Does this sound like a dying thermostat?

Have had my Disco on the road now for a few weeks.. when I had the engine replaced, I had them replace all the hoses, all the belts, radiator, and the thermostat. Engine came with a new waterpump and the T-stat was a 'warm weather" kind that opens at 85c.

Up until recently it was running at about 84 to 87c depending on ambient.. it would get a little warmer sitting at a light, but would quickly drop back down once I started moving again.

Last couple of days it has been running warmer.. like 90 to 95c.. and it depends on engine RPMs, not speed. If I let the truck idle, it will climb into the 90s.. but if I rev and hold it over 2000 rpms.. it drops to 84c. If I drive normally, it will rise into the 90s.. but if I hold it in 3rd (or 2nd depending on speed) so that the RPMs stay above 2000.. it stays around 84 or 85.. but the moment I let the revs drop, the temps climb.

This is a new fault to me, one I have never seen before and one that I am thankful for Torque for.. the stock guage would have never let me see this issue. It seems to me that the issue is one of flow. I am not losing coolant, it is just getting warmer than usual when engine speed is slow

I'd personally be looking at water pump, but historically i'm not great with diagnosing cooling issues if they aren't head gasket related.

Get a much less accurate gauge. Anywhere between 80 and 110 or so is acceptable.

Fresh engine will create more heat.

To me, sounds like time to seriously eyeball the cooling fan clutch. Try this: get a big shop fan, put it in front of the grille but don't turn it on. Let the engine warm up to 95c or wherever, then flip on the shop fan. Temp drop with shop fan on = fan clutch starting to crap the bed. No temp drop = look into coolant flow problems.

the Fan clutch is one of my fears.. I have thought about getting one of the electric fan kits from the UK for the Disco. My father ran a similar unit on his old 1 ton diesel pickup and it worked great. As far as the cooling system goes.. the only things not replaced with the engine were the reservoir, heater core, and fan clutch.. everything else was binned and replaced at my insistance

drove it today.. and the problem is getting worse. In playing with the fan, I can get half a revolution out of it if I give it a hard spin, and it has no wobble.

I did notice something though... my coolant is black. There is still a tinge of blue from the coolant itself, but overall, the stuff in the tank is contaminated with something. I am betting that the engine was never cleaned before being sent out and the coolant passages are/were full of machine oil, oil, and other chemicals. Tomorrow I am going to give it a good flush and try running just straight water for a day to see what happens.

Does the temp climb and remain steady at low rpm, or does it cycle up and down?

Did you have torque before the rebuild, or did you start using it after the rebuild?

You may be seeing warmer water from other parts of the engine cause the rise as they flow past the temp sensor at low Rpm, but at higher Rpm there is enough circulation mixing to avoid locally warmer areas.

As for the comrinated coolant, I'd show the shop that did the rebuild before flushing anything. Give them on opportunity to stand behind their work, and avoid possibly shifting the responsibility for any future failure to you.

I had torque way before the rebuild. Even with a dying engine, it would run at 91c. This engine, until a few days ago, would run at 85 (with a cooler thermostat) even on 90+ degree days.. now as long as the revs are low.. does not matter if I am sitting still or doing 55 down the highway, the temps climb, but as soon as I allow the revs to reach 2 grand, the temps start to drop back down to 85ish

It's likely the dirty coolant you talked about. Like GTXVette said, you need to show the company the coolant just in case changing it out doesn't fix it.

this is what came out of my Disco's cooling system.. it looks like it has 300,000 miles on it.

I have her filled with distilled water at the moment, going to run that till my next day off and flush again. I will add real coolant and some water wetter after that.

edit running nice and cool now.. 86/87c at 30mph and dropping to 84c at 50mph.. idles at 90c.. just like she did when I got her back

In reply to mad_machine:

I think you need to have the cooling system checked for the presence of exhaust gasses ASAP.

Drain and add a box of baking soda refill run until warm let cool run again and drain and flush. Then refill with water and get a exhaust gas test kit as happyAndy suggested.