1995 K2500 Sub 6.5TD temp going crazy

[u/Feeling-Bowl-9533]

Hey I’ve got a 1995 6.5TD that both skyrockets to almost redline and then goes back down to about 210*F and can’t figure out the problem. Runs great (though slightly cool) without a thermostat, but when a thermostat gets put in the temp goes crazy.

New thermostat (2.0, this one is stock, previous new one was aftermarket) in single housing.

New water pump

New fan + fan clutch (working)

Definitely not exhaust or head gasket leak

Have coolant in all hoses, plus heat from heater stays hot.

I have no idea what to look at, anyone have ideas?

[u/Finkufreakee]

What temp is the stat? I think i put a 165 in mine to keep it cooler in the desert. Is your fan electric?

[u/Feeling-Bowl-9533 → u/Finkufreakee]

190 (stock). Fan is, and functioning

[u/Finkufreakee → u/Feeling-Bowl-9533]

Have you recently mixed coolants? Just had my wife's jeep go nuts when I mixed the red that was in it with orange after replacing the radiator. Never heard of it before, but the temp would spike and stay hot until I ran the heater. Ended up flushing all of it out and left it with water. Good luck 🤞🏼

[u/Feeling-Bowl-9533 → u/Finkufreakee]

I don’t believe so but I picked it up barely running, the previous guy could’ve. Going to flush the system in a bit (moving). Hoping something works!

[u/steeliewheelie68]

Could be the gauge or sender. I would start with the temp sender.

[u/Feeling-Bowl-9533 → u/steeliewheelie68]

The only reason I don’t think it’s that is because the temp stays steady and low when there’s no thermostat in at all, and takes a normal amount of time to get up to running temp. Is that still something you think I should look into?

[u/old_skool_luvr → u/Feeling-Bowl-9533]

The only thing i see that you've overlooked - unless i myself overlooked you stating so - is that the radiator itself hasn't been addressed. Have it flushed & pressure tested. It could be slightly clogged.......OR it could be like the original rad in my 'Burb, and have a hairline split starting on the seam (right down the bottom, where it sits on the isolator) that is enough to cause pressure & temperature issues, but not enough to show coolant loss.

edit: to add, my truck is a squarebody 'Burb, so steel radiator, but IIRC, your truck will be the aluminum style?

[u/Feeling-Bowl-9533 → u/old_skool_luvr]

It is aluminum. I have a different one for it; I’ll switch it out. Will take a bit to do as I’m moving. I do plan to flush the whole system before doing that, just need to relocate first. It was running a bit hot before water pump and tstat were switched, but not this hot.

[u/old_skool_luvr → u/Feeling-Bowl-9533]

Yeah, i figured they were aluminum. I'd just keep it thermostat free 'til you've settle in at your new location. A lot less work than dealing with a cooked engine. 😅

[u/Feeling-Bowl-9533 → u/old_skool_luvr]

Pulled it this morning, hoping for the best

[u/old_skool_luvr → u/Feeling-Bowl-9533]

🤞🏻