I'd be deeply appreciative if any reader had any suggestions as to what this fault might be. I'll try and explain everything.

Porsche 944, 2.5, 1985, normally aspirated.

Car is regularly serviced, genuine 39k on the clock. No problems.

Purchased a year ago, I’d been driving it for journeys up to 4 hours, with no problems. It had it's last long run about a week before this problem, that neither I or two garages can solve.

One day, it suddenly started overheating with white vapour smoke. The coolant tank was emptying, but other than the exhaust vapour, there were no water leaks. To set the scene, it is a genuine overheating with loss of coolant, not a faulty temperature gauge.

There are no oil leaks anywhere. And there is no water contamination in the engine oil. The engine oil has remained pristine, having been changed less than 1000 miles and 6 months ago. It’s not using oil either. The engine level has remained constant.

Oil and air filters were changed late 2022.

The car still runs perfectly. Starts on the button, no missing, both when standing or driven, no black smoke, no bangs, no wallops etc.

The 3 year old water pump and thermostat was replaced with a new pump and thermostat. The problem persisted, so we replaced the new thermostat with another new thermostat, this time rated 2 degrees hotter. Problem persisted.

We used a CO sniffer to test for cylinder gases both at the coolant and exhaust ends. No CO detected. We tested the CO sniffer on a car at the garage that did have a blown head gasket, just to make sure it was working. It was working fine. We returned to the 944, tested a second time at both ends and again it showed as clear.

We then did a compression test, each cylinder was perfect, as it was when we tested during the March 2023 service.

We’ve removed the spark plugs, all fine.

There is no obvious damage to the radiator, and I understand the radiator is only a few years old.

The radiator fan works fine. Albeit we replaced the switch, just in case. Afterall, we’d replaced everything else.

All hoses have been checked for any splits, leaks, kinks etc. Every hose is perfect and near new anyway.

We drained the radiator, and the water was clear. We added Holts radiator flush and drained it, again clear. We added a second radiator flush (Worths), drained, and same again. The water was 99.9% clear.

We have replaced the temperature sensor. No difference.

Now for some clues that might help someone.

When starting the car, the water vapour being emitted from the exhaust becomes progressively worse over 25 minutes. Roughly how long it’s taking to overheat. By overheating I mean reaching the yellow blob at the right of the dial.

When it reaches the overheating point, the radiator top hose is too hot to hold.

The exhaust manifold, n. 1 manifold (nearest the bonnet) is hot. Nowhere near as hot as the top radiator hose, but hot nonetheless. Not too hot to hold. N.2 and n. 3 manifold pipes are merely warm, and n. 4, whilst not cold, either has nominal heat or has warmed up from the engine bay. In fact I had to grip it in my hand to ask myself is it warm or not. I am not an engineer or mechanic, but I’d have assumed that the four manifold pipes should be more or less the same temperature?

The coolant tank is barely warm and the cap is cold. Oh, we have a new Porsche coolant cap too.

Then at the end of this saga, the coolant tank empties steam and via the overflow.

It feels like the water isn't circulating, but ultimately I have no idea.

The car has never in my ownership seriously overheated or run dry.

Any pointers, even straws to clutch at would be appreciated.

I was wondering about the oil cooler, but at the rate I'm replacing items, I'll soon have enough removed parts to make a second 944.

Thank you.