Yesterday, my tech drove my wifes 85 300D and suggested the vacuum pump is going bad. I have searched the archives and still have a few questions I hope you all will answer.

1. Do I replace or rebuild? Also, I have read about a check valve;what's up with that?

2. I read something about don't remove the bolts one at a time. Look or feel if the bearing is under tension and rotate the engine until tension is released. Yet, the Haynes manual says to just unbolt and remove.

3. Do I need any special tools? "T" handle allen wrenches were mentioned.

Thanks in advance,

Mr. BILL

90 300SE
85 300D
84 399SD (sold)

The vacuum pump rebiuld is a DITer, I can't remember all the detail steps, you will need to remove the pump, change out the diaphragm. You will need to clear out the check valves, they are behind the small plate in front. Be careful, it is spring loaded behind the plate. These two valves must be installed back the same way, you will know if they are wrong when you can't shut off your engine.

No special tools required.

jack

(1) For 85 300D, it is just a matter of replacing the o-ring, valves, and gaskets if other parts (check the engine side and the old pump) are still okay . The diaphragm is for older diesels, not the 85 model. The valve consists of a plastic case with a rubber disk. It can go bad.

(2) The reason you want to rotate the engine, I think, is to make the installation easier. If it is under tension, it is hard to line up the screw holes and to seat the pump properly.

(3) You need a 4 to 5 inch allen wrench. The "T" handle allen should work if the length is not too long.

David

You can rebuild, but you are only replacing the pump parts. There is a roller bearing on the rear that has failed before, when it does, it can get into the timing chain and ruin an engine. I'd replace with new.


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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

Great info! I also saw a post that recommends a vacuum chech before doing anything.

I found a vacuum tester today,but,I'm not sure what to look for. Is it possible to gve a primer on vacuum testing?

Thanks to all!

Mr. BILL