Strange 951 overheating problem.
 

I have a 1986 951 that heats up very quickly and runs hot. Previous owner thought it was the headgasket and after purchasing I pulled the head and found the HG not to be blown. I replaced it anyway. I have also replaced thermostat, turbo thermostat, radiator thermofan switch and cooling fan relay. I have gone so far as to check all the coolant hoses and the coolant manifold for obstructions, I have checked the block for obstructions, I have drained and flushed the block, I have tried running the car with no thermostat and it still runs hot. Intercooler, oil cooler, ac condenser are all free of debris. I have also replaced the temp sender and gauge cluster. I have several 951s and none of them exhibit this problem. Car has good compression and leakdown figures and lots of power but im getting a little frustrated with the running hot issue. The car is running a little rich at the moment so it is not a lean fuel mix issue. Does anybody have any friggin ideas here?

PO had the oil cooler o-rings replaced before I bought the car and I am wondering if something strange happened like a mechanic left a piece of rag in there...

I've seen a water pump so rusted and old that it had no blades anymore, not on a 944. Maybe the water pump is not "pumping"

You didn't mention anything about the actual radiator. What condition is that in? They can build up a lot of crap inside the fins that will make it not exchange heat well at all. Also, what about properly bleeding the cooling system? It can be a pain, but there are several write-ups about performing that.

Radiator is new. Cooling system was properly bled. I have often wondered about the water pump. It looked very new and spun smoothly with no play when I changed the belts, but who knows. maybe something is wrong with the impeller.

I had the same problem, change everything and nothing fix it, at the end it was the negative battery cable that goes to the engine!!!!! change it and that fix it. The ground was faulty so when the fans need power it was loosing it through the ground. Try checking that out!!!!!!

Check the pressure cap, maybe replace it. Yes it could also be a spun impeller or a clogged radiator. If there is no way to drive it and stay cool enough it's probably the water pump.

Do you have the correct radiator? The 968 one has the 1" port blocked off on the inside, you'll have to drill it out for use in a 951.

You obviously have such a good understanding of these cars that an "out of the box" approach seems required. Please excuse any obvious questions.

When you say your car runs hot, are you basing that on what the dash display says? I know you replaced the dash cluster and the temperature sender (presumably the gauge temp sender and not the DME temp sender). To get rid of any sensor or electrical connection issues, do you have any independent verification that the temperatures you are worried about are actually seen in the engine? Again, sorry for the dumb question, but based on your already pretty thorough investigation, nothing should be off the table.