Coolant overflow
 

Let me describe my car's symptoms and see if you can help me figure out a fix.

The car pukes out coolant between the radiator reservoir and cap when the car is run hard (like a 25 minute warm day track session). The water temp gauge goes to about 3/5 up from the bottom. Not too hot. Oil pressure starts at 5 bar and drops to 4 by the end of the session. This happens when the coolant level is at min according to marks on reservoir, but stops if coolant level is a little lower.

Car is a '86 NA
Engine has 170K+ miles. (not sure how much more)
Connecting rod bearings and oil cooler seals were replaced 30K ago after an oil cooler seal failure.
Oil pressure is normally 4.5 to 5.0 at highway speeds.
Oil looks very good and clean. 20W50.
Coolant looks good. No milkshake.
Cap looks good. Rubber is free of cracks.
I'm so sure the system is purged of air. Bled system with nose up.
All radiator hoses are less than 3 years old.
Radiator has unknown history. Reservoir looks original.

All advice is appreciated. Thanks. :)

1st thing is replace or at least pressure test the cap. The spring weakens over time.
If that doesn't help, rebleed the coolant. Then start suspecting the radiator :(

I'm betting you need a new cap (OEM )

Good Luck!

Late cars use a 21 pound cap / 150 KPA.

I do not see any mention of thermostat or radiator fan switch replacement.
Are both your fans coming on with the AC on?

Recent thread....

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/608831-coolant-system-acting-up-got-logic.html

GL
John

Fans are coming on and stay on as normal after the engine is shut down after a hot run. The radiator fan switch was replaced 3 years ago. I'll start with a new cap.