Engine/Transmission (1994 - 1998) Best Thermostat?

[MMcCallie]

After going through about 14 thermostats within 2 years I gave up. All but one thermostat stuck shut and the engine overheated except one that stuck open while off road near Estes Park, Colorado and I could barely keep the engine temperature above freezing while driving down the mountain. This engine ran hot for years until I finally discovered this summer that I had the tiniest air leak in a coolant hose letting air get into my system which propagated frequent overheating, but not enough of a leak to let coolant escape anywhere but through the radiator cap.



I'm now in Missouri and to my surprise with all of my highway driving and temps only down into the 30s thus far, my coolant is rarely as warm as 140 degrees F. In the past few weeks I've noticed a change in my exhaust smoke. When cold at an idle I have light gray, almost white smoke coming from the tailpipe. Even after driving for an hour when it's cool out during acceleration when the smoke used to be black, it is now very light gray, just a thin haze, almost white.



When the ambient temperature is over 60 degrees I see no haze at idle and exhaust smoke is darker, almost black. Engine coolant is also warmer then, usually above 140 degrees. I am frequently seeing coolant at highway speed reading around 120 degrees when the weather is cooler. When I last checked my engine oil I noticed the slightest blueish tint which I think is diesel. I'm assuming that because my engine is not getting warm enough I'm getting an incomplete fuel burn and diesel is washing past the pistons. Isn't this correct that a cold diesel smokes white because it is not warm enough to completely ignite the fuel? Does this sound like a likely scenario?



Power is still fine and boost quickly reaches the mid 40s but with a very lightly colored smoke. When the weather was warmer I saw 15 mpg and am now down to 13. I am also wondering if it could just be that I dialed back the star wheel quite a bit. I have the AFC and plate full forward but the anneroid dialed way back to decrease smoke. Is part of the reason the smoke is so light that I have it dialed back?



I have a massive 32 lb alternator making the thermostat very difficult to change. I siliconed the large o-ring to a thermostat I gutted. I am contemplating the possibility of somehow installing a thermostat in the upper radiator hose so that replacement is easier if I continue to have problems with new thermostats. Does anyone have any ideas on how to go about this idea? I tried Cummins, Dodge, NAPA, Auto Zone, Advance Auto, Car Quest, and other off the shelf thermostats with no luck. I've also pondered designing a manual thermostat but it would worry me if someone else unknowingly left it shut and overheated.



I read a post of someone being very pleased with a Mr. Gasket 180 degree thermostat. I have noticed there are quite a few aftermarket brands and I'm wondering if anyone has experience or an opinion on what brand should be most reliable. Preferably a model without the big rubber sleeve that gets caught up and jammed. I figure 180 is what Cummins intended but am interested in temperature suggestions and reasons why as well. I remember reading articles about fuel economy competitions in which higher coolant temperatures even around 300 degrees aided mileage but I hear most dodge people say the Cummins performs better the cooler it can run. Thoughts?



Thanks,

Matt

 

[MMcCallie]

Forgot to mention no sign of coolant in the engine oil and no oil in the coolant. Oil pressure is around 88 psi when cold, will drop to single digits when engine is hot, like near 200 with A/C and exhaust brake at just over 500 RPMs. Now that weather is cooler oil pressure is usually just under 60 psi. I'm curious if my 17. 5 degree timing could have retarded to cause white smoke. Or perhaps a clogged injector or one that is not up to the right bar?

 

[GAmes]

Other than the sometimes annoying temp fluctuation I'd recommend trying another Cummins T-stat. In all these years I have only had one fail. I do replace them when I flush the system and replace the coolant. My heater always works, even in cold of northern Montana and central Alberta in the winter.

 

[Dr.Bob]

I like the ones that fluctuate, at least I know that it's getting up to 190.

 

[ata]

A diesel engine will show bluish-white smoke when the engine temp is below 140 and out side ambient temp is in the 30's or below. Plus it will put carbon and fuel in your oil. Diesel engine's DO-NOT like to be run under 180 degree's. I have had the best luck with Cummin's or Dodge thermostats.

 

[MMcCallie]

So is a 180 degree thermostat really sufficient? 180 is when it is fully open, not when it starts to open, right? How much of a temperature swing do most of the genuine Cummins thermostats yield? What about my idea of installing a remote thermostat in the upper radiator hose?

 

[GAmes]

If you give the serial # of your engine to a Cummins parts person he or she can look up the correct thermostat for you. I can not tell you if mine is a 180, 185 or 190 because it isn't stamped on the t-stat. I think your remote t-stat idea would be very difficult to make work properly.

