Honestly, they were a bit surprised and Mike-the-mechanic got a bit defensive when I asked my first question: what did the codes tell you? And from there it was interesting to hear the mumbo jumbo about special test, it could be the EGR valve but the air flow goes up, down, around downtown, then a miracle happens, ... and he couldn't be sure until he opened everything which seems fair. He also claimed that taking the pipes out was tedious.
The mechanic knew of the vacuum tube test but he didn't do it. He said he used some Merc test "something" called SDS or something like that.
Anyway, where we have left it is that my bet is it is the pipe (he also thought it was very likely) and he can open he valve and we go one step at a time. But I will wait until next week until Iget a second opinion.
Funny thing is when I talkd to the owner, his response was if it was up to him he'd just disconnect the check-engine light and keep driving the car for a while!!!!!!
The info you all provided last night was EXCELLENT.
Thanks a bunch.