May sound funny but I had the same problem and turned out it was the gasket on the gas cap. Made a new gasket and the problem went away.

Incidently does this happen only when the car is over 1/2 to 3/4 full of full? If so, bet you anything that is the problem.

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89 190E
2.6L
Arctic white
Grey leather
Sunroof
Pirelli P400 rubber
Smoked corners