Anyone have any opinions on what to use and how to go about this process? Thanks.

I have tried 2 napa's. The last was I think a 375190, still got the box somewhere. It matched the part number several people said to use, could have the number wrong though. Seems it works for most folks. There does seem to be alot of people that have this issue so we are not alone. But it bugs me that most folks dont have the problem.

Dave, Does the Napa 375190 stay steady on 190* for you? I take it that it doesn't but I thought I would check before I go and throw more money at something.

It does not stay steady for me. But it is the same one several people including Mopar uses with succes. I might have that number wrong, I keep forgetting to get the box out of my tool box at work. the in mine now also swang wildly just like the rest until I came over the mountain and the temp hit 215 on the gauge and it now works better than all the rest I have used. I took that one out a month ago and installed a cummins 180(part # 3967195) and it was only keeping between 150 and 170 and swinging alot. So i renstalled the one from Orielly's and it still holds in the 180's 85% of the time. It is a Murray part#4289, says made in Isreal on the box, but germany on the stat. Every time I have chaged the stat the block is full of water. I have thought about taking one of the stats that swings and boiling it in a pot. It seems the Murray worked way better after it was heated to 215. If you have an old try and see what happens. I still have four of last ones I bought and think I will give it a try. The jiggle pins i have heard people speak of are apprently the pins in the tstat. and they are there to let air bubbles thru from what I saw when i googled it. If the cooling system was to air lock as some have suggested, it seems to me you would go into an over heat condition since the system is locked. I had gotten used to this situation and just living with it. But now it bugs the heck out of me and I want a solution. I talked Mike the other day and next ime he changes one when he changes his coolant I am going to get him to send me his old one and see what it does in mine. Enough of all this, there has to be a solution/answer to the problem. I just dont know what it is yet.

Placing the thermostat in a pot of hot water may let you know what it doing unless its moving slower than you think and then you're not going to notice any differences. You'd think that there is no normal reason engine temp would fluctuate up and down either unless the thermostat was bogus, but logic would make you think that would be rare too since there isn't much to a thermostat.....so there is likely another reason this is going on.Unfortunately I didn't save the box when I replaced mine so I cant tell you positively what temp it was nor what brand it was but I want to say that I remember it being a 195* and I know it was a NAPA. I called the NAPA I bought it from and they can only say that they carry NAPA brand, but think that Stant makes them. I say 195* because I remember researching what temp to get simply because old school gut instinct was wanting me to get a cooler one, although I had to listen to reason while understanding the requirements of the 24v ISB being cold blooded (per say) and choose 195*. I'm glad I did and the truck runs 190* all the time unless I'm towing and then it will lean toward 195*, which is exactly like it should.What do you all think about this.....Maybe running a thermostat colder than 195* in the 24v is keeping the thermostat partially open to early and the excess circulation of the radiator water is causing such engine temp variations. I say that for good reason and follow me here as I try to explain what I mean..... In knowing that the 24v requires more head temp than the 12v because of the increase in airflow (which is the reason the 12v thermostats are available in colder rangers than 24v thermostats), maybe if the thermostat in a 24v isn't hot enough then the engine is no longer capable of maintaining a constant cylinder temp and is easily overcome by cooler radiator water. The thermostat being so close to the open/close setting of the thermostat could be allowing the thermostat to ride right in that "not really open but not really closed" state. Remember that even though cool air builds HP by creating a denser combustion charge, diesels need cylinder heat for proper combustion which means that they need the proper thermostat temp per the application to achieve correct cylinder head temperatures. So one would think that maybe what most of you engine temperature fluctuations guys are complaining about is a result of the wrong thermostat temp setting (to cold), and/or that the thermostat itself is simply opening sooner than its stamped setting. Just throwing that out there.....but it sounds completely logical to me. :)Edit: Backing my theory would be why all these thermostat issue arise in the colder weather.....because during warm weather, the engine is able to create enough heat to maintain cylinder head temps and the engine temperature should remain more constant.

