Fuel line disconnects from pump when tank is under half full


Hello, I have a difficult problem with my BMW X3 from 2004. The fuel pump has been replaced twice in the last year, and it has been in the shop 4 times also, because the engine loses fuel pressure. The mechanic told me that he thinks he has found the pattern of the problem - that the fuel pump in the tank works fine, so long as it is fully submerged. The moment that the tank is low enough on gasoline, and the fuel pump is exposed, then it is vulnerable to having the fuel line separate from the pump. The mechanic has never seen this before. He has called ACDelco for advice oh, and they have not been able to say anything useful. Are there any laws of physics that might apply here? Other than Murphy’s Law of course? I really appreciate your advice for what is otherwise a good car.

The physics don’t support the mechanics reasoning. If the pumps bouyancy keeps the fuel line connected, pressure would blow it right off. So no.

There is something else going on. What, I do not have enough information to give you a good guess.

Maybe you need a different mechanic.

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My vote is with Mustangman of finding another mechanic. I do not comprehend his reasoning at all and if he is the same guy who has replaced the pump twice + the 4 visits then another mechanic is a must.

Just a quick couple of questions. Has the car quit on the road and had to be towed and is this line disconnection external to the fuel tank and if so; small or larger line?

Do you know exactly which pipe disconnect’s?, they can suffer from some strange in tank problems when the charcoal canister starts to block. Being a plastic tank the fuel pump vacuums the tank shut due to no air being drawn back in via the canister, I’ve worked for bmw for years and the symptoms can range from split pipe too completely crushed pumps.


Like mentioned directly above, possibly the tank is getting pressurized (either positive or negative pressure) due to some fault or blockage with the evap system.

Might have them check the vent valve solenoid to see if it’s failed closed.

Yes, the car has quit on the road and needed a tow. Thank you for answering

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I will defer to Darron here since he’s the BMW guy. I’ve worked on BMWs as they came into the shop but I’m far from an expert on them and have never seen this problem. Did see something similar on a SAAB once and it was a return line problem.

I tend to agree with a tank venting issue though. VW had a problem years ago with something similar but what would happen is that the fuel tanks would get sucked flat.
People would run out of gas with the gauge showing full and it would only take 2 gallons to fill the tank. They would then proceed to run out of gas again when that 2 gallons was consumed. It was a tank venting issue.

The only other options I can think of is that there is hose fitment issue since you mentioned an AC Delco pump which generally a GM brand. My assumption is that the pump fittings are the same size and there is no issue with a loose fit or retention problem.

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I concur w/the idea that the vapor pressure (above the fuel) inside the tank may be involved. The vehicle’s evaporative emissions system is what controls that function. The evap system prevents gas fumes from getting into the air, which would cause air pollution. The way it works can be pretty complicated. But basically when the gas tank is being filled the evap system is supposed to open & close various valves to send the fumes into the charcoal canister, to be stored there for later use. As the vehicle is driven other valves open & close to route the fumes from a canister into the engine. In some vehicles there’s also an air pump that periodically pressurizes the system as a test of the evap system’s ability to hold the fumes. Given the problem seems to only occur when the fuel pump is exposed to the evap space, first step, check for evap diagnostic codes.

If there are no codes and no obvious problems, ask your shop if they are able to safely disable the evap system , not as a fix, but as a test. If the problem goes away, then you’ll know for sure that the evap system is the culprit.

Thank you all for your ideas. I shared them with the shop and will let you know how it goes. They’ve had the car for a long time this time, so maybe this will help them finish soon.

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Sorry I forgot to post the solution. I thought they used a genuine BMW fuel pump, but they used a Delco, and the measurement was a little off for the return hose. They owned up to it, and replaced it with a BMW fuel pump, and it worked just fine after that.