The obvious symptom is the car leaking lots of oil! Our car didn't really go through any coolant, it would just leave oil everywhere it went. This is caused by the headgasket shrinking away from the front timing cover seal and thus it creates a space for oil to come out. The headgasket itself is still functioning, but I have heard it can recede back to the point where coolant may enter cylinder #1, thus scoring the cylinder walls and causing damage. At this point, you will notice coolant slowly going down, possibly a smell, and possibly a larger than normal amount of vapor coming out your tailpipe.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro