No worries Rick, although I can't take the credit for the marked up diagram, someone else's useful handy work.

Have a look at this thread regarding tank vents (this is a 123 but relatively similar)

E300 diesel issue, again! - Mercedes-Benz Owners' Forums

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David


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

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

I could see that happening if a vacuum was pulled into the tank but my situation is just the opposite. pressure is building.If I open the fuel cap on the tank a burst of air rushes out and if the tank is full a bust of diesel gushes out!
There were times that while driving the car if I were to come to a stop the engine would quit running but I could re start it.
For that past few weeks or so I have been driving the car with the fuel cap screwed on only half way (not tight) to allow the air to escape and this solved the engine stalling problem.

You should not be getting pressure in the tank with a clogged tank vent, vacuum yes pressure no. How fast does the pressure build? Are all fuel lines routed correctly? Which line has air bubbles and why do you think the bubbles is the cause of pressure in the tank?

Have you unclogged the tank vent yet? That should have been the first thing to do.

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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 pressure will build within 10/15 minutes of driving. I believe the fuel lines are routed correctly only because I have not touched them. The line going from the lift pump up to the fuel filter is the one where I can see bubbles. I am not saying that the bubbles are the cause, it is just something that I noticed after taking a closer look with a very bright flash light.
I will unclog the tank vent as soon as I Figure out where it is located.

Here's some info on your tank vent 93 300D 2.5L Turbodiesel Power Loss

Take it out, you should be able to blow into it from either end. If not, it is clogged. I'd soak and agitate it in hot soapy water till no more dirt comes out and you can blow through it.

This may help with the pressure stalling the engine problem but you should still find out why you are getting pressure in your fuel tank.

You should post pics and videos of the air bubbles and fuel leaks, neither should create pressure in the fuel tank but should be addressed and fixed. I do not own a W124 (I have a W123). Yours has a fuel heater and elec idle control but a similar arrangement of a mechanical lift pump with a pre filter and secondary filter.

Edit: After taking off the tank vent, put an extension hose to the vent line and blow into it to make sure the line is not clogged also.

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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

How do I gain access to the tank vent, from the trunk?

It should be under the car, at least that where they are located on my W116 and W126 300 SDs.

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

Have you found it yet? As Alec said, it should be under the car. Did you open the link I posted? The diagram shows the tank vent routing clearly.

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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 can understand vacuum forming in the fuel tank. As the fuel is consumed in the engine, that volume must be replaced by air which can only enter via the vent system.

I don't understand how pressure can be produced. How can the fuel pump (lift pump, injection pump) add air? Is an air leak on the lift pump intake side allowing air to be introduced, which is then pumped into the fuel tank?

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Certainly an interesting problem. I was going to suggest, after he unclogs the tank vent, to place a balloon over it and observe how fast it inflates, then take the balloon off and sniff it's content, which may give a clue.

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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

No I did not find it yet and yes I did open the link (thank you for that) I can't see much from laying on my back looking up. I may have to take it to a garage that has a car lift.
Last night I filled the tank with fuel and stopped before it started coming up the fill pipe then I put the fuel cap on and backed it off some which has solved the engine stalling problem. This morning I started the car and let it warm up for about five minutes and noticed a 24" diameter puddle of fuel on the driveway, I looked under the car and could see fuel dripping from a hose that I think is an over flow tube that runs up to and under the fill cap.
I can understand that a clogged vent would cause this because the pressure has to escape from somewhere and I do plan on inspecting the vent but sure am having a hard time figuring out what is causing the pressure.
I ordered a new lift/feed pump and I doubt that is the problem but I thought it might be worth a try.

I'm not able to remove the fuel lines from the lift pump. I pushed the connector all the way in then pulled up as hard as I could on the fuel line and it will not come off.
What am I doing wrong?

Give the pipe a twist while pulling, it needs to be vertical and not angled as tight fit.

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David


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

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

All information needs to be in your profile up on the right hand side of your post. So that when other members are viwing it know how to answer ,and help you . .Its blank ,nothing about your car or engine type. Hope you sort out the problem .This pressure can screw up the tank, like a crushed bear can as all the air is sucked out of it .

I bet it is vacuum, not pressure in the tank.

Pressure in the tank wouldnt affect the vehicle running.

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