My 1995 E300 has started building air pressure in the fuel tank whenever the engine is running. The engine has stalled a few times on me when I stop at a stop sign. I opened the fuel cap on the tank a bust of air rushed out.I have since been driving the car with the fuel cap not screwed down tight and the stalling problem has stopped.
What would cause this pressure to build in the tank?

Thanks for your help.

Probably a plugged tank vent valve or plugged tank vent line.

See post # 13 on this thread.
fuel tank vent?

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

More likely a vacuum situation, rather than pressure, caused by the blocked tank vent line or valve mentioned by alex300SD. As the fuel is withdrawn from the tank, a partial vacuum is created since the venting system cannot equalize the tank to outside atmosphere, making the pumps work harder and eventually stalling the car. You've bypassed the clogged venting system by leaving the fuel cap open.

Regards,
Tom
'95 E300D

It's a pressure situation for sure. If I fill the tank drive for a few miles and open the fuel cap I get a diesel fuel bath!

it's your lift pump pulling a vacuum through the fuel line O-rings. you likely have a clogged pre-filter. it's sucking air in past the O rings and it ends up trapped in the fuel system.

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CENSORED due to not family friendly words

Suggest replacing the lift pump.

Rgds,
Chris W.
ex 95 E300D, 467K

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Objects in closer are mirror than they appear.

What is a "lift pump" and where is it located?

The lift pump is the mechanical fuel pump on the side on the injection pump.
Possible but unlikely that the crush washers on the lift pump are leaking.

The pre-filter can be clogged or its o-ring way have deteriorated.
The o-rings on the fuel lines can fail to seal as they deteriorate over time.
Fuel may not leak out , but air can leak in if they are dried out from age and engine temperature.
Perhaps just replacing the o-rings and/or the pre-filter with its o-ring is what is needed?
The car is 21 years old, when were they last replaced?

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Does this have a pressure re-circulation pump for the gas tank like the 95 C280 ? Driver side inner fender toward the headlamp. Cylindrical pump that exhibits "heart-beat" vibration when grabbed.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

I have the same issue. Pressure release when taking the cap off an empty tank (doesn't always happen). While I agree with the vent being blocked (caked with dirt), I'm interested to see about the lift pump pulling in air.

I'm up for a secondary filter change and I'm seeing some slight weeping at the center bolt. I'll pull the primary as well and see if I have any issues to note.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

Check the tank strainer for gunk, many look like this including mine.

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David


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

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