Quote:
Originally Posted by Silver-Bolt
Herr & Teague,
This is good stuff. Sounds like we can rule out the membrane failing as we would end up with fuel in the oil and likely a bunch of smoke out the tail pipe. The parts do appear as if the have been hot but gasoline will auto ignight at 495f.
I have also read about many of the low pressure intank pumps either failing of becoming noisey. Perhaps the real issue is not sufficient volume of fuel reaching the HPFP causing it to overheat. If the intank pump is not replaced, replacing the HPFP will just be a bandaid. It will continue to fail until is has a constant supply of fuel.
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I agree.
I speculated a couple of months ago (looking for thread now) that the HPFP is failing as a result of loss of pressure from the in-tank low pressure fuel pump (LPFP).
I believe BMWs split fuel tank contributes somewhat to this problem. When a car has a full tank of gas and goes around turns, goes up and down hills, stops and starts it’s not much of a problem. But as the tank level gets low the fuel “sloshes” from front to back and side to side – away from the in-tank pump fuel pickup. Getting down to a quarter tank or less really makes this worse. The engineers came up with a solution and that is to put the in-tank pump in a slosh cylinder or canister that’s open on the bottom to receive fuel, but as the tank tilts with movement some of the fuel is captured within the cylinder to prevent the pump from going dry.
This is a good design, but BMW has defeated this somewhat but using spit fuel tanks. The advantage is a more compact car design, lower center of gravity, etc. However, a low fuel problem is compounded somewhat by splitting the supply between the two sections of tank. So when you get down to a quarter tank of gas, you’re really only have one-eighth in each tank. They sort of solved it by using a remote pickup, but. . .
A low fuel supply is the primary reason in-tank electric fuel pumps fail and that’s due to a lack of fuel for lubrication, damaging the internal bearings and they overheat. You can imagine that over the lifetime of a car, in-tank full pumps experience hundreds if not thousands of low fuel situations but they are built tough to withstand it for 70- 80 thousands of miles before they fail.
Now let’s fast forward and talk about the Z4’s HPFP. We know the LPFP sends a pressurized flow of fuel to the HPFP, which in turn further boosts that pressure to a monstrous 2900 PSI (as HerrK discovered) for the direct injection system. To say the HPFP is entirely dependent on the LPFP is an understatement. Not only does it impact the fuel supply to the injectors, but IMO and more importantly, has a major impact to the life of the HPFP.
Finally seeing the HPFP guts exposed shows IMO that it’s severely over designed, and I agree with HerrK that is could be very “sensitive”. A pump of this type must have extremely close mechanical tolerances, and imagine it must maintain those tolerances over the life of the unit, nearly impossible in less than idea conditions. IMO a low fuel situation, results in LPFP cavitation and surging within the fuel line prior to the HPFP. This results in shock waves in the fuel supply line, thus starving the HPFP of much needed lubrication and the pump overheats. The little pistons and cylinder walls are scored, seals are damaged and the unit no longer meet spec to enable it to produce 2900 PSI. Whalla – long cranks and limp mode.