DougM

Finally had a chance to drive the car around the neighborhood after a complete engine out refresh. Engine started to overheat after a few miles and spewed coolant. After cooling down, added more water and burped the system. Tried it again and the same thing happened.
WP is new, not rebuilt
All seals and o rings in the water bridge are new as well as the thermostat.
A couple of observations right after shutdown.
Upper rad hose hot
Lower rad hose not quite as hot.
Radiator warm.
Once cool, it is easy to squeeze the lower hose to burp the system.
When it was hot, right after shutdown, it was difficult to squeeze the lower hose like it was full of pressure???

pjg

Check for radiator blockage.

IcemanG17

thermostat backwards...................................don't ask how i know

Mrmerlin

did you have the heads off?
Did you backflush the radiator with a hose after you removed it?

to fill the 928 cooling system,
with the car level
fill the bottle to just above the seam in tank,
then start engine turn on the heater to full hot,
run till the fans come on or for 10 mins till the thermostat opens,
both hoses are hot use an IR gun to test for temps. looking for about 185 top hose and 165 lower hose.

replace the cap drive the car for 5 miles with the heater on,
park it let it cool,
top up the system as necessary

DougM

I was wondering if that could be it. Will pull it out and check tomorrow.

DougM

Never took the heads off because they had just been off the car and freshened just before I got the car.
Radiator is a newer(2 years old) aluminum 928 Motorsports one. I did flush it with water when removed months ago.

hacker-pschorr

yup.

Did you replace the seal that goes behind the thermostat?

IcemanG17

Mine ran sorta cool with it backwards...only after 2 hours on track in reasonable temps did it finally overheat.....of course having Mark Anderson himself on your team helps...."Is the thermostat in correctly?" he asked...me "I dunno"...it wasn't....be fine ever since...

ColinB

First check the thermostat and gasket as others have suggested. If that's not your problem consider this: I had the same symptoms on my '89 car. It would overheat within 5-10 minutes. Turned out the coolant wasn't circulating because the water pump impellor had become loose on the shaft. I was told by the mechanic that it was a fairly common occurrence even with OE water pumps.

HTH
Colin

DougM

I will first do as MrMerlin suggests and try the proper procedure on filling the system tomorrow. If that doesn't work, I will pull the thermostat and check.

Lizard928

Pull the thermostat first and test in boiling water.

You will then know its in right and working correctly. Then fill as per MrMerlin.

Mrmerlin

where did you get the waterpump?
what brand is it?
did you replace the rear seal for the thermostat in the water bridge?

DougM

WP (Lasso? I think)was ordered by a Porsche service owner friend of mine that orders only OEM from the main US distributors. Not EBAY and not rebuilt.
All the seals were replaced in the bridge and the smaller rear one as well.

Mrmerlin

OK then
check thermostat first,
then refill as directed

Bill Ball

If it overheats that quickly, my money is on the water pump.