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Originally Posted by archwerks
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The Continuing Saga of The Achille's Heel of "The International Engine of The Year Award" Winner
My internet travels have uncovered a hypothetical cause for the frequent recurrence of HPFP problems:
It may be due to the low pressure fuel sensor in the fuel tank, particularly if the fuel tank is less than 1/4 full. The sloshing fuel is either physically damaging the components or the insufficient level of fuel to keep the components @ consistent operating temperature.
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I have a theory which may be close to that one but from a different angle. Keep in mind this problem affects a couple n54 equipped models with distinctly different fuel tank designs, but they are typically a split design ove the drive shaft which I feel contributes to a low fuel effect.
As we know the in-tank mounted primary fuel pump has a much lower failure rate than the secondary engine mounted HPFP. The secondary pump receives fuel at boosted levels in order to increase the pressure to accommodate piezo valve direct injection nozzles. Essentially, this means the secondary pump is completely dependent on the primary pump to do its job properly.
Another factor not widely understood is that fuel pumps today require the fuel itself to cool and lubricate the pump's mechanical parts. This is particularly true of the in-tank pump that is submerged in fuel, but also true of the engine mounted fuel pumps.
In my theory these two factors play a role in the HPFP failure.
- Normally, tanks have a "slosh can" cylinder around the fuel pumps to prevent sloshing away from the fuel pickup during a low fuel situation, during braking and going around turns. But the slosh can is limited during low fuel levels in the tank and can't prevent cavitation in the fuel stream being delivered from the in-tank pump to the HPFP. Cavitation essentially causes bubbles and vapor “gaps” that form shock waves in the fuel stream, but also rob the pump of much needed lubrication. Cavitation results in significantly higher friction in the pump. Excessive wear results and tolerances within the pump are compromised.
- As noted excessive heat resulting from poor lubrication and friction (No. 1, above) in the pump and lack of fuel during cavitation required to cool the HPFP causes the pump’s seals to fail prematurely.
Excessive tolerances plus failed seals result in a HPFP that can no longer produce sufficient pressures necessary for the direct injection system, i.e., long starts and limp mode. Said another way, a long start results from the HPFP's inability to produce enough pressure for the direct injection system to operate properly. Direct injection system must produce enough pressure to operate under high cylinder compression levels. I'm also sure there a fuel pressure sensor that's not letting the engine fire off if fuel pressure is too low.