I have been lurking in this forum for over a week now, in search of a solution for my 300SD hard shift problem and engine problem. I thought that it was time for me to share my experience and bring my small contribution to this community.

1)- 1985 300SD California emission (service campaign completed)
2)- 159k miles on the original engine an transmission.

We purchased the car about 10 days ago and from the get-go the transmission had some hard shifts and engine lacked boost.

Here is what I did:

1)- Change oil + filter
2)- Change fuel filters (fill up filters with diesel engine cleaner)
3)- Replace the Air filter
4)- Remove and clean the boost line from the manifold to the Change over valve.
5)- Remove the boost line fitting from the manifold and clean up.
6)- Replace the "changeover /boost protection" switch. ($50)
7)- Took the car for a long hwy ride..... (Boost problem solved)
8)- Verified no vacuum lines unplugged.
9)- No vacuum leak on transmission line and modulator.
10)- Replace the small in-line filter (Purolator F21124, $3) in vacuum line.


The car is performing extremely well now, boost and shifts like a dream....

I took the car for lots and lots of test drives between steps and after taking the car for a 200 mile long hwy trip it really improved the lack of boost.

It is only at step 10 that the hard shifts disappeared. That small $3 filter was in bad shape, I don't know how long this thing had been in the system but I am going to keep an eye on it from now on.

Problem I encountered while working on the car:

1)- Broke the boost line from manifold to boost valve, very brittle,
2)- Hard time finding new 1/4 inch clear boost line.
3)- Original Manual primer pump leaking bad (updated pump coming soon)




I'll take some pictures and post them here very soon.




I wanted to thank everybody especially "ForcedInduction" for their help. This is my first Mercedes and without this forum I would not have been able to understand and solve any of these problems.

Now, I enjoy driving this car so much, it is amazing, 25 years old and it is a better ride that our 2003 Toyota..

Thanks again.