Block heater melt down

Why was the block heater on if the engines were running?

This was my first thought as I was reading. Then secondly, even if left on and even if the thermostat in the heater failed and stayed full on I would think it wouldn’t be enough to overheat the engine. The block heaters are usually like 1000 or 1500 watts. Your cooling system should overcome that. I think more likely culprit here is something in that heater failed and caused you to lose coolant/water and that’s why you overheated.

Yes, I agree that in theory the thermostat should simply shut the heater off as the normal engine cooling temp is well above where the thermostat kicks off. However, I have always been in the habit of turning the heaters off before I start my engines. There is no reason to leave them on and this issue is possibly a perfect example of why not to leave them on. Not to mention you are adding load to your gen if the thermostat fails and keeps the heater on. And if you have external heaters plumbed in you should have valve protection that you can close so you don’t lose coolant in the event of a failure of the heater, hose or connection.

I agree that Id pick the first one. That heater is designed to get hot, handle the temps. My guess as well is that the heater failure lead to a fluid discharge that allowed the engine to overheat.

Running with the heaters on should be avoided, but it also shouldn't cause damage. More hours unnecessarily on a heater shortens its life span, but it shouldn't cause damage unless the type of failure you experienced takes place.

Oh man, so sorry to hear. How many hours on that engine?

+1.
The opposite scenario of an engine overheat for some other reason, melting the heater as a consequence, is totally implausible.
Temp gauge and alarm (which BTW are connected to separate probes, on that engine) should have gone through the roof well before the coolant becomes hot enough to melt the heater.
Assuming that the coolant CAN get hot enough to melt the heater, which I'm actually a bit skeptic about.

Sorry to hear that.
Can you remember what sort of temp the coolant reached, and possibly also oil temp if available?
Besides, for how long approximately did the engine run hot?

Ok, but in principle the engine should withstand a very short burst of everheat, without consequences serious enough to find coolant in the oil.

What I would fear is that the coolant dump was so fast that the temperature probes (both the one for the gauge and the one for the alarm) remained dry before even sensing the coolant temperature increase.
And in this scenario, by the time the alarm goes off and the temp gauge show a higher temp, it's actually the whole engine block that is overheating rather than "only" the coolant, if you see what I mean.

Anyway, involving an MTU specialist is certainly the best course of action.
I for one would be curious to hear his opinion, if you don't mind keeping us updated!
In the meantime, all the very best for his findings.

Do your engines not have low coolant level alarm? If heater ruptured and dumped coolant, why didn’t the low level alarm alert you to a problem?

I believe the reason for wanting the heaters off when running is the possibility of low coolant and/ or air entrainment causing hot spots on the heater element and thus they give up working or "worse'?

On MTU 183's they don't. If you ever snap the belts on a 183 at cruise rpm's. The engine will overheat and grenade before the coolant temp alarms ever go off. It was one huge inherent known flaw with them.

Correct.
They fitted a bronze raw water pump which runs forever and never needs an impeller replacement, but God only knows what they were thinking when they made it belt driven.
MAN engines based on the very same block have a more basic rubber impeller pump, but gear driven.

I’m still interested to hear if these engines have a low coolant level alarm and why that didn’t alert to a problem?