1998 Boxster with 5X,XXX miles. Car has been well maintained.
The original complaint was coolant spraying out of the coolant tank cap (valve in cap opens at 1.8 bar of pressure.) Eventually coolant level would drop, triggering the low coolant light.
Pressurizing the cooling system produced no detectable leaks. Dealer had replaced the following attempting to rectify the problem
Coolant tank cap replaced 4 times
Coolant tank replaced
Waterpump replaced.
Bleeder valve on coolant tank replaced.
Unfortunately, non of the above rectified the problem. Previously, my shop had filled, bled, and pressure tested the cooling system as well with no leaks detected and no "clues" as to what the problem may be.
The last time the car came into the shop for some routine maintenance, we finally detected a coolant smell coming out of the exhause (it was not there before)
Rotating the motor to TDC on all cylinders and then pressurizing each cylinder to 70 PSI indicated that air was leaking into the cooling system in #6 cyl. This indicated one of the following:
Bad head gasket
Cracked head
Cracked cylinder
After removing the motor, and removing the head, we found that the head gasket was indeed bad, but for a completely different reason: The cylinder sleeve had pulled. The total distance was less than 0.25mm - but just enough to break seal and allow combustion gasses to enter the cooling system, but not enough to allow coolant into the combustion chamber until the head gasket started to develop significant cracks in the sealing surfaces.
The first picture is cyl #6 - my thumbnail is resting on the split between the cylinder and the sleeve.
This next picture is #6 again, I ran a marker around the sealing surface of the cylinder - the white line you see is the area where the liner pulled away.
This next shot is #6 again - notice the concentric lines in the cylinder- that is where the piston "walked" across the face of the cylinder because the liner is actually pulled out more on the bottom that the top.
Here is Cyl 5 for comparison
Here is a shot of the head bolts - Difficult to see in this picture, but they are approximately 1 foot long.
And lastly, here is the engine compartment, without an engine.
Tomorrow, we order a new engine.
AFJuvat