Losing rail pressure

Slowvert135i

Corporal
Oct 24, 2017
112
43
0
Shelton, ct
Ride
08 135i e88
Ok so for months I’ve been having fueling issues with my car. Under boost the rail pressure drops to as low as 800psi..... car has st2 lpfp, brand new index 12s from fcp maybe a month old (200miles), brand new hpfp from fcp maybe 200miles aswell. Car starts fine, idles fine, and drives fine but log shows the rail pressure drops out under boost. Not sure what the issue could be now. I’m going to change the rail pressure sensor. But other wise I’m lost with ideas there’s no fuel leaking anywhere in the bay or under the car. Any one have any input?
 

KevinC39

Corporal
Jun 27, 2017
226
115
0
Ride
335i
E30 at 22psi on a big turbo...wouldn't you expect the rail pressure to crash?
 
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MoreBoost

Sergeant
Jul 27, 2017
334
135
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335i
First check if your lpfp is dropping. If it is then the hpfp and pi wont be getting enough pressure.
If lpfp looks good then it may be the hpfp unfortunately. I know you said you've replaced it but a lack of rail pressure is usually the sign.
What hpfp did you get? I replaced mine 2 weeks ago. HPFP was holding 750 psi under light throttle but dropping to 300 psi under throttle and boost.

(Note this is a JB4 log. HPFP value is multiples of 150 psi. So 2 = 300 psi. 12 = 1800 psi)
06-11-2018.jpg


After changing the pump all is good again. Now holding 1800 psi under WOT (with meth) and going as high as 2550 psi during ramp up.

graph2.jpg


There are alot of cheap copies of the lpfp. Aliexpress has new ones for as little as $150! That's probably a ticking time bomb.
I suspect there are many Chinese copies passed off as genuine.

I've seen lpfp sensors fail and when that happened the lpfp just read a fixed value but didn't impact driveability. I don't know what impact a hpfp sensor would have. It's worth replacing as they are cheap. Just see if it solves your issue.

I don't know enough about port injection to diagnose that.

Good luck.
 

Slowvert135i

Corporal
Oct 24, 2017
112
43
0
Shelton, ct
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08 135i e88
My port injection is not plugged in as I wasn’t having this issue so I left it unplugged. The hpfp is from fcp euro I’ve replaced the hpfp 5times now. Hpfp reads 900are at idle and goes up to about 3k as I get into boost then drops to 800area
 

GhassanAutomotive

Private
Platinum Vendor
Apr 8, 2017
48
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E60 M5
Upload a pull log to data zap, that'd give us most of the information needed to diag your issue. As mentioned above, make sure your LPFP is not crashing causing the HPFP to starve.
 

Slowvert135i

Corporal
Oct 24, 2017
112
43
0
Shelton, ct
Ride
08 135i e88

PFS

Corporal
Premium Vendor
Nov 19, 2018
166
207
0
Ride
335i
With a reasonable A/F target, 22 psi on a single stock hpfp on e30 feeding a 6266 is out of capacity. Not really a problem (with hardware), you're just out of fuel. This'll get better when you turn on PI (with your stage 3 installed).
 
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Jroms135

New Member
Jun 25, 2019
8
0
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2011 BMW E82 135i
Was hoping for a little help. I have a 2011 135 (n55) fuel it stg2 lpfp, hpfp is 6 months old. I have been running a custom E50 tune for about 5 months no issues. 2 weeks ago I went to the track and Im losing rail pressure between 3500-4700rpm when I hit 18psi I’d boost. The car start to “buck,stutter,fart” whatever you want to call it. If I feather the throttle I can get to 4700rpm and go WOT to redline with plenty of rail pressure. But if I go WOT before then. Rail pressure drops. Any help, ideas or suggestions would be awesome. TIA.
 

Weehe

Specialist
May 21, 2019
84
39
0
Ride
2011 335i
Was hoping for a little help. I have a 2011 135 (n55) fuel it stg2 lpfp, hpfp is 6 months old. I have been running a custom E50 tune for about 5 months no issues. 2 weeks ago I went to the track and Im losing rail pressure between 3500-4700rpm when I hit 18psi I’d boost. The car start to “buck,stutter,fart” whatever you want to call it. If I feather the throttle I can get to 4700rpm and go WOT to redline with plenty of rail pressure. But if I go WOT before then. Rail pressure drops. Any help, ideas or suggestions would be awesome. TIA.
E50 is on the ragged edge of what the stock pump can handle. I'm guessing you are over boosting and running out of fuel. Probably should start a different thread in the N55 section.
 
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