ST N54 Low rail pressure

tlinford

New Member
May 2, 2022
6
1
0
Took my car out of winter storage after installing a 535lph LPFP in it and started getting a new tune from Doc Vu. I'm seeing big drops in rail pressure. My tuner thinks its the HPFP and so do I but since I just installed the new LPFP I was wondering if there's anything I should check before I drop $1400CAD on a new HPFP. The car doesn't seem to have any issues other than the lack of rail pressure at WOT on the log and cutting spark when it drops. MHD with JB4. Low pressure is fairly steady just below 70psi but does drop to 60psi when I get near peak boost. High pressure starts at 7, peaks at 15 before dropping to 5 around peak boost. Recommendations for diagnosis?

Details: Gtw3684r, on 18psi atm and no port injection.

Log
 
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Torgus

Colonel
Nov 6, 2016
2,284
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Boston
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ACF 6466 E92 + METH
Took my car out of winter storage after installing a 535lph LPFP in it and started getting a new tune from Doc Vu. I'm seeing big drops in rail pressure. My tuner thinks its the HPFP and so do I but since I just installed the new LPFP I was wondering if there's anything I should check before I drop $1400CAD on a new HPFP. The car doesn't seem to have any issues other than the lack of rail pressure at WOT on the log and cutting spark when it drops. MHD with JB4. Low pressure is fairly steady just below 70psi but does drop to 60psi when I get near peak boost. High pressure starts at 7, peaks at 15 before dropping to 5 around peak boost. Recommendations for diagnosis?

Out of curiosity what address did you send him the money? Still going to the stripper's account so he doesn't have to pay taxes?

and yes please post the log.
 

Maroon_e90

Specialist
Sep 30, 2021
57
11
0
23
Northwest Indiana
Ride
2009 335i - xdrive 6MT
Took my car out of winter storage after installing a 535lph LPFP in it and started getting a new tune from Doc Vu. I'm seeing big drops in rail pressure. My tuner thinks its the HPFP and so do I but since I just installed the new LPFP I was wondering if there's anything I should check before I drop $1400CAD on a new HPFP. The car doesn't seem to have any issues other than the lack of rail pressure at WOT on the log and cutting spark when it drops. MHD with JB4. Low pressure is fairly steady just below 70psi but does drop to 60psi when I get near peak boost. High pressure starts at 7, peaks at 15 before dropping to 5 around peak boost. Recommendations for diagnosis?
Yeah gonna need more information. Logs preferably, What boost are you running, what turbo and any port?
 

RuskiRacer

Captain
Vendor
Jul 17, 2019
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Out of curiosity what address did you send him the money? Still going to the stripper's account so he doesn't have to pay taxes?

and yes please post the log.
Duhhh its going to misty lol. Im not sure what the issue with being a tuner and being on Instagram is? If all Instagram tuners sucks throw ken wedge and motiv in the same boat there on insta as well
 
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Torgus

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Nov 6, 2016
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Duhhh its going to misty lol. Im not sure what the issue with being a tuner and being on Instagram is? If all Instagram tuners sucks throw ken wedge and motiv in the same boat there on insta as well

I agree to a degree. I just trust companies more that have a website and a legit business in their name. Not some stripper's paypal so they don't have to pay taxes. I'm probably showing my age.
 
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RuskiRacer

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Jul 17, 2019
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I agree to a degree. I just trust companies more that have a website and a legit business in their name. Not some stripper's paypal so they don't have to pay taxes. I'm probably showing my age.
I hear ya there. I don't have much of an issue with not wanting to pay taxes. Some of the largest fortune 500 companies pay little to no taxes because they somehow manage to write everything off. And not like the United states makes efficient use of our tax dollars anyways. Vu tunes my darton sleeved block and I haven't had any issues with his tunes personally and he responds pretty quickly. Also if your having a huge shit show he'll call you and try and talk you through the issues on the phone which no other tuner seems to be very keen on doing. I agree vu should legitimize himself more and make a business page.
 
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wheela

Sergeant
Jun 4, 2021
407
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2015 e84 X1 35i Msport
I hear ya there. I don't have much of an issue with not wanting to pay taxes. Some of the largest fortune 500 companies pay little to no taxes because they somehow manage to write everything off. And not like the United states makes efficient use of our tax dollars anyways. Vu tunes my darton sleeved block and I haven't had any issues with his tunes personally and he responds pretty quickly. Also if your having a huge shit show he'll call you and try and talk you through the issues on the phone which no other tuner seems to be very keen on doing. I agree vu should legitimize himself more and make a business page.
Yeah, I really shouldn't poke fun as I have no reason to doubt his ability, and to the contrary there seems to be people putting down huge numbers with his tunes. I just can't forget that octagon thread where new users started appearing out of thin air to come to his defense, the whole thing was comical.
 