 

[MMcCallie]

I read in another post that the Cummins thermostat #3934373 is the one that is slow opening. Does that mean the engine will run warmer with that one or that it is less prone to failure because it doesn't receive as much of a shock from the change in pressure once it opens, therefore also not swinging to such wide temperature variances? If it is slower to open, doesn't that mean you will have to wait longer to have heat in the cab?

Tonight the air was as cool as 37, supposed to be about 24 tonight. My truck was plugged in all day, was just over 100 when I started it and in the 90s once I started driving. On the way to town on the highway for about 30 minutes coolant got up to 121 degrees. I left the engine revved to about 800 RPMs during errands with my vernier throttle cable with the exhaust brake on. I got the temperature up to 177 in town. On the drive back home coolant dropped back down to about 129. Just thought I'd document some results.

 

[fest3er]

I think the slower opening t-stat is simply less prone to 'wild' temp swings. A fast-opening t-stat will cause the engine temp to drop quickly, then the t-stat closes and the temp rises again. A slower opening t-stat is more conducive to a steady rise in engine temp.

Diesel 101: a diesel engine will not heat up if it's not working. Make it work, and it'll warm up PDQ.

If you're finding the engine not warming up in colder temps (and you have a known good t-stat in it), block off 1/3 to 1/2 of the radiator. That'll reduce the heat transferred to the air, keeping the engine warmer. When it gets much colder (frigid), block off 3/4. When it gets down to -20 or so, block the whole thing off.

That assume an 'unladen' vehicle. If you've got a decent load, the engine will heat up and stay hot as long as the engine works. By way of example, my engine would cool to probably under 140F when idling in neutral at -10. But if I let it idle in gear (auto trans), it would stay hot.

If you have an exhaust brake, you can use it to make the engine work and stay warm while idling in neutral.

At -24F, 5-10 minutes of 1200 RPM in gear will warm up my engine and trans. But it'll cool right down once I start moving and the radiator isn't blocked at all. It's a matter of finding the right balance between heat generation, heat dissipation, and work performed.

 

[BDaugherty]

When the factory t-stat stuck open at 140k, I bought a 190 degree Stant from Advance auto. Before paying for it, I pulled it out of the box and verified "made in USA". It opened a little faster than the stock t-stat, but ran for the rest of the time I had the truck with no problems. Also, it was supposed to stick in the "open" position if it ever failed... I just never got to test it. It may have cost about $14.

 

[NIsaacs]

I would install a thermostat back where it belongs. Now that you have found/fixed your overheating problem it will probably work fine. High temps of overheating day after day will damage them, might be why they would not last long. On my three diesel pickups, I have only changed each one, once.



Nick

 

[1tuffram]

10 years ago I purchased the updated Cummins slow open t-stat to stop the temp gauge needle yo-yo effect in my '95 truck. My only complaint with the updated t-stat is I would like more cab heat and better defrosting in the winter. Does anyone know what the temp rating is for the Cummins slow open t-stat? I know it is lower than what came in the truck and I have been considering changing back to a higher temp t-stat with my next coolant change.



Has anyone tried the T-stats Geno's sells or would it be safer to go to Cummins and get a 180 or 190 t-stat?

 

[MMcCallie]

I very much like the failsafe that the thermostat, if it fails, would revert to being stuck open. I would feel much more confident before installing knowing that. What 1tuffram said is one of my concerns though, that I will have even less in cab heat. My Excursion has a whole lot of windows to thaw and interior space to heat. At least now even when my engine is running at 120 degrees I have some heat in the cab.

I just looked at Geno's and they sell a Gates 180 with seals for $16. 95. I didn't realize Gates made thermostats. I also checked Stant's website and they sell a 180, part number 14248. It's sold at Napa and Car Quest making me think surely I've tried them already. Reading about their SuperStat however did make it seem quite appealing, if that number is a SuperStat, which was unclear on their website.

I wonder if the frequent overheating caused most of my thermostat failures but shouldn't they survive 250 degrees or so? I remember my last one wasn't opening until about 350 degrees when we heated it in a can of water we heated with a torch. Yeah, no wonder my engine was overheating! My dad was telling me his 1930 Model A originally came with a thermostat mounted inline in a radiator hose. I'm still hoping to find out more on that possibility but in the meantime I'd appreciate more brand related suggestions.

 

[NIsaacs]

I am not sure how having a thermostat could give you less in cab heat v/s none.



I know you said that with your alternator, access to your thermostat was difficult and have suggested a radiator hose install for ease of replacement. However that might become very difficult if you should need one on the road due to availability. Napa and Carquest are everywhere. I also doubt that Cummins makes their own or that there is one made that stands out over the run of the mill Gates or Stant.



I can tell you are head shy with just the word "thermostat" due to all your overheating issues of the past. Just get one and install the sucker. There are literally millions out there working as designed.



