Hi everyone,

I’ve been trying to track down an issue with my Mini Cooper S 2007 for quite a long time now, but I’m honestly running out of ideas. Ever since I bought the car (3 years ago), it has had the same problem: once the fuel level drops to around half a tank, the car starts to stutter, lose power, and eventually becomes almost undrivable.



Refueling is also a problem. When I try to fill up, the pump keeps clicking off repeatedly even though the tank is nowhere near full. This already made me suspect something odd with the fuel system or tank ventilation.



To verify whether the fuel level reading was actually correct, I removed the rear seat and took out the fuel pump and siphon assembly to check the tank itself. Turns out the gauge isn’t lying: the left side of the tank is still completely full, while the right side is empty. So it seems like fuel isn’t being transferred properly between both sides of the tank.



So far, I’ve replaced the fuel filter and carefully checked all the fuel lines and hoses, but nothing looks damaged, blocked, or out of place. The fuel pump also appears to have been replaced at some point in the past, so I’m not sure if that’s the root cause either.



At this point I’m a bit stuck and would really appreciate any insights. Has anyone experienced something similar or has an idea what could cause this behavior? Any help or suggestions would be great. Thanks in advance! 😄

 

Are any fault codes present?

You may want to carefully inspect the tank siphon jet pump, tank expansion line, and fuel tank breather valve for restrictions.

Below is an AI assessment of the issue:

In a 2007 Mini Cooper S, fuel is transferred between the two lobes of its saddle-style tank using a mechanical suction jet pump (siphon) that relies on the Venturi effect created by returning fuel from the engine.

The most likely causes for failure of this transfer process include:

• Clogged Suction Jet: Debris or sediment in the tank can obstruct the small nozzle of the suction jet, preventing it from pulling fuel from one side to the other. If this jet is damaged or permanently blocked, the entire fuel tank may need replacement as the jet is often integrated into internal piping.
• Weak In-Tank Fuel Pump: If the primary low-pressure fuel pump (located in the left lobe) is weak, it may not generate enough return flow to drive the Venturi siphon effectively, leading to fuel starvation even when the tank shows fuel remaining.
• Blocked or Kinked Crossover Lines: The internal lines connecting the two tank lobes can become kinked or obstructed. This often occurs during fuel filter or pump maintenance if the lines are not seated or routed correctly.
• Clogged Fuel Filter: A heavily restricted fuel filter (located in the right lobe) reduces the overall system flow, which in turn diminishes the return flow needed to operate the suction jet pump.
• Improper Seating/Seal Leaks: If the internal fuel lines were recently serviced, a failed gasket or a line that didn't "click" into place can cause the system to lose the necessary pressure to siphon fuel.
• Tank Venting Issues: A blocked fuel tank vent or charcoal canister can create pressure imbalances that hinder the natural or mechanical movement of fuel between the lobes.

Diagnosis Tip: These issues typically manifest when the fuel gauge reads roughly 1/4 to 1/2 full, but the car stalls as if it is out of gas because the fuel is trapped on the side without the primary pump.

 

Thanks for the quick reply! The only thing I haven’t checked is the tank valve. Do you maybe know where that is located? And how I can check again for clogged or kinked lines? I know there are two pipes that go from left to right. One light grey hard one and one brownish flexible pipe. The grey one just loosely clicks into place while the brown one has an actual connector. There isn’t much information or schematics that I could find about this. Thanks!