Dear MrCjames: thanks a lot for your information.

Dear M.B.DOC: your reply is right on the spot!

I have replaced both oxygen sensors and the MAF (AMM), so their codes are gone. However, a new code P1170 has started popping up, just as you said in your first reply.

I don't know much about the door/trunk/A-C vacuum system in this 1994 c220, and don't plan to replace the vacuum pump at this time. Is it possible for me to just use some golf tees to block some vacuum lines so that the fuel trim management is back to normal (i.e. no more P1170 code)? Thank you very much.

Best regards,

Eric