Fuel/Starting issue 86 300SDL
 

My baby, Sadie, has appx 205k on her...a glitch now and then but overall, near mint...runs great as long as she doesn't have a cold...
About 6 weeks ago, she wouldn't start...from experience, it seemed like fuel filters, so I changed them both out, but she wouldn't start. Killed the battery trying. Jumped her, she finally started, and ran fine. Started/stopped/restarted fine all weekend, then Monday morning, she wouldn't start. Played with her, jumped her again, got her going, took her to my Mercedes shop for a physical.
Glow plugs tested bad, so swapped them out with new. She fired fine. next morning, dead again. Mechanic sent a flatbed for her, got her there, he got her going.
Same thing next morning.
At this point, my wife mentioned the incline in the driveway. I told her it shouldn't make a difference, but asked Peter anyway. No, sealed system, never done this in the year prior...
Five times back and forth to his shop, and as long as she sits level overnight she is fine. He replaced the fuel filters again to be sure all was ok, tightened and checked all fuel lines, etc. After sitting at the slight incline in the driveway, in the morning, the main fuel filter is 3/4 empty. top it off and after a good few cranks, she fires up runs all day fine.
Peter finally suggested parking her nose-down in the drive way and seeing...she starts every single time, even after being gone for a week on vacation...
Any ideas?

You have an air leak in your fuel system. Unusual to empty main fuel filter, maybe the seal in the top of the filter, the center bolt, injector return lines. Need to make everything air tight. Make one change at a time, that way when you cure it you will know what the problem was.

Exactly mine/Peter's thoughts...have gone through them all, he even put clear lines on to see if we can see air/bubbles, and nothing...I'm a good all-around mechanic, he is a MB specialist, and we're both scratching our heads...he has serviced Sadie since 1992, and I know he knows what he is doing...yet she still leaks air...

Try clamping the fuel supply line before the primary filter one night and see if that stops it. If it does you can gradually move further up the system until you find it. I would have thought the one way valves in the lift pump would stop drain back. maybe thats the problem. When you changed the primary filter, maybe a little bit of crud got in. I have seen a hair sit across those valves and allow drain back.
You wont see air bubbles if the air leak is at the top of the secondary fuel filter. You could try putting a bead of RTV around the edge of the center bolt on the secondary one night & see if that fixes it.

I'll do just that...thanks! First, have to replace the master cylinder so she's safe again before I even move her...I hate sloped driveways!

The primer pumps on the lift pump can sometimes leak and cause a similar issue. Might try a new crush washer on the primer pump assembly, or just give it a little tighten.

An OM603 doesn't have a primer pump.

I agree.

It is almost a certainty that an air leak is occurring at the secondary..........either on the bolt or the filter itself.

That was our thinking, thus Peter replaced the filter 2 days after I had replaced it, including the 2 O-rings...I am wondering if there is somehow a very small crack in the mount itself...will continue the quest, but backing in to the drive does the trick for the moment...although my next door neighbor thought I had expired tags or something...:eek:

Show's what I know about MB's beyond the 617.:o

Have you checked all the rubber conections under the car and at the tank?

Yep...all of them and probably some that have nothing to do with the issue, but hey, I'm under there...:cool:

Also make sure that all the rubber lines are soft.

When the rubber hoses get old they dry up and no amount of tightening will make them seal properly.

Keeping the secondary below the fuel supply will certainly solve the problem..........of course.

..............makes it difficult to take the vehicle on any trips overnight, however.........

If I had an SDL, I guarantee it would sprout a primer pump, but, the factory decided it didn't need one.....