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BMW 3-Series and 4-Series Forum (F30 / F32) | F30POST > Technical Forums > N55 Turbo Engine / Drivetrain / Exhaust Modifications > 119508 Rail Pressure Sensor Plausibility: Pressure Too Low Before Engine Startup
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      03-04-2020, 02:52 PM   #1
rickyf10
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119508 Rail Pressure Sensor Plausibility: Pressure Too Low Before Engine Startup

i know it's the wrong forum, my motor is s63tu on f10 m5, but there are not many people active on m5post so i decide to ask for some advice here

So few days ago in the morning, i started my car but CEL poped up, "drivetrain malfunction" poped up on screen and engine died soon after rev tried to stablize around 800 during cold start. Then i scanned the code and got:

118011 Mixture control, Across bank mixture too Rich. (this one doesnt trigger CEL)
119508 Rail Pressure Sensor Plausibility: Pressure Too Low Before Engine Startup

I flashed back to stock tune (was running bm3 stage 1), and tried to start again. The rev fluctuated around 800 again but car didnt die (along with kinda rough idle). After driving off, the car just ran fine.

So today I kept eye on rail pressure and notice that rail pressure was very low (around 100 psi) before i started it up, and after I started engine, it rise to 1800 psi (is it normally that high????), along with rough idle as last time.

note: all 8 injectors were replaced half a year ago due to leaky injector

anyone has idea on this problem?
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      03-04-2020, 03:48 PM   #2
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I've had a similar code when my HPFP was starting to fail to keep up with my tune. Even happened while idling for an hour, tried to drive off and the drivetrain message came up but was able to restart the car immediately after.

So check your HPFP of maybe the LPFP.
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      03-04-2020, 03:51 PM   #3
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well first thing, reset the adaptions. Make sure you dont have any weird leaks in the intake side of things or anything that could cause bad sensor values.. next id change the upstream 02 sesnors.

Thats what I would do. Im also wondering if you have a leak somewhere or a pump thats dying and rail pressure is dropping until it builds back up. Does it have trouble starting on the first try?
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      03-04-2020, 07:45 PM   #4
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Quote:
Originally Posted by Munchi435i View Post
I've had a similar code when my HPFP was starting to fail to keep up with my tune. Even happened while idling for an hour, tried to drive off and the drivetrain message came up but was able to restart the car immediately after.

So check your HPFP of maybe the LPFP.
HPFP, yep, this is what I suspect too. Just not sure how to test HPFP too see if they are actually faulty.
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      03-04-2020, 07:49 PM   #5
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Quote:
Originally Posted by Phantom10R View Post
well first thing, reset the adaptions. Make sure you dont have any weird leaks in the intake side of things or anything that could cause bad sensor values.. next id change the upstream 02 sesnors.

Thats what I would do. Im also wondering if you have a leak somewhere or a pump thats dying and rail pressure is dropping until it builds back up. Does it have trouble starting on the first try?
Yep! I'll be clearing adaptations today to see if it helps. i dont see any leak in the engine bay tho. My mechanic suspects O2 sensors are the cause (since im running catless downpipe) and he's gonna put on O2 bypass today. I'll be updating this later
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      03-04-2020, 07:55 PM   #6
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Quote:
Originally Posted by rickyf10 View Post
Yep! I'll be clearing adaptations today to see if it helps. i dont see any leak in the engine bay tho. My mechanic suspects O2 sensors are the cause (since im running catless downpipe) and he's gonna put on O2 bypass today. I'll be updating this later
I'd say try the 02's first since that would be cheaper than HPFP. I'm not sure how to test the HPFP but systemically start from the least expensive points would be my suggestion.
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      03-04-2020, 08:48 PM   #7
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Quote:
Originally Posted by rickyf10 View Post
Quote:
Originally Posted by Munchi435i View Post
I've had a similar code when my HPFP was starting to fail to keep up with my tune. Even happened while idling for an hour, tried to drive off and the drivetrain message came up but was able to restart the car immediately after.

