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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53007
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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My 1984 Mercedes 380SL leaks liquid fuel at the gas cap

Customer Question

My 1984 Mercedes 380SL leaks liquid fuel at the gas cap after filling it up. Last time I filled it up I opened the gas cap and ~1 ga. of fuel came shooting out. I have checked that all lines are clear and I replaced the overpressure valve thinking that fixed the problem. I just filled the car up again after several weeks and the same thing is occurring. I have been doing a restoration with the help of a very good Mercedes mechanic. He got the car running by replacing the fuel tank, air-flow meter and rebuilding the warm-up regulator and fuel distributor. My mechanic does not have any ideas, but I have not taken it in. Do you have any ideas?
Submitted: 4 months ago.
Category: Mercedes
Expert:  Marty replied 4 months ago.

Hi, the only way that fuel can be pushed out of the filler neck is if the tank is not venting. You need to check all of the vent lines and also the vent pipes into the tank for blockage. You should also replace the charcoal canister for the vapor system. The is not a fuel injection system problem or a fuel line problem this is a vapor line problem. I have seen the very small vapor pipes plug up with rust or with beads from the charcoal canister. There is a vent valve at the fuel tank that you should check. Not sure if that is what you are eferring to when you say "overpressure" valve.

Let me know if you have any questions.


Customer: replied 4 months ago.
Where is the vent valve at the fuel tank? The valve I am referring to is coming in the photo. Its origin at the fuel tank is on the passengers side near the battery. This line is clear, but I am not sure as to the functionality of the vent valve. The line between the valve in the picture and the charcoal filter is clogged. Do you have any recommendations for "unclogging" or "replacing it.
Customer: replied 4 months ago.
Specifically what are the vent lines you are referring to in the attached diagram? Thanks, Chris
Expert:  Marty replied 4 months ago.

Hi Chris, a blockage anywhere in the system that you illustrated in 47-110 can cause pressure in the tank. The tank needs to be able to vent. If there is a blockage in the expansion tank system or in the charcoal canister the pressure will build up in the tank. You can disconnect some of the lines and see if it allows the tank to breathe. You could start by disconnecting #51. If the problem goes away your problem is between 51 and the canister up front. If you still get pressure then your problem is between the tank and #51.

Let me know if you need anything else.

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