erratic coolant level and pressure

[jvandyke]

'87 924s with an '83 944 engine;
coolant level changes dramatically
if expansion tank cap is off and car is fired up it will often push water out the tank immediately
suspect: HG failure but;
car runs great, doesn't get hot (at least according to the gauge), compression test says all is well, home made leak down test (pumping air into each cylinder one by one) reveals nothing amiss (expect bubbles in coolant tank due to HG failure but nothing happens other than a little hissing through oil filler tube (air past rings I'd think) it sometimes sound like hissing in intake but if a valve is open there a bit you'd expect that, tried test at TDC and coolant level weirdness and pressure could't happen on the intake side anyway..
plugs all look fine, no apparent "steam cleaning"
have a HG kit but before I open that can of worms, is there anything else we're missing? We have tried to bleed the daylights out of the system but that doesn't mean we got it, even so if it pressurizes instantly on start up, what else could it be but a "mild" HG failure? cracked cylinder wall?

[Costan944S]

I have a similar problem with my 16v, it's missing coolant and have no clue why. No water under the car, no wet hose. It's true that I did not checked all you checked, but the coolant level is rising after opening the cap. It's missing around 200 ml of fluid every 50kms, more or less.

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[djnolan]

Start with a cooling system pressure test you connect to the expansion tank cap. You can borrow a tester from your local auto parts store.

[jvandyke]

Might have solved this:
on advice of an x Porsche mechanic we drained off the pure water and added in about 70/30 coolant/water, the water was apparently boiling off which explains almost every scenario we had. 70 miles today and no worries, was it really that simple? Apparently.
Boils at temp causing massive air pockets which makes the level erratic and gauge sender ineffective (reading voids, not coolant). After today's run the level stayed constant, engine temp held steady.

[thomasryan]

coolant concentration should run in the 45-55% range. most manuals state this as coolant doesn't carry away the heat. the freezing point goes up after 60% coolant.

if it pushes the water out of the tank when you start a cold motor, it isn't boiling around the exhaust valve seat that quickly.

[jvandyke]

Fighting it again. Broke the car out of hibernation and have issues as before. Temp gauge doesn't complain, car runs well enough (there's an issue there too, different thread) We keep adding coolant (a mix) we loose some on blow over, then it drops way down, filll it up, bleed (by Clarks, using raised front end, pressurizing the expansion tank, release from bleeder, probably 5 rounds of this to date, just all over the place, no evidence of steam from the exhaust or anything, went 25 miles on Friday, it dumped some coolant mid way after shut down, wrong cap? Running 16lb. Remember, 83 engine in an '87, no idea if that matters. One calls for 13lbs, one for 16.......getting frustrated. What happens if the water pump just doesn't pump? is that a thing?