Interesting problem
 

Has anyone encountered this problem before? I have 2 cars that I am currently working on, and both have the same problem: Even after the car sits for a few days, you can remove the coolant tank cap, and get a pressure release, now here is the twist..I just replaced the radiator and resealed the oil cooler block off plate on one car, so the cooling system has been opened and drained, I cracked the bleeder screw open (prior to adding coolant) and got yet another pressure release.

Seems to me, that if pressure was trapped between the thermostat and the engine, that pressure would have bled off when I removed the lower radiator hose, and if pressure was trapped between the thermostat and the coolant tank, that pressure would have bled off when I removed the upper radiator hose.

One car (the radiator job) is a turbo, the other is a 968. http://forums.pelicanparts.com/suppo...s/a_frusty.gif

Have you tried venting it while the front of the car is elevated? Put the front of the car on blocks and crack the vent and let it idle and warm up...air should find its way out if there's any.

Otherwise, perhaps a small headgasket leak, pressurizing the system with exhaust gas?

It might just be vapor pressure of the coolant itself, especially if it's hot outside or the car was in the sun.

I wouldn't expect the thermostat to hold back any pressure in the engine, given that the other end is not blocked off.

The only other thing would be if there was a headgasket or oil cooler leak pressurizing the coolant chamber.

Thought of those problems, compression and leakdown tests show negative on a bad HG, and as I said, I had pressure at the bleeder screw AFTER I flushed the cooling system and drained the block (at the block drain), AFTER I had removed and installed a new radiator, and AFTER I had changed the seal around the oil cooler block off plate and o-rings on the OPRV. Just can't figure out where the pressure at the bleeder screw came from after the complete cooling system had been open for 2 days, and the engine hadn't even been turned over for a week.

Very interesting, flush cooling system, and drain block, remove radiator, install radiator, remove oil cooler block off plate, install oil cooler block off plate, PRIOR to adding ANY coolant, PRIOR to cranking the engine, crack bleeder screw and get pressure!!!! WTF???

Only requires venting when installing coolant, the turbo has a Lindsey Steam Vent kit installed...open vent, add coolant, close vent, start engine and it self bleeds. But as I said above, complete cooling system had been open and engine hadn't been started in over a week.

I'd think I was either going crazy or hearing things, but the owner was standing there and heard it too.