Resurrecting an old post, I know, but this has been happening to me on my 87 300E lately.

Investigations reveal that the fuel tank design was changed from 1988 on to improve the splash baffles round the output line, due precisely to this problem.

With the old tank, the only way round the problem is to keep the tank at least half full if you're planning trips uphill.

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1987 300e manual 250,000 km (sold)