Go the local hardware store and see if they have the "gasket" pictured.

I can only find plastic ones now, used to be a composite material.

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Well... if I can find the old aluminum washer. I was working in the dark last night and wasn't taking any special steps to save/preserve the old one. It may be lost by now. We'll see.

Like I said, I'll keep you all posted. Thanks.

Do you have a spare filter housing you can steal one from?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

No reason yet to go to more sophisticated test lists. Take a magnification glass and examine the filter holder for cracks around the bolt hole though when installing the washer.

I think others have experienced that. Beats frustration of why will the washer not seal if that problem continues. Things like check the valve lash are pretty far down the list yet. You wanted a list though. Watching others add to it is interesting. Plus we all learn something.

For example I never knew there was an air purge bleed valve item on the injection pump. Instead I thought the pumps self purged well. I even wonder why they did this. Unless otherwise there is always a small pocket of air we leave that is trapped in the pump.

To me if this is so it is normally doing no harm. Much like the static bubble in the primary filter.. Actually if that existed I would want to leave it in there anyways. To act as an additional pulse attenuator.

If you can locate an 02 filter head, that does away with the crush washer, as it has two rubber "O" rings on the banjo bolt.
First pic is of an 01 head.
Second of an 02 head.
Third of the two "O" ring banjo bolt.
You may get away with just that new style bolt.

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The 02 filter housings are less prone to leaks when changing filters. I have two spare ones if anyone needs one.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

...possibly... I have a spare engine out of a '79, but I've managed to avoid pulling any parts off it. Personal OCD quirk. I'd consider that if I can't come up with anything else.


I wouldn't think so either. But, sometimes my tendency to fear the worst overrides my need to take a logical Occam's razor approach to a problem. It's just a little maddening to spend 3.5 years working on a project, get a VERY NICE week of driving out of it, and to have this happen with no notice.

Note to self - buy a magnifying glass. It'll also help for me to look at this in daylight.

Don't get me wrong, I'm very appreciative of all the diagnostic suggestions. I just want to get my ducks in a row so I can fix this. With any luck, some combination of playing around with that filter washer is all I need.

Good to know. I'm hoping it's not the housing as I'd like to fix this ASAP.

Actually the physics of the secondary fuel filter leak have always interested me. Unless it where a very heavy leak it should not have much effect. Other than it reducing fuel pressure but that would take quite an obvious heavy leak.

This with the filters contents under constant pressure when running as it is after the lift pump. Now when the engine is off air may enter there and enable back flow of the fuel if the lift pump valves are not essentially perfect. If air gets into the system while running it almost has to be before the lift pump.

After sitting there may be air to purge out in the fuel system ahead of the lift pump with a secondary filter leak.

I never commented on this before because the majority seemed to think leakage there was a running factor. . It just never added up to me. I would welcome any other opinions or observations.

Where I have been going is any leak after the lift pump may allow some drainback. Many of the complaints of hard starting in cold weather I always wondered might be low initial fuel supply quantity initially.. Ideally when you shut the engine down residual fuel under pressure should remain present if the lift pumps two check valves are good.

I was wondering and may have expressed that adding one more check valve to the system overall might not be a bad ideal. Volkswagon does it around the rear axle area on their 1.6 diesel cars. I have run one of their cars without it myself after driving on a gravel road smashed it.

I just borrowed a small piece of hose from the engine compartment. Removed the smashed check valve and substituted the hose and was on my way. So I suspect they have a check valve up ahead and this is just a secondary one at the back of the fuel line. Perhaps a back up if the front one is expected to deteriorate. Running without it made no difference in that car and it remained in service for a long time.

What I feel is repairing that leak might solve his problem as it seems to have done for others. At the same time unless I have missed something I do not understand the physics of it. I am just missing something I guess.

Not stealing your thread either. Just a filler until you get the leak repaired and that may be all that is required hopefully.

Hmmm... That doesn't sound 100% correct to me. I may be misundstanding. thought the cigar hose was the return-to-tank line. Let me see if I understand this correctly.

At the top of the filter housing:

• the small braided line is the return-from-injectors line
• the cigar hose is the return-to-tank line
• the clear plastic hose is ALSO a fuel tank return line? (if so, what route does it take?)

I hear where you're going with this, and your logic makes sense. But, I see a fuel leak, and I have a poor running condition. It makes sense to try to connect those two first. If solving this leak does not affect the running problem, then we know it wasn't necessarily a factor.

Think in series. You want a clear line on the 8 mm 5/16" barb of the filter tee, spliced in series to the cigar hose or in place of it. The path of the cigar hose returns to the tank. Is that any clearer?

edit: This clear line is custom, not any of the OE clear lines.

With your leaking filter center banjo bolt, it would drain down overnight and you would see a bubble (with engine off) at the highest point of that clear line (should be solid fuel if system was tight), indicating a problem.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Much. You're talking about adding some clear line into the mix. Interesting suggestion... not something I ever would have thought of.


For what it's worth, here's my car when I bought it in 2012, followed by what it looks like as of this past Sunday:

























Hopefully, you can see why I want to get it running right

Nice transformation! You did a lot of work to it.

The clear line I am describing is the best diagnostic aid for air in fuel in a diesel. It gives you an instant indication if you have air in fuel problem or not while the engine is off or idling (and while running if you rig up a remote cam).

Easiest way to install a temporary clear hose is to replace the cigar hose with a length of 1/4" i.d clear PVC tubing from a hardware store. Soak the ends in boiling water to soften, stretch it out with long nose pliers and slip over the 8 mm barbs, use rubber bands as clamps (not really needed if the clear line is temporary since the fit is so tight).

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I thought lost weekends where a booze thing. You have proved otherwise in this case. Hell of a lot of work done. Looks really good as well.

What you are experiencing now is absolutely nothing in comparison to what you have already done. The chances of it being a bad fault are tiny at best. Actually just a get familiar with your fuel system that is never a waste. The better you know it the easier to fix and maintain it. I am partial to the 616 engines and 240d cars although I have many others.

Actually when this problem is over. I will post my impression of a 240ds fuel system tuneup maintenance requirements. It is a very cheap thing to do and is also well worth at least checking a few things. Many of these fuel systems other than filter changes have never been maintained.

To me it is a credit to the manufacturer of them that was not Mercedes Benz.. That they seem to hold together for longer then the manufacturer ever expected the cars to. Even better if the need is indicated they can be restored easily. This includes two minor improvements where I think the original system designers failed to consider the longer term picture.

Well, it looks like I got lucky on this one, in a couple of ways. I found the old aluminum crush washer on the ground next to where the car was parked. I cleaned it up and smoothed it out before reinstalling it. I worked the primer pump a bit and started the car. It ran somewhat better - still not perfect.

Then, I ran out to the store to get a new battery. The battery had given up the same night I had the rough-running problem. I got home, installed the battery, and then discovered another showstopper issue.

When I was working on the cooling system, I replaced the hose that goes from the water pump housing to the heater control valve. As the previous owner did, I just routed a long piece of straight hose. Well, the hose had fallen against the exhaust manifold, and as I was running the car in the driveway, it burned through and started spraying coolant. I cut out the bad section of hose and rerouted it.

When I was sure everything was OK, I took the car out for a test drive across town. It was still running a hair rough for the first couple of blocks, but it smoothed back out after that. I'm guessing that some air still needed to work its way out of the system.

The drive across town went well as did the drive back. The car stayed smooth. The last two test... I guess, are whether it's still smooth upon the next cold start, and how it behaves after the next highway run.



So, apologies for the long thread over what may have been just a leaky washer.