Early oil cooler not working but not leaking
 

2.2L engine re-assembled after larger pistons installed. All seemed okay
both before changing pistons, and now until a hotter day (~80F).
I found oil temp going to about 230F after 30+minutes on highway. It stays below
200F when outside it was ~55F. I checked that the thermostat was working (on
914 I could remove it while engine was still hot).

Can engine-mounted oil cooler have really poor efficiency if there were previous
top-engine oil leaks that got on the fins? Before re-assembly I could see through
the cooler, there is no blockage per-se. But I did have quite a bit of oil late last
year that turned out to be breather hose. Maybe being colder outside then I just
didn't notice any problem.

Thanks

I would be looking for a dead mouse or a rodent nest on the cooler.
I run safety Kleen over the finning then blow air from bottom to top, then more safety Kleen.
You know the thermostat is working, change out the temp sensor.
Bruce

Well it's only been together for a few weeks, so I'm doubting mice at this
time. But yeah, I was thinking the easiest thing to do first is spray it a lot
and see what comes out. Then, I need to check, but I think it is possible
to remove without really pulling the motor.