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MB Paul
MB Paul, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3567
Experience:  Qualified Mercedes Benz Diagnostic Technician - 20+ years experience - Workshop Foreman for Mercedes
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Fuel pump continuously shuts off during filling. Sometimes I

Customer Question

Fuel pump continuously shuts off during filling. Sometimes I have to invert the hose/nozzle to fill up without interruption. Recently replaced right side fuel sending unit to clear up stored DTC. Fuel guage still not fixed. Randomly stops working.
Submitted: 1 year ago.
Category: Mercedes
Expert:  Lou P. master tech replied 1 year ago.

Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

Expert:  Lou P. master tech replied 1 year ago.

just so i have it here, what is the vin and the build date, i know by now i should have it memorized.

Customer: replied 1 year ago.
WDBHA28E5VF509869 10/96
Customer: replied 1 year ago.
That link isn't working.
Customer: replied 1 year ago.
No, there is a small tank there but no canister. #2024700589
Expert:  Lou P. master tech replied 1 year ago.

ok, so are those the hoses on that tank that you took loose?

Customer: replied 1 year ago.
No, i took the hoses on the charcoal canister loose to blow air in them.
Expert:  Lou P. master tech replied 1 year ago.

ok did you leave the hoses loose and try to fill fuel ?

it will be tricky as we need to watch for fuel to spill out, but you should be able to test it that way.

Customer: replied 1 year ago.
Is this how you test the canister?
Expert:  Lou P. master tech replied 1 year ago.

well, it can be tricky, even though compressed air flows through it, there is not that much pressure behind the air that bleeds out when filling the tank, no different then if you ever had to empty a gas can, when there is the small hole at the back of the can it flows smooth, when there is no hole it glugs, same in the filler neck. the fuel backs up and clicks the nozzle off. make sense?

Customer: replied 1 year ago.
Kinda. So you want me to disconnect the hoses to the canister this evening?
Expert:  Lou P. master tech replied 1 year ago.

yes, and then see if you can fill fuel , just a few gallons so you know if it helped or not, again just be careful of the loose lines and make sure no raw fuel comes out. the hoses should be connected to a internal line if the filler neck that runs down to the top of the tank so fuel should not come out.

Customer: replied 1 year ago.
If I'm filling up the fuel will come out of the neck and where the canister is connected?
Expert:  Lou P. master tech replied 1 year ago.

Please restate that last one.

Customer: replied 1 year ago.
After I disconnect the hoses from the canister, then refuel, the fuel will spill out of the neck? Will it come out of the charcoal canister lines as well?
Expert:  Lou P. master tech replied 1 year ago.

ok, so we are looking to see if the fuel fills without the nozzle clicking off, there should be no fuel coming out of the canister, and we are just being cautious to watch for fuel coming out of the lines from the neck.

Customer: replied 1 year ago.
From the neck? Am i disconnecting the lines from the neck or the canister? Or from both??
Expert:  Lou P. master tech replied 1 year ago.

ok the canister is right there at the neck correct? in the wheel well?

Customer: replied 1 year ago.
No. Its opposite side of neck. Driver side wheel well.
Expert:  Lou P. master tech replied 1 year ago.

ok good, even better, so just undo the lines at the tank not the neck.

Customer: replied 1 year ago.
Which tank?
Expert:  Lou P. master tech replied 1 year ago.

oh my, why do we keep getting off the same page here.

there is no tank or canister next to the fuel neck correct?

Customer: replied 1 year ago.
There is an expansion tank next to the fuel neck.
Customer: replied 1 year ago.
am i disconnecting hoses from expansion tank or charcoal canister?? Also, am i to assume i am filling up with a gas can or should drive to the pump after i disconnect?
Expert:  Lou P. master tech replied 1 year ago.

ok, so the tank next to the filler neck, lets start there. again. are there any lines from the neck to the tank or is it one piece?

Customer: replied 1 year ago.
One piece
Expert:  Lou P. master tech replied 1 year ago.

ok, then this is going to be difficult. i dont know now man. let me think on this.

Customer: replied 1 year ago.
Well, i disconnected the canister last night and drove it to work. I got a CEL after a few miles. P0422 DTC. I will try to fuel in an hour or so.
Expert:  Lou P. master tech replied 1 year ago.

ok, the check engine light is expected, how did the tank disconnect?

Customer: replied 1 year ago.
Two air hoses.
Expert:  Lou P. master tech replied 1 year ago.

ok perfect, then try and fill fuel at the fuel station. see if the nozzle clicks off.

Customer: replied 1 year ago.
Just topped off fuel. Clicked off after a few seconds. Had to invert nozzle again to fill up.
Customer: replied 1 year ago.
Here is a pic of the filler neck and expansion tank.
Expert:  Lou P. master tech replied 1 year ago.

ok, do you see that line on the expansion tank? lets remove that one next.

Customer: replied 1 year ago.
The single narrow line near the bottom?
Expert:  Lou P. master tech replied 1 year ago.

yes sir

Customer: replied 1 year ago.
Can't open attachment as png.
Expert:  Lou P. master tech replied 1 year ago.

are you opening it on the phone or computer?

Customer: replied 1 year ago.
Got it. Now, if i remove this line, should I expect fuel to pour out?? What are we expecting to happen?
Expert:  Lou P. master tech replied 1 year ago.

fuel should not pour out, that should just be a vent line.

BUT i am not 100% so caution. sorry for my delays man, super busy as well.

Customer: replied 1 year ago.
after i disconnect it -- how do i test/troubleshoot?
Expert:  Lou P. master tech replied 1 year ago.

see if you can fill fuel again. we need to get the fuel tank to vent , the reason the nozzle is shutting off is because the air is not venting out.

Expert:  Lou P. master tech replied 1 year ago.

either that or someone shoved crud in your fuel neck and there is a clog.

Customer: replied 1 year ago.
Hmmm, do you thinking replacing the neck may be an option?
Expert:  Lou P. master tech replied 1 year ago.

i do,

Customer: replied 1 year ago.
Well, i took the hose off. The expansion tank and the filler neck appear to be one piece. I believe it is separate in other models. Is there a sensor inside the neck/ expansion tank? How does the venting work?