping wide open

I took a look at Rockauto, Vin number D is up to 98 and Vin number 6 is 98 to 2010, this is the bit I couldn't remember

I wonder if the one you got from Rockauto had just been misboxed as in a 12v thermo in a 24v box

What about ordering one for say a 2008 or thereabouts if as Rock says on some thermos the correct 24v goes from 1998.5 to 2010

 

Also if you check the fitment on the Mopar thermo.... part number 52028185AC this is listed for 1997 to 2003, I'd say you would be able to cross reference this number in the parts lists on this very site

 

And to answer your question about a thermo sticking... yes possible, way to test is in a pan of water bringing it up to the thermos opening temp and see if it opens gradually or snaps open way over where it should do, thermometer would be handy too

Make sure the thermostat isn't laying in the bottom of the pan it should be suspended In the water and the thermometer need to be near the copper pellet. When it lays on the bottom the heat of the burner will be the hottest and hard to measure good temp.

I did check on years up to 2008 and they all show the same numbers but you get pictures of the small one and the big one. When I ask about the small one they said that the cant order them but for stant they both have the same manufacturer number. Per gates motorad and gates are just repackaged stant the other ones are not brands that I know so I chose to stay away from them. Tomorrow I will do a side by side test of the old one that I saved ( it did still work but I had the cooling system removed and drained so I replaced it). I plan to call genos tomorrow and see what the can do for me.

Called genos today and found the oem cummins tstat so after talking to the rep I ordered it. Looks completely different the what the parts house show in their database. I did test both of the tstats that I have and they both open the same amount at the same temp but the larger one flows way more when open. I will post a picture comparing them when it get here

Be careful with Cummins OE they don't last long and may split apart. Cummins doesn't manufacture thermostats. 

I finally got the Cummins OE thermostat and it looks way different. I did a coolant flush,water pump, thermostat and removed the radiator to clean it. I am happy to say that it didn't need it after 6 years with MPM breather mod. As you can see the one on the left in the picture below is the one I removed, the middle one is what is available from parts houses and the one on the right is the OE. So far it looks good but it is only 70 degrees out today but it should be in the 80s by wed and I am going the hookup my trailer and find a good hill and see how it does. 

This is the original (thermostat) that came out of my '02 and the one I replaced it with (box). The one that went in looked like the pic on the box. It's been three years since I replaced it. Don't remember why I replaced it. Wasn't giving me problems. I think I was flushing the cooling system so I replaced the thermostat while I had it apart. Looked different from the original, but has been working fine.

My local NAPA sells the Stant style thermostats not the Robertshaw like Cummins.

 

As I lookup right off my Prolink. As you can see my wholesale price to because I'm a vendor as well. Never had a NAPA thermostat fail.

The thermostat was still working but I was noticing a slight rise in temp and a bigger swing in temp so being I had it apart i decided to change it. The OE thermostat appears to be a 180 degree so I will run it for the next month and see how it does. If I decide that I don't like it I will try the new larger one and see how it goes.

All 98.5 to 2002 are setup with 190°F not 180°F. 12V Cummins are setup with 180°F.

 

Hotter thermostats are suggested for better thermal efficiency. Heat is attracted to cold so cooler thermostats will net less MPG and less power because percentage of the explosion is dumped back in the coolant. Hotter thermostats don't dump as much energy into the coolant jacket.

 

Did you know there is a 200*F thermostat for the 6.7L that does fit our 2nd Gen 24V. Yup I've got a 200*F thermostat in the shop for winter time. Warmer IAT, better MPG, and better performance in cold.

 

Typically my truck runs right at about 195*F to 197*F in coolant temp. Towing it could go as high as 225*F depending on how close or past 1,200*F EGT's. Typically a 7% grade with my RV I only see maybe 1,000*F to 1,100*F. 

8 hours ago, Mopar1973Man said:

Did you know there is a 200*F thermostat for the 6.7L that does fit our 2nd Gen 24V

I ran one for a while (20-30k miles?), only took it out because I am one of those that suffers from Hot Fuel and didn't want to do anything stupid to my VP44 computer while searching for a little extra MPG.  And yes, the heater does work VERY well with it!!!!

Most likely picking up hot return fuel from the sender?

52 minutes ago, Mopar1973Man said:

Most likely picking up hot return fuel from the sender?

I would assume so, as we've discussed before.  At the time I took out the 200F thermostat I figured going back to the 190F thermostat could only help lower the cylinder head temperature, and thus the return fuel as well.  It sure wouldn't raise the temp, so I swapped to my 190F Super Stat that I love so much - just smooooth temp changes, up and down several degrees showing on my Adrenaline screen as I traverse the countryside.