Test drive it with the cap off the tank as long as you don't have more than 1/2 a tank or so in there.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I drilled a tiny (0.63mm) hole in the tank cap to fix a problem where pumping from my aux tank in the trunk into the main tank would cause it to build up significant pressure. It should help for vacuum as well, but the proper way to fix it would be to clear the tank vent, I think it has a check valve in it, because I never had a problem with vacuum, only pressure...

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Ich liebe meine Autos!

1991 Mercedes-Benz 560SEL | Megasquirt MS3-Pro | 722.6 transmission w/ AMG paddles | Feind Motorsports Sway Bar | Stinger VIP Radar | AntiLaser Priority | PLX Wideband O2 | 150A Alternator | Cat Delete
1981 Mercedes-Benz 300SD | Blown engine, rebuilding someday...
1981 Mercedes-Benz 300SD | Rear ended, retired in garage.
2009 Yamaha AR230HO | Das Boot

Excessive speeding? It ain't excessive till I redline!

Busted Benz, unfortunately I was unable to get the air out of the fuel delivery system and thus not able to restart the car. Sent it to a knowledgable garage to sort out.

Prior to that I was removing the fuel cap after each 10 minutes of driving.

300SDI, I inspected the bell shaped valve on the third fuel line (close to the drivers rear wheel) and could not detect any issue. However, I did notice that there is a crack in the final 90 degree bend, most likely admitting air into the system. I believe this to be a non issue as my fuel tank had a vacuum (not over pressurized).

Thanks for the help...

I had significant vacuum in the fuel tank of my 97 E300D a few years ago. I didn't address it and it ended up partially collapsing the fuel tank (gave me about 4 gallons less capacity). I found the vent tube underneath the fuel tank and gave it a blast of compressed air (with the fill cap removed). Solved the problem...it was as simple as that.

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Mark in MA
05 MB E320CDI 402k Granite Grey Metallic
05 MB E320CDI 267k Black
05 MB E320CDI 232k White
05 MB E320CDI 209k Tectite Grey
99 Dodge 2500 Cummins 5sp 148k
62 Jeep CJ-6 120k

Thank you. I had the car transported to a garage yesterday. His diagnosis is not yet complete, but I will update when the project is complete!

Flawless

I really believe the hand pump is your problem not some clogged vent. If a diesel sucks air into the fuel system, it will give you many starting problems. Repair is very inexpensive with the new-style (black) priming pump. IIRC, ~$30.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Funny, the shop recommended the same solution! New pump should be in tomorrow. I'll be pleased with a fix, but am still not able to wrap my brain around how that malfunction has created a vacuum in my fuel tank!

Anything more than a slight vacuum, whatever the cause, should be relieved by the tank vent.

If the fuel line that eventually terminates in the bell shaped valve has a crack, might that create / enhance / or have no effect on the vacuum present?

An opening in the vent line would relieve any pressure differential.

As the lift pump sucks fuel from the tank, air must displace the fuel that was sucked out. That air come via the vent. If the vent is clogged, a vacuum is created. Make sense?

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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

Yes, and thanks. I was confusing the fuel return line as a line that would have relieved the void created by the fuel removed. Two lines, one in and one out, I thought would have allowed for a balanced system.

All you need for that system to work is an engine that consumes no fuel. You would always have a full tank.

Just got a call from the shop to come and get my car. They replaced the purge pump (next to the injectors), and the line leading from the fuel tank to the bell shaped valve. Apparently the #3 fuel line (leading to the bell shaped valve in the drivers side rear wheel well) was clogged. $200 later, I am assured of peace of mind.....?