6.5L k3500 getting hot all of a sudden

[Angelo]

It's a 1996 4x4 ambulance, so is basically always pulling a load. I've weighed it at 12,630 pounds total. Is that a lot?

I did the HO water pump, duramax fan swap from Leroy diesel last May, 2022. Been doing fine until now. Driving up lots of hills, going slow the temps stayed below 210 almost always. Occasionally if I pushed it too hard on mountain grades, I'd pull over if the temp got to 210 and give throttle to circulate coolant and let it catch up. The temps would lower quickly.

Now, all of a sudden it is getting over 210 on flat ground when I am not giving it throttle. If I give it throttle it slowly catches up and lowers the temp. Never had this problem of temps going higher than 210 and taking this long to recover. I used to hear a loud noise when it'd get to over 210, is that the fan clutch engaging? I don't hear that noise anymore.

I can turn my fan by hand and it doesn't move on its own if I try to spin it. Hate to think the clutch has gone bad this quickly. I don't know how to test the fan clutch.

It's only been about 10k miles since the HO water pump and duramax fan, severe duty clutch was installed. I also installed a 190 ac Delco thermostat in December of 2020. Could any of these components be going bad this soon?

My radiator fins are looking pretty bad, and the radiator is a 2007. But for these symptoms to start happening all of a sudden tells me it can't be the radiator. I did a flush this past summer as well. I'm thinking about putting a 180 thermostat in to see if that fixes it since its the cheapest to try. And thinking I should have been running the 180 this whole time since I'm always pulling a load. Anyone agree?

What could have gone bad all of a sudden?

I've attached pics of the gnarly looking rad

 

[Schurkey]

12,630 lbs is about double what an ordinary pickup would weigh. Yeah, that's a lot.

If you used to hear the fan clutch engage, and now you don't...start there.

You DO NOT need a lower-temp thermostat to cure an overheating problem. You MIGHT need a non-defective thermostat of the correct temp rating. But I'd start with the fan clutch.

 

[termite]

Plus one for Schurkey's line of thought. Check coolant level and condition as well, assuming you haven't already.

 

[smdk2500]

When is the last time you cleaned the cooling stack? Is the ac condensor, rad, trans and oil cooler fins clean? If not try washing them out. I would agree with Schurkey on the clutch but its a good idea to check the front side of everything else as well. Its amazing all the crud that can get trapped in there.

 

[Angelo]

....
If you used to hear the fan clutch engage, and now you don't...start there.

You DO NOT need a lower-temp thermostat to cure an overheating problem. You MIGHT need a non-defective thermostat of the correct temp rating. But I'd start with the fan clutch.

I'm gonna try replacing the tstat with another 190° for now I guess? It is 5 months after the 24 month warranty on it expired. It'll arrive Thursday tho. Cheapest fix to try.

When engine is warm and I try to spin the fan by hand it moves very little after I let go. What other way can I test the fan clutch? It seems fine. I would always hear that noise when I was climbing mountain grades pushing 2k+ rpm. Now it just overheats on flat, going slow, and giving no throttle. Let me know if there is some other way to test the fan clutch.

The rad is clean, and coolant is filled to fill line in reservoir.

 

[Schurkey]

If the fan clutch doesn't engage...that's pretty-much your answer.

About the only other things that could cause that symptom without it being a faulty clutch, is blocked airflow through the rad, or a failed radiator that doesn't heat the air going though it. If the clutch isn't heated by radiator air, it'll never engage. Possible, but seems unlikely.

 

[Angelo]

Well I'm surprised that it was a stuck ac Delco 190° thermostat. I figured out it wasn't the fan clutch by clamping the fan to the nut on the water pump and testing with same hot result.

The bad t stat was installed in December of 2020 so it was right after the 24 month warranty was expired. Part number 131-91 from Rockauto. I ended up replacing it with the same part number because it keeps my truck cool until it failed. Hopefully this one lasts longer than 2 years though

 

[termite]

I'd toss the removed t-stat in a pot o water and thermometer to see where it actually opens at (and if it opens at all).

 

[Angelo]

I'd toss the removed t-stat in a pot o water and thermometer to see where it actually opens at (and if it opens at all).

Yeah I was curious too. Going to try that. My guess is that it opens at 210+. The truck would always eventually cool down after being at 215-220. Will post those results here...

Edit:

Boiled in water, doesn't open at all

And in regards to me not hearing the fan clutch kick on. I guess that's a mistake on my part. Maybe the fan was engaged the whole time while hot and didn't I pay attention to hear it kick on or off.

 

[someotherguy]

Yeah I was curious too. Going to try that. My guess is that it opens at 210+. The truck would always eventually cool down after being at 215-220. Will post those results here...

Edit:

Boiled in water, doesn't open at all

And in regards to me not hearing the fan clutch kick on. I guess that's a mistake on my part. Maybe the fan was engaged the whole time while hot and didn't I pay attention to hear it kick on or off.

Sure sounds like time to switch thermostat brand. Motorad has been a trusted name for a long time (I hope that hasn't changed) - Motorad 423195 should be the correct one for you. Also 7332195 is their fail-safe design, but in the case of the 6.td even failing open may not be enough to save your engine, so next up, please consider...

Since your application is a heavier-than-normal vehicle: https://www.thedieselpage.com/features/dualstats.htm - though it'll cost you. It just might save you the expense of total engine replacement, though. You probably already know about this upgrade, but if you haven't read this particular article, please do. Heavy trucks like ambulances are specifically mentioned as to why this upgrade was engineered by GM.

Richard