So check your HPFP of maybe the LPFP.
HPFP, yep, this is what I suspect too. Just not sure how to test HPFP too see if they are actually faulty.
if you have any tuning software like mhd or bm3 you can take logs. and I think you can pull up the info real time using a BT obd2 dongle and the torque app, just not sure if you can log and save the data
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      03-04-2020, 09:06 PM   #8
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Quote:
Originally Posted by DreamerJoe View Post
if you have any tuning software like mhd or bm3 you can take logs. and I think you can pull up the info real time using a BT obd2 dongle and the torque app, just not sure if you can log and save the data
i do have datalog recorded by bm3, it's a cold start up.
http://www.bootmod3.net/log?id=5e5dae3dd10b4369882e10bf

i have consulted bm3 support and they say afr seems way off but they can't tell which part is mechanically wrong

Last edited by rickyf10; 03-04-2020 at 09:11 PM..
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      03-04-2020, 09:22 PM   #9
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Quote:
Originally Posted by rickyf10 View Post
i do have datalog recorded by bm3, it's a cold start up.
http://www.bootmod3.net/log?id=5e5dae3dd10b4369882e10bf

i have consulted bm3 support and they say afr seems way off but they can't tell which part is mechanically wrong
Need a proper log. You need to do the log in 3rd if 6MT or 4th if 8AT, start at about 2500 RPM, floor it (dont roll into it) and log until about 6500 rpm and stop the log. Get 2 or 3 logs like that. Also, make sure DSC is fully off and if auto transmission, make sure you are in manual shift mode not just sport shift.
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      03-04-2020, 11:28 PM   #10
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Quote:
Originally Posted by Munchi435i View Post
Need a proper log. You need to do the log in 3rd if 6MT or 4th if 8AT, start at about 2500 RPM, floor it (dont roll into it) and log until about 6500 rpm and stop the log. Get 2 or 3 logs like that. Also, make sure DSC is fully off and if auto transmission, make sure you are in manual shift mode not just sport shift.
DSC fully off????? Iím 7dct btw.
Iím now installing o2 bypass and reset mixture adaptation later to see how it will be going
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      03-05-2020, 10:25 AM   #11
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do a full throttle log from 3rd gear to as high as you can go.

make sure traction control is off.
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      03-05-2020, 10:50 AM   #12
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Quote:
Originally Posted by rickyf10 View Post
Quote:
Originally Posted by Munchi435i View Post
Need a proper log. You need to do the log in 3rd if 6MT or 4th if 8AT, start at about 2500 RPM, floor it (dont roll into it) and log until about 6500 rpm and stop the log. Get 2 or 3 logs like that. Also, make sure DSC is fully off and if auto transmission, make sure you are in manual shift mode not just sport shift.
DSC fully off????? I’m 7dct btw.
I’m now installing o2 bypass and reset mixture adaptation later to see how it will be going
put the car in sport +, then hold the DSC button for 5 seconds. you should then get confirmation on your iDrive that it's off.
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      03-05-2020, 02:43 PM   #13
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Definitely gonna do a log but here in Vancouver is rainy... Iím gonna do it when sunny day comes
So here is a little update. 2 o2 bypasses are in place now. Startup this morning seems smooth but still a little fluctuations (within 770-830ish) when rev stablizes around 800. Before startup, I notice that my rail pressure is the same as one in low pressure system (88.4 psi). Is it normal??
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      03-05-2020, 03:24 PM   #14
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Quote:
Originally Posted by rickyf10 View Post
Definitely gonna do a log but here in Vancouver is rainy... Iím gonna do it when sunny day comes
So here is a little update. 2 o2 bypasses are in place now. Startup this morning seems smooth but still a little fluctuations (within 770-830ish) when rev stablizes around 800. Before startup, I notice that my rail pressure is the same as one in low pressure system (88.4 psi). Is it normal??
Might be an odd question but how old is your battery?
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      03-05-2020, 03:42 PM   #15
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Quote:
Originally Posted by Munchi435i View Post
Might be an odd question but how old is your battery?
Itís replaced a year ago
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      03-28-2020, 12:51 AM   #16
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so here is a little update on this. it turned out to be a faulty postcat o2 sensor.
the bank2 post cat o2 sensor sometimes loses voltage significantly (a little question on this: post cat o2 sensor is supposed to be monitor sensor but how come it affects air-fuel ratio ) my mechanic buddy swapped o2 sensors between bank1 and bank2 and the rich fuel mixture problem on bank2 is gone
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Thanks everyone here for giving advice!

Last edited by rickyf10; 03-28-2020 at 01:32 AM..
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      03-28-2020, 06:20 AM   #17
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If you have BM3, is there an option to disable/ignore the O2 sensor that's post cat?
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      03-28-2020, 09:34 PM   #18
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Quote:
Originally Posted by Polo08816 View Post
If you have BM3, is there an option to disable/ignore the O2 sensor that's post cat?
I believe you mean ďCEL deleteĒ, the one option that removes cat efficiency code from DME. Yes I do have that option for my catless downpipe
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