which cap? and weirdness
 

I understand the expansion tank cap differs from early to late cars, what if you have an '83 engine in an '87 924s? I'm thinking early cap, yes?
"Why am I asking?"
Glad you asked.
Symptoms:
heat sink after shut down, noticed a very warm engine many hours after shut down
erratic coolant levels
most recently after engine pull and reinstall (belts and seals), while fighting belt whine we're running sans power steering belt and alt belt and covers while checking belt tensions, brief 10 second runs between several minutes of tweaking belt tensions, suddenly had coolant leaks popping up, coolant system had become pressurized but engine is cold, suspect HG failure and pressurizing it, but we also have oil around oil pressure sender so am now doing oil cooler seals (man do I wish we had done that last week while the engine was on the stand!!!)
tonight we decided to just leave the cap off the tank, immediately upon start up it was pushing coolant out of the tank
we never did get a decent coolant bleed either though, only a meager attempt prior to short 1 mile test runs before going back in the shop, car runs well and don't have other HG failure symptoms, a bit of "scum" in the coolant expansion tank (cooler seals?) but that's it,
Since we're pulling exhaust to do the cooler I'd like to do the HG too if it needs it.
I know, I know, should probably have rebuilt the entire thing while we had it on the stand but it was running fine (pulled it for release bearing and just did front main and belts while it was out).
So, first question, why isn't the cap bleeding off pressure? New cap sought, which one? 13lb I think.
HG? I hope not.
Thanks for reading.
http://i1148.photobucket.com/albums/...psjd8gyxkf.jpg

Before you worry about HG, get a new BEHR cap. I've tried the LAP brands with no success.

You poor guys!

No need to pull the exhaust to do the oil cooler - you may just open a new can of worms. As long as the PS pump is out of the way you'll have enough room.

I'd make sure the HG is compromised before diving in, but it shouldn't push coolant out the overflow tank on startup (if at all, really).

if you are getting coolant blowing out the tank on cold startup you've got a bad headgasket blowing exhaust into the coolant.

if you were able to swap the engine and do the oil cooler you will have no trouble doing the head gasket. it's about 2-3 hours to remove the head from the block.

as far as the cap goes...it doesn't matter. the early and late 944/924S cooling systems are 100% identical with the exception of the cap pressure spec, so a different cap isn't going to affect the system in any meaningful way aside from the direct effect of raising the coolant boiling point.

"i am guessing" that porsche decided it was worthwhile to up the pressure spec from 13 or whatever psi to 21 or whatever psi because they were finding coolant boiling under certain circumstances in the earlier cars, but the engine block, water pump, hoses, radiator, and coolant tank are all the same.

I would do a full system bleed sounds like you have air trapped in the system,my experience is that these cars don't bleed easily,always seems to take time to get right.