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      10-14-2015, 08:18 PM   #1
capcadet05
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HPFP vs. LPFP explained

This may sound like a stupid question but after some extensive searching I can't find the answer to this simple question. What is the purpose of the HPFP and the LPFP? What functions do they perform and how to they interact with the car? There seems to be plenty of explanation of n54 turbo topics but none about the HPFP (other than the fact that it breaks all the time 90% of posts out there). Also, when you start the car which pump is it using to prime the engine? I assume the HPFP because of the posts about signs of impending HPFP failure. But couldnt this also be the low pressure pump? Any light on this topic would be helpful!

John
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      10-15-2015, 10:37 AM   #2
Snertz
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LPFP sits in the tank and feeds the HPFP a constant 72psi; HPFP is mounted on the engine and is chain driven. HPFP takes this input and bumps it to 700-2500ish psi depending on load. LPFP is what primes when you first start the car, that's the whirring noise that can be heard when unlocking the car.

If the HPFP dies, the car goes into limp mode but can still run off the LPFP. If the LPFP dies, the car won't run at all.

Some more in-depth reading is available here, check out the engine management PDF : http://www.e90post.com/forums/showthread.php?t=38856
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Last edited by Snertz; 10-15-2015 at 10:43 AM..
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      10-16-2015, 12:30 AM   #3
capcadet05
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Diagnosis.....HPFP or LPFP

Thanks for that excellent response! I read that link previously and there is a lot of good stuff there! I am trying to diagnose potential start up problem of intermittent long cranks after replacing the plugs/coils/oil and cleaning carbon build up. There were misfire codes on a few of the cylinders before replacing the plugs, have not checked since then. I'm pretty sure its not the fuel injectors because that would mean all of them would be bad since the car is all together not cranking. Hence suspecting LPFP or HPFP. I can here the LPFP pressurize as you described in your post, so this make me wonder if its the HPFP (which has been a reported symptom). Does the OBDII give your real time diagnostic of LPFP and HPFP fuel px during a start? I would imagine that would be a little too much to ask for lol. 2010 135i E88 by the way!

Much appreciated.

John
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      10-25-2015, 10:50 AM   #4
flinchy
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ALso.. LPFP = low pressure fuel pump

HPFP = High pressure fuel pump

Low>High>Fuel rail.

your issue will be HPFP 99/100 times, not LPFP
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