My reasoning for boiling the tstat was not to watch it operate, but to put it under some heat stress. Mainly because the one in the truck now was exposed to 215* temps on my pull over the mountains. It is the only one i have that has been exposed to that and it sarting working better after that exposure.My oe stat was a 190* and every one i have purchased has been a 190 except for the last which was a cummins 180. Figured I would give it a try since the 190 non cummins were not working right. It acted the same way except it ran about 10* cooler while fluctuating.I need to think about the 3rd statement a little. But the oe used to hold very steady at 188 or 189, just below the 190 mark on the temp gauge for 6 to 7 years before it went.My temp fluctuations happen year round. The only differance being when I stop and idle in very cold weather it does cool faster. Other than that it fluctuates year round and holds in the 180's while driving down the interstate empty and 3 t0 4 degrees warmer if I am in hilly country if I am pulling the trailer.

My reasoning for boiling the tstat was not to watch it operate, but to put it under some heat stress. Mainly because the one in the truck now was exposed to 215* temps on my pull over the mountains. It is the only one i have that has been exposed to that and it sarting working better after that exposure. My oe stat was a 190* and every one i have purchased has been a 190 except for the last which was a cummins 180. Figured I would give it a try since the 190 non cummins were not working right. It acted the same way except it ran about 10* cooler while fluctuating. I need to think about the 3rd statement a little. But the oe used to hold very steady at 188 or 189, just below the 190 mark on the temp gauge for 6 to 7 years before it went. My temp fluctuations happen year round. The only differance being when I stop and idle in very cold weather it does cool faster. Other than that it fluctuates year round and holds in the 180's while driving down the interstate empty and 3 t0 4 degrees warmer if I am in hilly country if I am pulling the trailer.

I may be wrong but I'm betting that your thermostat started working better (for your situation) only because, for some unknown reason, its no longer opening at the same "cooler" temp than it was before. Maybe that 215* jump forced the thermostat to open fully and changed its set limits. Not saying that 215* is overheating but I have heard that overheating an engine sometimes means also having to change a damaged thermostat from the overheating. I still stand firm on my theory though..... I think the 190* thermostats are to cool for the 24v. But.....in saying that, I'm sure there is a reason a 190* is offered for the 24v. And that reason is probably for the HO application because HO engines run hotter cylinder head temperatures than SO's. Thats why they run different valve seats than the SO motors. So maybe the HO is more capable of maintaining the hotter temp and can get away with a 5* cooler thermostat.....but the SO cant. I am betting that if you install a 195* thermostat that your engine temperature fluctuations will stop.

I may be wrong but I'm betting that your thermostat started working better (for your situation) only because, for some unknown reason, its no longer opening at the same "cooler" temp than it was before. Maybe that 215* jump forced the thermostat to open fully and changed its set limits. Not saying that 215* is overheating but I have heard that overheating an engine sometimes means also having to change a damaged thermostat from the overheating. I still stand firm on my theory though..... I think the 190* thermostats are to cool for the 24v. But.....in saying that, I'm sure there is a reason a 190* is offered for the 24v. And that reason is probably for the HO application because HO engines run hotter cylinder head temperatures than SO's. Thats why they run different valve seats than the SO motors. So maybe the HO is more capable of maintaining the hotter temp and can get away with a 5* cooler thermostat.....but the SO cant. I am betting that if you install a 195* thermostat that your engine temperature fluctuations will stop.

I am thinking the same thing on my current tstat. Running thru that heat cycle did something to it and it now functions better. As far as a 195* tsat, I have tried several differant 190* and a cummins 180*. whats $30 more for a 195. Next chance i get I will throw one in and see. When its all said and done I will soon be opening a used tstat store. Come by and have look at our inventory. Drip's Used Thermostat Store Fluctuation is our specialty

Now for the cost of a few bottles of distilled water at Wally World...why not? I doubt that you'll need to buy more than that but you can always use the remainder for your BATTERIES. I have three Model A Fords and use distilled water in each one. I go through about 12 gallons/year in them. Figure out how much a radiator flush or replacement costs and you'll have your answer. A replacement radiator for a restored Model A costs in the range of $450 (lots more for your truck). How many gallons of distilled water can you buy at about $ .65/gallon? I say use distilled.My humble opinion.