Our 240D has been running well after a recent fuel tank drain and a dose of biostat.
We added a bottle of redline at last fill up. After fifty miles or so (the past couple of days) the motor got noisy at idle. Today I took it out and drove it very hard for twenty miles and the roughness went away.
Now however there is air in the fuel line, that is, a very large bubble in the clear plastic fuel filter, and when I operate the manual fuel pump, small bubbles of air well up from the fuel pump side of the filter.
It's like an air lock. But the air seems to be coming from the fuel pump.
There aren't any loose fuel connections. The in tank fuel filter is clean, and both motor side fuel filters are less than a month old, as is the manual pump.
Does this sound like anything anybody has run into before? Is there some kind of check valve in the fuel pump that might have failed? Thanks for any suggestions.