Yesterday, I replaced the vacuum pump diaphragm on my ’68 220D (W115). It had torn and was sending oil towards the intake manifold. This is my first time doing any significant work on this newly acquired vehicle, and I wanted to share my experience in case there are other “beginners” out there with diaphragm-based vacuum pumps. To give you a sense of the difficulty of this job, I do not have much experience working on cars other than passing my dad a wrench when I was young. In other words, if you are a beginner, but have the confidence to tackle this job, I believe anyone can be successful.



I decided to leave the vacuum pump on the car while replacing the diaphragm. This was a strategic decision that I put a lot of thought into ahead of time. I felt I had a greater chance of success by removing fewer fasteners. On the downside, I didn’t get to inspect the bearings or “roller coaster” cam. Right now, I feel I made the right decision. But if the pump fails in 2 weeks, I will obviously reconsider.



I started the day by cleaning the engine around the vac pump, which made the whole job more pleasant. My vehicle doesn’t have power steering so access to the 8 flat-head screws that hold the diaphragm was pretty easy. I used a flat head screwdriver fitting in my ¼” ratchet which worked well. I didn’t need to remove the fan or fan shroud to get reasonable access. I replaced the check valves and diaphragm, following the general procedure outlined in the FSM, on this forum, and elsewhere.



My plan was to rotate the crankshaft to ensure the new diaphragm was fastened under a slight bit of tension. But I got lucky and it ended up in a favorable position when I parked the car. The two metal disks (washers?) that push/pull the diaphragm were the same size in my pump. I think that is an error by whoever last did this job, because the FSM clearly shows one to be larger than the other. I just put it back how I found it (rounded sides toward the diaphragm) and hopefully it will be okay. Elsewhere on this forum, a few people found it hard to tell which way to orient the metal disks, but it is very clear which side is supposed to touch the diaphragm.


Via this forum, I had been given the advice to not overtighten the center allen bolt. The correct torque is 9 Nm, but I tried to “guess-timate.” In hindsight, I know I undertightened it. So at this point, I am relying on luck and a little blue locktite to keep it all together. I need the car to visit family this weekend, so I hope I don’t regret not re-opening it all up to properly torque it. After the fact, I used a friend’s bicycle torque wrench on a different fastener to get a sense for what 9 Nm feels like, and I definitely did not achieve that. I am going to keep my fingers crossed on that one.



One surprise occurred when opening the diaphragm repair kit. The kit contained a small rubber o-ring and I had no idea where it was supposed to go. I put it on the center bolt behind the inner (or back) metal disk/washer. That was based on a hunch and hopefully it won’t backfire. I could not find any mention of this o-ring in the FSM.


One decision I made on the fly I now regret. The FEBI kit I bought came with four flat-head screws. I decided to replace 4 of the 8 old screws with these new ones, figuring that metal fatigue could occur in a 45 year old fastener. Unfortunately, I partially stripped a couple of the new screws, while the old ones all held up fine. I think they new ones are much softer (weaker?) than the old ones. In hindsight, I should have just re-used old fasteners instead of replacing with the new ones. Lesson learned.



All in all, the job took about 2.5 hours (I am fairly slow) and it appears to be working (for now). I had two main take-away lessons from this project. 1: Find the right tool for a job before you start it and 2: Check your pockets for parts before you “finish” a job. I had an extra washer which I now need to go find a home for. Oops.


My idle is not steady (it hunts or lopes from 800-850 rpm). I had hoped that this exercise might solve that problem. It didn’t. But at least the oil should no longer be getting to the intake and my brake system is safer. Thanks for all of your contributions which helped me have the confidence to tackle this job.Next up is a valve adjustment. I am nervous about that one, but if I learn anything helpful for beginners, I will pass it along.

This is exactly how we build self confidence over time. We all or many have worked our way up over the years in our comfort zone of what we can handle.

The old Clint Eastwood expresion almost defines it. A man must know his limits. You expand your limits everytime you take something new on that represents a challenge at that time.

This site is instrumental in helping an individual develop self confidence further. There is almost someone here that is kind of watching over your shoulder to consult. They have either done what you are attempting before or intuitivly know what is required.