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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: 2 months ago.
Category: Mercedes
Expert:  Lou P. master tech replied 2 months 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 2 months 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 2 months ago.
WDBHA28E5VF509869 10/96
Customer: replied 2 months ago.
That link isn't working.
Expert:  Lou P. master tech replied 2 months ago.

here

Customer: replied 2 months ago.
No, there is a small tank there but no canister. #2024700589
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months ago.
No, i took the hoses on the charcoal canister loose to blow air in them.
Expert:  Lou P. master tech replied 2 months 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 2 months ago.
Is this how you test the canister?
Expert:  Lou P. master tech replied 2 months 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 2 months ago.
Kinda. So you want me to disconnect the hoses to the canister this evening?
Expert:  Lou P. master tech replied 2 months 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 2 months 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 2 months ago.

Please restate that last one.

Customer: replied 2 months 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 2 months 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 2 months ago.
From the neck? Am i disconnecting the lines from the neck or the canister? Or from both??
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months ago.
No. Its opposite side of neck. Driver side wheel well.
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months ago.
Which tank?
Expert:  Lou P. master tech replied 2 months 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 2 months ago.
There is an expansion tank next to the fuel neck.
Customer: replied 2 months 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 2 months 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 2 months ago.
One piece
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months 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 2 months ago.

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

Customer: replied 2 months ago.
Two air hoses.
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months ago.
Just topped off fuel. Clicked off after a few seconds. Had to invert nozzle again to fill up.
Customer: replied 2 months ago.
Here is a pic of the filler neck and expansion tank.
Expert:  Lou P. master tech replied 2 months ago.

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

Customer: replied 2 months ago.
The single narrow line near the bottom?
Expert:  Lou P. master tech replied 2 months ago.

yes sir

Customer: replied 2 months ago.
Can't open attachment as png.
Expert:  Lou P. master tech replied 2 months ago.

are you opening it on the phone or computer?

Customer: replied 2 months 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 2 months 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 2 months ago.
after i disconnect it -- how do i test/troubleshoot?
Expert:  Lou P. master tech replied 2 months 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 2 months ago.

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

Customer: replied 2 months ago.
Hmmm, do you thinking replacing the neck may be an option?
Expert:  Lou P. master tech replied 2 months ago.

i do,

Customer: replied 2 months 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?