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tlinford

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May 2, 2022
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I added a log. As far as the conversation about my choice in tuner goes, we will see. Really couldn't care less if he's putting my money directly into a strippers waistband, as long as he writes a decent tune. So far he's been really responsive and that's about all I can say.
 
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wheela

Sergeant
Jun 4, 2021
407
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Twin Cities, MN
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I added a log. As far as the conversation about my choice in tuner goes, we will see. Really couldn't care less if he's putting my money directly into a strippers waistband, as long as he writes a decent tune. So far he's been really responsive and that's about all I can say.
That's great to hear. It seems like I've seen a lot of posts from different people complaining about unresponsive tuners, or just not being able to even get a tune. Or if they do, the tunes seem kind of half assed. If you've found somebody that's responsive, and takes pride in optimizing your tune, that's a good find.
 
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RuskiRacer

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I added a log. As far as the conversation about my choice in tuner goes, we will see. Really couldn't care less if he's putting my money directly into a strippers waistband, as long as he writes a decent tune. So far he's been really responsive and that's about all I can say.
I mean hpfp failures on n54s are super common so he may not be wrong. Did you add the log into this thread ?
 

Maroon_e90

Specialist
Sep 30, 2021
57
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23
Northwest Indiana
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2009 335i - xdrive 6MT
Based off what I’m seeing the lpfp is getting on the lower end that I’d be comfortable with and you could solve this problem marginally with adding more LPFP, but almost immediately after that you’re going to run out of HPFP. The 535 on e40 prolly tops out at 575 WHP while the stock HPFP may be lucky to see 475WHP. I would agree with the tuner that you need to upgrade by adding port or add more HPFP.


You could check your FPR but I seriously doubt that would be an issue.

*Edit: I want to clarify you probably dont need a new HPFP since it appears to be working properly. I would say you need MORE fuel with an overdrive kit as the upgrade for HPFP
 

RuskiRacer

Captain
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Jul 17, 2019
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Based off what I’m seeing the lpfp is getting on the lower end that I’d be comfortable with and you could solve this problem marginally with adding more LPFP, but almost immediately after that you’re going to run out of HPFP. The 535 on e40 prolly tops out at 575 WHP while the stock HPFP may be lucky to see 475WHP. I would agree with the tuner that you need to upgrade by adding port or add more HPFP.


You could check your FPR but I seriously doubt that would be an issue.

*Edit: I want to clarify you probably dont need a new HPFP since it appears to be working properly. I would say you need MORE fuel with an overdrive kit as the upgrade for HPFP
The issue is that guys think that can make high 500s sometimes 600hp and think they can get away with a single pump. Sure maybe if you have a dialed in aftermarket fpr and a conservative tune you can. But your usually gonna see fluctuations, not sure why people don't just run dual 450s and a hobbs switch or use jb4 to activate it. In my opinion the only way to get steady pressure with no odd drops or fluctuations.
 

Torgus

Colonel
Nov 6, 2016
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The issue is that guys think that can make high 500s sometimes 600hp and think they can get away with a single pump. Sure maybe if you have a dialed in aftermarket fpr and a conservative tune you can. But your usually gonna see fluctuations, not sure why people don't just run dual 450s and a hobbs switch or use jb4 to activate it. In my opinion the only way to get steady pressure with no odd drops or fluctuations.

This.
 

Msoden82

Lurker
Sep 2, 2021
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5
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I agree with above, definitely add another 450 and maybe an overdrive kit. Before replacing the HPFP I’d replace the LPFP sensor to see if that helps the rail pressure.
 

tlinford

New Member
May 2, 2022
6
1
0
Based off what I’m seeing the lpfp is getting on the lower end that I’d be comfortable with and you could solve this problem marginally with adding more LPFP, but almost immediately after that you’re going to run out of HPFP. The 535 on e40 prolly tops out at 575 WHP while the stock HPFP may be lucky to see 475WHP. I would agree with the tuner that you need to upgrade by adding port or add more HPFP.


You could check your FPR but I seriously doubt that would be an issue.

*Edit: I want to clarify you probably dont need a new HPFP since it appears to be working properly. I would say you need MORE fuel with an overdrive kit as the upgrade for HPFP
E85 is not available in my area and I don't particularly want to run meth so there's really not much point in port. ~500whp area is plenty for a street car imo anyways. Car just runs good ole Canadian 91 with octane booster.
 

tlinford

New Member
May 2, 2022
6
1
0
I agree with above, definitely add another 450 and maybe an overdrive kit. Before replacing the HPFP I’d replace the LPFP sensor to see if that helps the rail pressure.
I don't see the point in adding more LPFP since I don't run port or ethanol.