Hypothesis- Internal leak within the secondary fuel filter housing that causes return air from the cigar to mix with supply fuel. There appear to be crush washers between the fittings on all three ports (2 brass fittings on the side, three way fitting on the top). Beyond this, anywhere else to look? Alternatively, barking up the wrong tree entirely?

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1981 240D 4sp manual. Ivory White.

Your cigar hose is new/ recently replaced? Check the sealing surface on the cigar hose ends for imperfections that may not provide a perfect seal. If you have a Mityvac with pressure capability, test the cigar hose under pressure to check for leaks.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The cigar hose should be full of fuel after you ran the car and all air in the system was bled out to the tank, same with the temp clear cigar hose. If there's a leak within the secondary fuel filter housing, you'd see a bubble in the clear cigar hose also. But you didn't? Only with the black cigar hose?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Never any bubbles in the clear temp hose and yes the cigar hose is new..

Afaik, diesel fuel (or liquids in general) do not compress. If the cigar hose was 100% full of fuel 100% of the time, what would be the point of the expansion space in that design? What’s funny is the engine did feel nominally smoother on the smaller ID temp hose. The overflow spring is to spec, the lift pump is fresh, would the further restriction of fuel leaving the IP have an effect beyond the Overflow valve?

In any event, I’m traveling for a few months so I’ll have to dig this up when I return. Thanks funola -looking forward to reading your thoughts.

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1981 240D 4sp manual. Ivory White.

The cigar hose is soft and expand and contract slightly with return fuel pressure, a regular fuel hose which is braided is rigid does not expand/ contract. If you wrap you hand around the cigar hose, you can feel the pulses of the lift pump with engine running. The cigar hose's purpose (supposedly from an old Mercedes mechanic I spoke to) is to protect the return steel line from rupturing. A lot of people run without a cigar hose with no issues. I have a factory cigar hose on mine and I never see bubbles in the short clear return line right before the cigar hose.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Actually that’s a good point, you do have a permanent window.
I still can’t understand how an air leak there would impact the other lines...
When I get back I’ll pull a vacuum and see where it’s at.

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1981 240D 4sp manual. Ivory White.

Sorry to Dig up this old thread but I'm chasing a similar issue where I need to hand prime every morning on my 300TD. @Shern, did you ever figure out this issue?

I did not figure this out... I also spoke to a few other people who currently experience the same issue of waking up to find an inch or two of air in their return lines.

I've replaced every single fuel component, and even pressurized my hard lines to look for pin holes.

At this point, the only thing I can imagine is that somewhere in the IP, there's some internal seal that's either marginal or gone.

-

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1981 240D 4sp manual. Ivory White.

Did you check the return and supply lines at the tank?

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Post #1

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1981 240D 4sp manual. Ivory White.

If you have an M Type Fuel Injection Pump some have claimed that air can be pulled in past delivery valve holder O-rings. On the M Type the only seals in or around the Fuel Injection Pump are the Crush Washer and O-rings up there around the Delivery valves and in the Lift Pump there is that little O-ring and the valves. Of course the lift pump valves has to seal and the valve seats and springs have to be OK. Then there is the crush washer area between the lift pump and the Hand Primer and the Hand Primer itself.The bottom of the element seal metal to metal against the aluminum housing.

On the MW Fuel Injection Pump there around the delivery valve is a Crush Washers.
The Elements on the MW are sealed with 2 O-rings but you cannot get at them with out a lot of complication because moving the element changes the calibration of the fuel quantity. Of course the in the Lift Pump there is that little O-ring and the valves. Of course the lift pump valves have to seal and the valve seats and springs have to be OK. Then there is the crush washer area between the lift pump and the Hand Primer and the Hand Primer itself.

When the Engine is running the return fuel gets hot. Hot fuel in the tank expands as does what ever air in the tank that absorbs heat. If your tank vent is not working correctly when the air and fuel in that tank cool and contract it will create a low pressure and pull fuel from the return and inlet lines if it can.
One way to find out about that would when the Car has been out for a good long enough to heat up the Fuel and Air in the Tank park it and remove the Fuel Fill Cap and let it sit over night. Before you start it in the morning check and see of the Fuel has retracted out of the Fuel Line you were speaking of. Before putting the Fill Cap back on start the engine to see if it has those symptoms you mentioned.

One way you can prevent Fuel In the Inlet Fuel Line from going back to the Fuel Tank is to put a Check Valve in the Fuel Line as in the attached picture. I have some like the one pictured and some that are plastic I got on eBay.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

It's interesting this. Something I've noticed for some time now (I think I wrote about it several pages back).

The thermal expansion you're describing is apparent. The size of the bubble is greatest in the early morning. Say I haven't driven the car and check it again around noon, the bubble will be much smaller.
The fuel level is rising and falling

Now of course I've done the tank vent thing: took it off, cleaned it out, made sure it blew in both directions, tried taking my fuel cap off right before shutdown, etc etc etc. No difference. If it isn't clear, I've done all of the obvious things and even some of the less obvious things. Which is what lead me to considering an internal pump seal. The thing I struggle with there, is that this still doesn't make any logical sense. As you know, anything past the lift pump is positive pressure.
I'm not talking with any certainty here, but I guess the venturi effect could be responsible somewhere in the chain?

I don't know, I've mostly given up on it.

The check valve however is a very cool idea... where exactly would you put it? In the engine bay before the primary filter? Or spliced into one of the rear fuel lines?

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1981 240D 4sp manual. Ivory White.

Shern ;

IIRC (stop laughing, it's possible ) in spite of the bubbles you can start the engine up and drive off so it's not an important detail .

My friend who was M-B Factory trained during the W123 era says it's normal and I should stop stressing about it so I did .

>PING<

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Despite this thread's resuscitation, I stopped thinking about it a while ago.
It's only an issue on the first start of the day.

It's not normal though.
If it was normal, this thread would be filled with photos of clear lines with an inch of bubble.

NBD either way.

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1981 240D 4sp manual. Ivory White.

Electric fuel pump is your friend.

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CENSORED due to not family friendly words