C250 Diesel shut-off valve leak repair

I am a lurker. First post.

Due to a few threads on this forum I was able to work my way through several problems on my 1995 C250 Diesel (non-turbo), with a 605 engine (295000kms if you're interested).

After these successes I have decided to describe my fix and post some pics, as it would have made my repair much easier if I could see the part and understand how to take it apart.

There was a major leak on the transparent fuel lines running from the filter - pump - shut-off valve(the black box on the side of the injector pump) - filter - injectors. It looked like diesel was streaming from the underside of the shut-off valve. Also the car was having a very hard time starting first time in the morning, and then would be OK-ish for the rest of the day. Until parked for 8-12 hours it would be hard to start again.

After searching through the forums and reading loads of posts, I got all the seals from the Merc dealer(8X part # MA601 997 03 45) and took all the pipes off, but did not remove the shut-off valve as I was unaware of the seal going behind the valve. Also broke one transparent pipe in the process but got a new one from my Merc dealer for ZAR180 (US$25).

Replacing the seals did not help. In fact, when I looked at the old seals I thought to myself that these seals are in pretty good nick. Luckily somebody who owned the car before me installed a priming pump (probably the person who replaced these seals previously, I've only had the car for 1 year). So while I was priming the fuel line it was squirting out from below the shut-off valve again like crazy.

Took it all apart again and this time also removed the shut-off valve from the side of the injector pump, and replaced the seal behind the pump. (seal part # MA021 997 75 48)

After assembling everything again and priming it, diesel was squirting out the side of the shut-off valve just as if I hadn't given it any love. I suddenly realized that I hadn't looked at the valve very closely before and just assumed that the leak was on one of the pipe connectors etc. But now I thought I had to buy a new shut-off valve. Due to most posts on the net regarding this problem talking about replacing the shut-off valve, I went to my Merc dealer with the idea that I'll just buy a new valve and replace it. Until I heard the price... ZAR2600 (about US$400!).

So I decided to have another look at my old shut-off valve. Now I noticed that it is possible to remove the two caps on the ends of the valve by driving out two pins on each side. This revealed two very brittle and squared-off o-rings. I went to my local o-ring shop (Bearing Man), as Merc told me they only sell the shut-off valve as a unit and have no parts internal to it. The guy at Bearing Man said that these squared rings must have been round-shaped around 1994 or so and sold me 2 o-rings (2-214 Viton o-ring, 3.5mm X 25mm) at ZAR20 (US$3). These turned out to be too thick, but I was able to install them(the shop being closed on Saturday afternoons), with much effort, grease, elbow-grease. I think a 2.5mm or 3mm thick o-ring should work much better.

After reinstalling the shut-off valve and cranking the engine for what seemed to be 2 minutes (30sec break after every 30secs) it started and has not dropped a single drop of diesel from that area. (10km test-drive)

Let's hope that it stays that way and that I'll have a car that starts easliy tomorrow morning!

If it starts leaking again I'll replace those two 25mm o-rings on the shut-off valve with 2.5mm thick ones first before looking at anything else.

I also learnt a few things, such as what huge difference it makes working on this dirty job with latex gloves. Never used it before but for this job it is definitely worth it!

I also think that the OM606 engine in the E300 Diesel (we didn't get that car down here in South Africa) has the exact same part and problem.

Below are some pictures of the parts in question for anyone else suffering the same problem:

Side-view, with shafts driven out, caps off, and (flat, brittle) o-rings removed: