Recently my 1992 300D 2.5 started shifting roughly, and feels a little underpowered off the line. I've only had the car a for a few weeks, but I feel like it just started doing this after mostly driving more or less normal (whatever normal is for these cars, I guess).

Anyway, I know I haven't given a ton of info to point to anyone system or component failure, but my first hunch is to evaluate the car's vacuum system and look for a leak or other problem, at least as something to rule out. I wouldn't mind also checking out if I'm making enough boost. The thing is, I have no idea what I'm doing - I mean, on one hand, it seems pretty straightforward: just hook up a vacuum gauge and measure away, right?

The thing is, I don't even know where to start, having never done this before. It seems like the older W123 diesels are well-supported by all kinds of websites offering DIY info aimed at the novice (DieselGiant, MerecedesSource, etc.), but I'm finding a lot less out there pertaining to my W124 OM602. I'm aware of the FSM, but I don't recall seeing anything about vacuum assessment and diagnosis.

Anyone have any suggestions or tips? Other than being trained by Mercedes back in the 80's, how are people figuring this stuff out?

Oh, and before someone tells me to "take it to a pro," get this: I emailed the guy in my area and he wrote back to tell me that he was too busy and not booking appointments! Go figure. I mean, he wasn't rude about it, and I guess good for him that he can afford to turn customers away, but it was a little weird. One of the idiosyncrasies of moving to an island, I suppose.