Nick

 

[Missouri Mule]

From 96 Factory service manual;

When the coolant reaches 181 F the thermaostats starts to open and is fully open when temps reach 203 F. Above 203 F coolant temp is controled by the radiator,fan and ambient temp.



Is your radiator fan running all the time? This could cause low temps.



Are the outer and inner seals for the thermostat OK (as in not letting coolant through).

Is your t-stat sticking open?



I'm running original T-stat(knock on wood) only had it stick close once (probably trash in system and the radiator cap was loose, coolant was low, just had it changed).

I have good heat even when it gets in the high 20's here and last year when I was in Kansas for Thanksgiving it was 28 degrees at 5 in the morning I had plenty of heat.

 

[Matt42]

Warning! Warning! Rubber Seal Diagram Is WRONG!

The diagram in the 1996 Dodge TSM (and maybe other years) shows the installation of the stepped (or two-diameter) rubber thermostat seal being installed BACKWARDS. It is possible to install it that way, but it requires some odd squeezing and forcing to make it fit. It fits backwards in a counter-intuitive way and mounting it in that backward way will actually reduce the temperature fluctuations shown on the gauge! But, the backward rubber seal installation will also cause overheating and may also cause the thermostat to stick open, once it opens! So you might think you have solved a problem, but you will have caused a much, much worse problem that can cost you an engine!



I have seen this incorrect diagram picked up and used in aftermarket service literature.



Be very careful here. You need to have a number of years of experience to recognize the essential and very important wrongness of this problem. If you bought the truck used, it is very probable that the rubber seal was put in wrong by a prior owner or service person. I bought mine new, so I had the benefit of the Cummins assembly to compare!



Bottom line: The easy way to put in the two-diameter rubber seal is the right way.

 

[MMcCallie]

Missouri Mule, thank you. The information you provided about the thermostat just beginning to open at 181 and being fully open at 203 is exactly what I wanted to know. I have a Horton electromagnetic clutch fan and and additional 17" electric pusher fan that I previously had activating at between 188-194 degrees. I raised the temperature and will now feel better knowing I don't have to worry about seeing over 200.

I looked at a 4 year old Mopar thermostat and it had the rubber sleeve around the body of the thermostat. Do al of the new Cummins thermostats still use this sleeve that is so prone to folding over and causing thermostat failure? I called a Cummins dealer 2 hours away being the closest and could not get an answer, they said the design has never changed and didn't understand what I was asking about a rubber sleeve around the thermostat, separate from the gaskets. Does anyone know?

The consensus about brand I'm getting from asking locally is that Stant makes the best thermostats. This is the brand NAPA carries. Unfortunately, they do not appear to make a SuperStat for the Cummins, just the OEM replacement model which I've already tried and had fail. NAPA claims they are a failsafe design but the guys behind the counter admitted from their own experience that they are not. Auto Zone only carries Duralast. I'm leaning toward the NAPA model if Cummins is still using the rubber sleeve.

I've noticed other local diesels also producing white smoke in their exhaust and after asking around I learned that St. Louis County and Jefferson County in Missouri use fuel additives that make gas and diesel vehicles run poorly, smell bad, and smoke. I was assuming such probability since I hadn't seen white smoke before arriving in Missouri.

 

[MMcCallie]

Does anyone know if the newer Cummins thermostat is still made with the rubber sleeve? Here are the Cummins part numbers I have, 3934373 for the thermostat itself and 3925466 and 3927305 for the gaskets. Anyone know what the part number would be for the Mr. Gasket thermostat that would also fit?
Thanks,
Matt

 

[1tuffram]

I was searching for a 190 deg t-stat and found Diesel Storm has a Cummins 190 deg thermostat (Cummins part # 3946849) that will fit '89 - '02 trucks. Anyone have any experience with this thermostat?



I emailed Diesel Storm to see if it causes the temp gauge needle to swing back and forth like the original that can in my truck or if it was more like the soft seat thermostat (#3934373). The response was they do not have any feed back to know and that it is a fairly new thermostat offering from Cummins.

 

[HEMI®Dart]

I was searching for a 190 deg t-stat and found Diesel Storm has a Cummins 190 deg thermostat (Cummins part # 3946849) that will fit '89 - '02 trucks. Anyone have any experience with this thermostat?



I emailed Diesel Storm to see if it causes the temp gauge needle to swing back and forth like the original that can in my truck or if it was more like the soft seat thermostat (#3934373). The response was they do not have any feed back to know and that it is a fairly new thermostat offering from Cummins.

89-02 Dodge 5. 9L Cummins 190 Degree Thermostat with 2 Sealing O-Rings [3946849,3925466,3927305] - $73. 99 : DieselStorm Technologies, Parts, and Accessories