Quote:
Originally Posted by jeremyc74
The low pressure sensor MONITORS the feed pressure, which is held steady at 5 bar. The low pressure pump is inside the fuel tank, and it's the same thing that's used on the 128i. It's a typical fuel pump that's not variable pressure.
The ECU determines what the pressure should be on the HIGH side of the HPFP, and then adjusts it based on the reading from the HIGH PRESSURE sensor.
You can't control the outlet pressure of a variable PD pump based on it's inlet pressure. The inlet side is a constant 5 bar, and the low pressure sensor is just making sure it's where it's supposed to be.
Again, I can't come up with any logical reason the low pressure sensor would have anything to do with this. It's just there to make sure there's proper pressure feeding the HPFP, not to control anything.
Edit:
Ok, I'm taking another look at this, and now I'm questioning whether or not the low pressure pump is variable speed or not. There's another diagram in the tech document that shows some of sensors, and they're showing a transistor where the electric pump is. They also mention that if the low pressure sensor fails the electic pump will continue to deliever fuel at 100%, which suggests that it's capable of something less than that.
IF that's the case, and the LP sensor were failing in such a way that caused the pump to run at a much slower speed than it was supposed to maybe the HPFP is being starved, which is leading to the repeated failures after a short period of time.
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Yes, That's what I was suggesting with this previous post:
"If the low pressure has something to to with overall high pressure, then it could be part of the problem.
I would guess if the low pressure sensor is causing the HPFP to be under supplied with fuel, that would cause a problem in that the HPFP would be working harder to keep fuel pressure up with an insufficient supply."
It's a possibility.
Hopefully, we'll get more answers soon from those doing the tests.