Thermostat: 180* vs 190*; does it really matter?

[Crunch]

Yesterday I was returning from a 300 mile trip when I noticed that my engine temp was starting to rise. No leaks, but the overflow bottle was full, and showed signs of spilling out the top hole. Determined that it was the thermostat, and the guy I drive for has extra hoses and other emergency items he sends with me, and that includes an extra thermostat. Since it was night I checked into a hotel, but also changed out the thermostat in the parking lot before I crashed.

When I hit the road this morning I noticed that the operating temp of the engine only rose to the middle of the "2" on the temperature gauge, where it usually rises to the left side of the first "0". So I figured that the replacement was the 180* thermostat and I had always used the 190* since I bought the truck. This started a long conversation between us, wondering why there are two different thermostats. He searched the Internet and found that the answers were all over the place. Some made sense and some didn't. One that made the most sense to him was that Cummins spent millions of $$ researching this, and decided that the 190* thermostat was the one to use. But the 180* thermostat I installed last night was also a Cummins thermostat, so who knows. Is there a definitive answer for this, or does it come down to a preference for the driver? And what's up with the price of thermostats? All my life this was just a cheap part that you got at an auto part store and as long as it worked like it should, you didn't think twice about it. These puppies are expensive these days!

One other thing: The thermostat that I installed had a rubber ring around the outside of the circle. The one that went bad didn't. Was there a change to the way they make them, or what?

 

[steve69]

Following. I have been wondering the same. I run a 190.

 

[CharlesinGA]

The rubber ring on the outer edge of the thermostat IS the seal, no gasket. What was keeping the old one from not leaking? Had a gasket been put in there before?

I think the 190° thermostat is correct, After having a aftermarket thermostat stick on me, I went back to a Cummins thermostat. No more problems. Cummins p/n 5292744, got it right off the receipt from July 2020 from Cummins South ATL for my 2003 5.0L HO. Was $52 then, its $75.99 from Cummins online now, (free shipping), $69.95 from Genos.

Charles

 

[seafish]

It is my understanding that 3rd gen trucks run cleaner and more efficiently at the higher operating temperature with the 190 thermostat.

 

[AH64ID]

Cummins has thermostats in the 160°, 180°, and 190° ratings. The application determines the temp.

The engine is more efficient with a 190° thermostat over a 180° one, but the main reason to stick to a 190° thermostat is the cooling fan. The fan is programmed around the temps that are associated with the 190° thermostat (cracks at 190°, full open 207°, normal grade at high GCW 215-217°, and max allowable of 225°). The 180° thermostat has a max allowable temp of 212°, meaning that then cooling system is above heat rejection efficiency before the cooling fan is really working.

 

[Crunch]

The rubber ring on the outer edge of the thermostat IS the seal, no gasket. What was keeping the old one from not leaking? Had a gasket been put in there before?

Not sure. It looked like that was how it was made, but there was never any leaking. I guess that speaks good for the metal to metal seal. Honestly, I've never replaced the thermostat on my truck since I bought it new. It's always worked perfectly until yesterday, and I have a thing about messing with anything that can result in leaks. I know it's not rational, but gaskets and seals just seem like natural fail points to me. When I was a teenager and did things like water pump changes I admit I was one of "those people" who used Monkey Snot (on the gasket, not in place of the gasket). I remember several repairs (mostly for chicks) where I had leaks without it. Once I got comfortable doing the work I dropped the snot and just hoped for the best, and it's pretty much worked out OK.

 

[Tuesdak]

So I figured that the replacement was the 180* thermostat

More likely it's just defective out of the box as WE are quality control anymore. Getting good parts is a struggle with the parts stores race to be cheap including dropping better parts quality brands from their availability.

That's a good run on the OEM part.

This isn't one of the applications like Cough Cough GM Diesels Cough where they didn't do their job so you need a lower temp thermostat say to buy time when you hit a grade waiting for the obsolete spring thermal fan clutch to get air moving. Use the correct thermostat for your year pickup and the computer will be happy for emissions, getting out of warm up mode, and as above fan control.

Summer so most forget a hotter thermostat means a heater that's hotter.

You also get less water and diesel buildup in the engine oil with a hotter t-stat as it evaporates or boils off faster with higher temps. Oil runs hotter than the coolant.

 

[Crunch]

Observation regarding the 180* thermostat: After a couple of weeks running with this thermostat I've noticed some definite differences while driving. On 95-100 degree days, when I am on the highway driving long distance the temperature stays right about 180*, i.e. barely touching the right side of the "2" in "200". But when I am driving around town in the same ambient temperatures, the engine temperature actually climbs higher than the 190* thermostat did. Those temperatures cause the needle to reach about a third of the way into the second "0" of "200". I'm guessing that's roughly in the 210-215 degree range, which is a bit higher than what I ever noticed with the 190* thermostat, which was pretty much right at the 207* mark. The only thing I can figure is that the 190* thermostat does better at maintaining a steady temperature because it opens later, and can therefore close a bit as it cools. The 180* thermostat would be fully open before 200*, and therefore loses the ability to close at normal operating temperatures, so the thermostat ends up running full open in city driving and the engine maxes out at the higher temps and stays there.

 

[AH64ID]

Based on the temps I posted above it’s running too hot with the 180° thermostat.

 

[Crunch]

Based on the temps I posted above it’s running too hot with the 180° thermostat.

It's certainly running at the top end of the safe range. It's only in the local, stop and go traffic, but I still might go back to the 190* thermostat and put this one back in the "spare box". Highway speeds it seems fine, just a bit cooler than I'm used to. Not happy about the $60-$70 price tag for a friggin' thermostat, though. That just seems wrong.

 

[Tuesdak]

I wonder if the cost of the thermostat will soon be forgotten when the age and condition of the expensive EV fan clutch comes into question. Water pumps are also a good item to inspect.

 

[Crunch]

I wonder if the cost of the thermostat will soon be forgotten when the age and condition of the expensive EV fan clutch comes into question. Water pumps are also a good item to inspect.

I dread that day, but so far it's been working fine. Water pump is only a year or two old, so I'm not worried about that. Plus, it's almost as easy as the thermostat to change. I even have a spare water pump.