M925A2

3up

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This is a 1989 just picked it up in Arlington Washington headed back to Las Vegas temperature gauge reads to 200 to 230 on the radiator one side is
cold one side hot and fan not starting, I think maybe the thermostat is not opening but I'm not sure. Headed to harbor freight to get some tools i have good tools back at home. So is there a way to bypass the stat?
 

Karl kostman

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3up by how you are describing this you may very well be correct or you have a blockage somewhere else in the system. The first thing I would do is to drain the coolant down below the thermostat and remove it and look for a reason it not opening it could be something as simple as cleaning it and scraping any residual scale etc. out of the thermostat body and scrape out the housing assembly at the same time. I would say that you have about an 85% chance that this could take care of your issue. While you have the thermostat out make sure you look at it closely looking for broken or stuck parts that need to be de-scaled and good luck! I do not know how to bypass this on the 939 series.
 

cbrTodd

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I would confirm the head and radiator temps with an IR gun before I went any further if I were in your shoes. These gauges aren't very accurate.

If you have the single thermostat version with the external bypass, that type of thermostat can't be removed - it will not force coolant to the radiator and it will overheat. So don't try that as a temporary fix - it won't help. I'm not sure what would happen if you tried removing one or both thermostats on the internal bypass version with 2 thermostats.

Some other things to consider, if it truly is getting hot at one side of the radiator but not the other, the radiator core could be clogged or the water pump impeller could be damaged. Do you get clean coolant out of the radiator bottom drain? Or a brown muddy mess?
 

3up

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Ok driver side rad is cold passenger is hot but not burning hot. Went got an IR gun and now its running 195 no issues both tanks on rad are hot now. I must have scared it running great now only about 900 mi to go 😁
Thanks guys.
if some one has a pic of the 2 stat motor would love to see it. I only see one on this one.
 

charlesmann

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i was having an issue similar and i drained my system, put in probably way more vinegar than need be, but along with some CLR in the radiator, changed out the water pump (what a damn headache that was) and replaced my stat. other erroneous reading from the temp gauge, things seem to be good now
 

Mullaney

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So could a person buy those stats at like napa?
.
Short answer is yes. P/N THM 613080 @ NAPA

Source for that information is the 5-Ton parts spreadsheet that is maintained by @simp5782.
Might be worth your time to download it if you happen to be on the side of the road and need a part from a local parts house.

------------- Apparently I didn't read well enough (EDITS Added!) -----------

That's the thermostat for an NHC250, not an 8.3. The two look quite a bit different, and the best I can tell, the thermostat for the C series is only used on the C series. It is Cummins part number 3913028 (assuming that 3up has the single thermostat option as mentioned above).

THANKS for the catch cbrTodd
 
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charlesmann

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So could a person buy those stats at like napa?
EDIT:

Not sure. You would have to give them the part number from the tech part manual or contact a cummins repair shop/dealer for their stocked p/n and see if napa carries it, or if the cummins shop/dealer has it in stock.

I got the pump, stat and gaskets from dieselpartsdirect.com and i think i typed in the tm pc part number in the search engine and they stocked that p/n or showed a current cummins p/n. I dont remember, that was over a yr ago. They stock the p/n out of the part catalog (Cummins part number 3913028) under ds-3913028

And as cbrtodd said, but i didnt check myself, napa doesnt carry the p/n.
They might be able to cross ref it, but not gonna hold my breath. Best bet would be, if you need it, might be best to source a cummins dealership and see about the gaskets and a water pump for your travels.

And that is IF you have a single stat.
 
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3up

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Its dark and raining here now so hoping tomorrow will be better. The gauge goes up to like 220 then back to 190 ish and stays. I havent been to place to IR the tanks but will tomorrow. Pertty sure its stat or gauge. Will do more looking in the am thank you guys for your help. Im kinda stuck and that pic really helped. Thank you
 

3up

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.
Short answer is yes. P/N THM 613080 @ NAPA

Source for that information is the 5-Ton parts spreadsheet that is maintained by @simp5782.
Might be worth your time to download it if you happen to be on the side of the road and need a part from a local parts house.

:cool:
Sweet thank you
 

cbrTodd

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.
Short answer is yes. P/N THM 613080 @ NAPA

Source for that information is the 5-Ton parts spreadsheet that is maintained by @simp5782.
Might be worth your time to download it if you happen to be on the side of the road and need a part from a local parts house.

:cool:
Mullaney, please be careful with the parts interchange sheet. That's the thermostat for an NHC250, not an 8.3. The two look quite a bit different, and the best I can tell, the thermostat for the C series is only used on the C series. It is Cummins part number 3913028 (assuming that 3up has the single thermostat option as mentioned above), and my brief search on the Napa website (which they seem to make less useful on a daily basis) didn't show anything that crossed over.
 

simp5782

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8.3s are cummins thermostats only or other aftermarket retailer. Truckpro fleet pride etc would carry it
 

3up

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Ok roger that IR gun shows 175 ish everywhere hoses rad ect, gauge showing 210 to 230 thinking bad gauge. What says you guys? Gauge will drop to 190 ish after a bit
 

Mullaney

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Ok roger that IR gun shows 175 ish everywhere hoses rad ect, gauge showing 210 to 230 thinking bad gauge. What says you guys? Gauge will drop to 190 ish after a bit
.
The sending unit could be bad.
Might even be a bad gauge or a bad ground to it.

Sad to say that the gauges are not really known for their high quality...
 

charlesmann

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Grants pass Or, on some river. Thank you all for your help truck is running great. Other than an exhaust smell in the cab from an exhaust leak I can't seem to find it's perfect.
I ran in to that in an old dodge company tck. Diesel exhaust smell in the cab, esp with the fan blower on. Tck shop could find a noticeable soot trail in the eng bay or under cab.
My 936A2 had an exhaust smell till i replaced the rusted out muffler.
 
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