knocking vacuum pump

[joebiodiesel]

I am looking at an '85 300D with 220K on the Odometer. The car starts, runs, shifts and handles well. I checked it over and it needs a couple of glow plugs and the windows don't operate properly..none of which bothers me too much.
However, there is a loud knocking noise coming out of the vacuum pump. It is RPM dependent, and is quieter at some times, but never goes away. It could easily be confused with a rod knock or REALLY loud combustion noise on one cylinder...but the stethescope tells otherwise. I am an experienced mechanic, with absolutely no Mercedes experience. Is there a common problem with these pumps? Are they rebuildable? Can they damage anything in the engine if they fail?

Thanks for any replies.
Joe

[Wes Bender]

If the vacuum pump fails it can take the timing chain with it, resulting in a very expensive fix. Most of these pumps are rebuildable. Do a search on this forum under vacuum pump.

I would fix the vacuum pump ASAP. Don't ask me how I know...

Cheers,
Wes

[lrg]

Joe,
The pump is pretty easy to remove and replace. If it's making noise as you describe it's unlikely it can be rebuilt. The later OM603 ('86 and later 6 cylinder) engines are almost always destroyed when the vac pump lets go but the earlier 5 cylinder OM617 engines (as standard on the '85 300D) have more of the pump body outside the engine so you have a sporting chance a total pump failure won't take the engine with it. That said, Wes is right, there is still a very good chance when the pump goes it will spit parts into the timing chain, breaking the chain and causing damage beyond what the car is worth. The fix may be as simple as installing a new vac pump or a bit more complicated. There is a concentric cam that runs off the injection pump that actuates a lever on the pump. Sometimes (fortunately not often) these cams can get damaged and need replacing. While not terribly expensive, it turns a simple pump replacement job into something more complicated (setting the injection timing and harder to access parts). As an experienced mechanic you should be able to make any repairs (including the cam replacement) without major drama. Don't, however, wait for the pump to totally let go because there's a good chance the next stop will be a junkyard. If you buy the car have the seller drive it to your location (that way if it blows up it's his problem). Then buy yourself a shop manual CD and fix the pump before you drive it again. Good luck.

[joebiodiesel]

Hi Wes and LRG,
Thanks for the advice. Any chance the vacuum pump from an '83 300D is the same? I have access to one if I need to rob some parts. Where can I find some good service information on these. LRG you mentioned a CD...who makes a good one?

Joe

[lrg]

I think the two vac pumps are the same but you should check a cross reference on the parts numbers to be sure. The CDs are all the same and are copies of the now out of print FSMs. Fastlane sells the CDs and you can also often find them on Ebay. Be prepared, the copy quality is not the best. If you are only replacing the pump, you can probably figure it out without the manual. If you get into IP timing, it helps to have some of the specs found in the CD. Haynes also makes a good general workshop manual for the 123 chassis (late 70s and early/mid 80s 300D) but is lacking in a number of areas, including IIRC the vac pump replacement. By the way, if you want to cross check the pump parts numbers, you can often do it by looking up the parts for the two models on Fastlane. If possible return them the favor sometime buy buying something from them since they sponser this site.

[joebiodiesel]

Thanks everyone for your help. I decided not to buy the car. I swapped the vacuum pump for the owner (I work with her) and the knocking went away. I decided, however, not to buy the car. It developed a high speed vibration, like when a drive shaft is out of balance, and the alternator would overcharge for short bursts, and actually load the engine enough to slow the car briefly. Too many headaches for me. It needs nearly $2000.00 in parts alone to fix the car. Time to continue the search.
Joe