zgoober

Pretty sure my thermostat has been stuck open for the past 3 months. Truck doesn't get above ~150 degrees.

Has this, or will this, have any negative side effects? Major or minor?

67HotRod

Besides lower mpg, I could see carbon buildup..

SIXSLUG

I changed mine out as soon as I noticed my temps were below normal.

carl48

more engine wear

m.t.hands

the "seal" broke on mine and was holding it open, takes all of ten minutes to change it out....i went with a napa, but i think most suggest finding a cummins replacement....

zgoober

Yes, I have replaced the thermostat.

I can find information about thermostats failed closed and the I'll effects of overheating. I can't find much about failed open. At least nothing specific...

Carbon build up? Where? Is that serious?
Engine wear? Why? Where?

rickf

One of the byproducts of combustion is sulfur dioxide, not as much now with USD but still there. When mixed with the condensation in an engine you get sulfuric acid. I don't think I need to say that is bad. When an engine does not get above 180 degrees then the water does not evaporate out of the crankcase and you have the acid formation. Plus diesels run better the hotter they are.

Rick

m.t.hands

i would think you'd experience incomplete and poor cumbustion, leading to higher carbon deposits in the valve-train

zgoober

Thanks. Good information to know, but not so great to hear. Will running any additive in the fuel or oil clean this up?

ellerman

Zgoober, change the thermostat and if you are worried about an excessive amount of acid that could have built up change your oil. If you are worried about carbon run the truck hard or tow something heavy. I doubt you did any damage.

gator_grabber

I'd highly suggest you attempt to get a Cummins thermostat, the aftermarket ones just don't perform as well. BTW, running an engine too cold is really hard on it and causes accelerated wear.

totalloser

+1 to cummins stat. Usually fails open. This is pretty rare with normal stats. Opens abruptly causing temp fluctuations, but has much bigger port.

IMO the main issue is the terrible fuel efficiency. But as to unhappy engine from running cold, the pistons will never get to op temps, and will remain tapered instead of swelling up to fill the bore. Potentially a little more wear on pistons, but I wouldn't worry much about it.

You are probably losing about 3mpg running at that temp. Add it up... ouch!

Midnite

Zgoober, I guess I'll be the lone dissenter here. I don't think you've suffered any engine damage whatsoever. I also don't believe the mileage difference is much at all, at least in my experience with a failed thermostat. It certainly wasn't noticeable in my case.

It's not like the engine is remaining at the same temperature as the surrounding environment. Even with a failed thermostat the engine gets up to 100 degrees fairly quickly, it just starts to taper off and take progressively longer to reach higher temps.