I recently had my hood and headlamp replaced due to a fender bender... when i got my car back i noticed some backfiring at take off.

I went back to the body shop and they said they didnt touch the engine, so i think when they washed my car then got water in it. If so would a bad plug or something cause this?

its a 1996 C280... i have no check engine light or anything... just a popping noise when i give it gas at idle.

Any help would be appreciated. Also if it is a bad plug or something like i suspect, then is there a way to determine which one has gone bad b/c i just recently replaced them or would i have to replce them all?