Very strange cooling system behaviour Topic is solved

[Clemens]

So after I recently found a small crack in the radiator of my 94 T5M I replaced the radiator as suggested.
Put everything back together, keeping sleddriver´s story with the burping coolant system in mind.
Filled it with 50/50 red antifreeze and massaged the big hoses. let it warm up under idle till the thermostat opened. Let it cool down and topped up the overflow tank.

Next day drove it around town for half a mile. stopped for an hour. afterwards when firing up the coolant reservoir lamp came on. turned off motor and rolled to the autoparts store, added 1 Liter of red antifreeze after letting it cool a bit (there was still fluid in the reservoir before topping up). Drove it around town. Came home, checked everything, realized that the gray (probably original) reservoir cap has a crack. I know they fail, so I wasn´t worried. Ordered one at the dealer. Meanwhile I wanted to drive it and superglued the crack in the reservoir. Tightened it carefully in order not to break it apart.

Drove on the highway at mild speed for a few minutes and then through town. Opened the hood after shutting off the motor and popped the hood. Reservoir was at max as it was before leaving home.

Suddenly fluid started escaping underneath the cap. Went to the store, came back, reservoir had emptied to min mark, quite some fluid on the ground.
Refilled to an inch above min. Tightened the cap good.
Drove home very carefully. Once at a slight incline under boost the warning light came on. Arrive at home. With the hood open i let it sit idle to check if the fan comes on. Coolant was at half inch above min. Suddenly the fan comes on, fluid level drops like a quarter inch or so. The surface in the reservoir seems to bounce a bit (like boiling).

Shut off the motor, level starts to rise. I tighten the cap really good and everything stays inside, but i can hear it boiling inside, sounds like alot of fluid comes from the little overflow line. Continued to boil for 5 minutes after the engine was off. Now, 10 min. later, the level is slightly above min.

I hate the idea, but is this a sign of a bad head gasket?

Thing is thought: if it were the gasket, why would it only start boiling after shutting off the engine? Because then there is no more pressure added to the cooling system? Cuz I think it would increase the level while driving alreaday because the gasses are added..


The water/coolant mix looks clean, the oil is as thin as warm 0W-40 ist on 85 degree days with a warm motor.
Motor ran fine before the radiator replacement, still runs fine.


P.S.: After I got my 96R I first had some coolant issues and after getting new coolant I also had a lost water situation once. Refilled it and then it stayed good. Must add that I had a new reservoir and a cap that was not cracked.

[kahl]

My first suspect would be the overflow reservoir for a crack. I have changed out the overflow reservoir on all the P80s. It seems you have a leak some where in your system. Also check the hose clamp on the bottom and top of the reservoir. The small nipple that the top hose slides on to can crack and leak. At this time I would not suspect the the head gasket has failed.

[jimmy57]

When you stop engine the retained heat and no more flow of coolant will allow coolant to get to 225 or above. If system holds pressure then you never have an issue. If the system doesn't hold pressure then that temp is enough to boil a 50/50 mix of coolant and it will get air pockets, burp, gurgle, etc. You have a cracked cap so that has to be replaced and is the most likely part causing the problem since you don't mention having a leak.

[Clemens]

Wondering if it is pulling air through the broken cap. I just drove it for 20min, hoping all the ait had escaped.
While the motor was running wveryrhing was fine (i checked 4 times), but as soon as i turned the motor off the boiling and overflowing through the crack in the cap or underneath the cap started again. Reservoir is at min now.

EDIT: saw jimmy's post too late. That explains everything. Car will be parked till new cap arrives.