It sounds like you are getting there, soapy water good idea, give it a clean at same time.

i would still make sure the fuel tank is unobstructed as if blocked could draw fuel back when switched off, you may even find this more pronounced now that you have fixed leaks up front.

What's the latest vent wise?

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David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

I have not looked at the vent yet. My plan was to run the engine with the tank cap off eliminating the vent as a possible cause and make sure there are no leaks up front. I would then tighten the tank cap and run the engine and if pressure builds in the tank or bubbles appear in the clear lines again I would assume the vent would be the last possible cause.

No worries using soapy water if it is under pressure, not so if under vacuum.

I do not know your lift pump setup well. If one of the 2 lines that still has bubbles is a suction line, you have to look upstream for the leak.

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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

I think I finally got it...or at least very close.
The last thing I did was replace the O-rings in the delivery valves, then started the car and let it idle for 15 minutes. There were bubbles in the two clear lines on the lift pump and when I would shut the engine off the clear line from heat exchanger to pre-filter would drain down (very slowly). I let the car sit for 30 minutes then start the engine and again there were bubbles in the two lines and the one line would drain down (but not as far). I did this three or four times, then I decided to take the car for a drive. I parked the car over night and this morning I looked at the line and it only drained back about 1/4".

I stumbled across this thread by accident, and yes I know it is several years old... but there was some misinformation posted earlier, back when the discussion was about tank pressure instead of delivery valve seals. Anyway:

Pressure in the fuel tank is normal, as @Diseasel mentioned back in post #46, especially in gasoline powered cars. If there is no "whoosh" of escaping pressure when you remove the cap, something is wrong. I can't recall offhand if diesels have similar, or less, pressure compared to gas cars. FSM job states that up to 50mbar (0.75 psi) is normal pressure inside the tank.

Vacuum in the tank is VERY BAD, and will destroy the tank by collapsing it. The "whoosh" should never be vacuum sucking air into the tank when the cap is removed. Vacuum in the tank is almost always caused by a faulty vent valve, described back in post #31. This valve is inexpensive, and should be replaced if original - it's cheap insurance. Especially now that all the 124 sedan fuel tanks are NLA new and can be difficult to source used, due to problems shipping fuel tanks (they are considered hazardous/explosive by many carriers).

Not sure if the whoosh is pressure or vacuum? Hold a small plastic bag tightly around the filler neck and open the cap with your fingers through the plastic. If there is pressure in the tank, the plastic bag will inflate. If there is vacuum, it will collapse the bag. I should get a video of the pressure inflating the bag, and add to my YouTube channel... seems to be a lot of confusion about the whoosh over several decades and multiple MB discussion forums.

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Dave
Boise, ID
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