Your mechanic is probably right.

However, the problem could also be a vacum leak or a bad mass air meter.

I'd have him search for a vacum leak first, and if none found, then swap in a known good mass air meter, which can be replaced in about 15 minutes.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".