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  1. #1
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    Low pressure fuel system issues, opinions please

    Relevant Modifications: Fuel system is completely stock, running 93 pump gas with meth injection. Car is FBO on stock twins. Pink floor mat retrofit.


    Background: Got a CEL the other day while cruising around town, don't recall the code but the description was something similar to "low pressure fuel sensor, signal missing." Car did not limp and drove fine. Cleared the code, didn't come back - log LPFP pressure and see that its indicating 58-60 psi at idle(72psi is target) - good the sensor is talking... I take it for a drive, notice that the pressure stays around 50psi while driving, going WOT it stays around 50 but dips as low as 37psi a few times. I assume its just my fuel pump getting tired, I order a new oem pump/bucket/fuel sending unit, whatever the hell they call it.

    Fast forward to last night. I install the new pump assembly, fire the car up. Runs great. Plug in the cobb, still indicating 58-60psi at idle - WTF. Take it for a drive, same behavior -save for the fact that i never see the fuel pressure drop below 50psi.

    So i'm a little pissed that I wasted money on replacing my fuel pump, but now i am not sure i just want to start throwing parts at the thing. In my mind, the most logical issue is the FPR is failing to regulate things (pussy). But the fact that i did also have a logged code indicating the fuel pressure sensor had an issue makes me think it could be a sensor on its way out. FWIW, i had the low fuel pressure sensor replaced about 20k miles ago, same time they replaced my twins and HPFP under the recall.

    So, what are your thoughts? Anyone have a similar issue?
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  2. #2
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    Dude, how many horsepowers with the pink floor mats? In all seriousness, you can go the old school way and tee into the LP fuel line going into the HPFP, idle car and check what the real pressure is with a fuel gauge; its probably about $20 to build a test tee. Check connection to the current LP sensor, even try swapping it if you have another one handy.

  3. #3
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    Before you do anything else reset your adaptations via BT or INPA.

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    The stock FPR is passive...it won't/shouldn't flow fuel back to the bucket unless pressures exceed its "setpoint" and won't restrict fuel flow. That said, it does have a diaphragm but I'd be really surprised if that is your issue here. Not ruling it out...it would just be very surprising and the first one I've heard of.

    Did you reset your INPA adaptions after putting in the new pump?

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    Well I went about replacing just about the entire damn fuel system, so I feel your pain.

    Would be inclined to think it's the LPFP sensor but it almost always is accompanied by the 29F3 code. So IDK.

    Another +1 for resetting fuel adaptations. Maybe check for kinks in the line?
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    lpfp sensor might be the issue. seen many similar situations with that as the cause.
    2008 e92 335i: JB4 G5 ISO/BMS back end flash, fbo, e85, inlets, Rb turbos, level 10 valve body + converter, water/methanol.....and a lot more....

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    Its most likely the sensor. The car actually doesn't even need that sensor to read properly to run, it just needs it to be plugged in, wont even throw a CEL if its present lpfp is reading 0, but lpfp is actually fine. I know this because during my HPFP testing, I have driven and tested the car with the lpfp sensor plugged in but no fuel going to it, car showed 0 psi on the AP, but ran fine, and didn't even throw a light, before you go any further I would try a sensor.

  8. #8
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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    Its most likely the sensor. The car actually doesn't even need that sensor to read properly to run, it just needs it to be plugged in, wont even throw a CEL if its present lpfp is reading 0, but lpfp is actually fine. I know this because during my HPFP testing, I have driven and tested the car with the lpfp sensor plugged in but no fuel going to it, car showed 0 psi on the AP, but ran fine, and didn't even throw a light, before you go any further I would try a sensor.
    I am tempted to believe it is the sensor as well, and i actually have a spare one that i could throw in. I just new to find the time to pull the intake manifold off. If that doesnt fix things i will probably do as @Tzu suggested and try to plumb in a mechanical fuel pressure gauge to verify the readings. Only other thing i could think of would be the EKP module, but as sensitive as this car is to issues with electronics, i would imagine it would generate a code.

    Thanks everyone for your suggestions.
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    Click here to enlarge Originally Posted by lulz_m3 Click here to enlarge
    I am tempted to believe it is the sensor as well, and i actually have a spare one that i could throw in. I just new to find the time to pull the intake manifold off. If that doesnt fix things i will probably do as @Tzu suggested and try to plumb in a mechanical fuel pressure gauge to verify the readings. Only other thing i could think of would be the EKP module, but as sensitive as this car is to issues with electronics, i would imagine it would generate a code.

    Thanks everyone for your suggestions.
    Do not pull the intake, just pull the TB off and its right there. A lot easier.

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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    Do not pull the intake, just pull the TB off and its right there. A lot easier.
    yeah, thats a hell of a lot better. Thanks for the tip, no homo.
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  11. #11
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    A bad lpfp sensor usually reads 87 psi...that said...as suggested it is an easy swap, especially since you already have one. Will be interested to hear if it resolves your issue.

    Got logs?

    As far as the DME not caring what the lpfp sensor reads...I don't really think that's the case and believe it will vary PWM based on feedback.

  12. #12
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    Click here to enlarge Originally Posted by SteveAZ Click here to enlarge
    A bad lpfp sensor usually reads 87 psi...that said...as suggested it is an easy swap, especially since you already have one. Will be interested to hear if it resolves your issue.

    Got logs?

    As far as the DME not caring what the lpfp sensor reads...I don't really think that's the case and believe it will vary PWM based on feedback.
    Steve, I can take a video if you would like. I stop all flow to the LP sensor as I block off its feed to run other things. The car not only starts and runs perfectly I have High horsepower logs showing 0 PSI LPFP and everything else is fine, no CEL, no anything. Doesn't make sense to me either. But I have done it multiple times now.

  13. #13
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    @VargasTurboTech

    I'm not disputing what you're saying. Click here to enlarge

    My concern with this is long term, as we know the DME compensates over time for the degradation of the LPFP and other variables.

    This is often exhibited by doing an adaptions reset. I had a customer that recently did an experiment that is running my stage 2 bucket and reset his adaptions via INPA. His car typically targets 82psi but after the reset targeted 72psi. After driving the car a few times, it was back to 82psi. Is this based on the LPFP sensor input or current draw????...not totally sure at this point.

    I'm also wondering if that reading doesn't have a little bit to do with the hpfp flow control valves pwm input...not totally sure...all hypothesis at this point.

    At the very least I wouldn't think the car will be too happy with it, because in the event of an HPFP failure, it's supposed to see and maintain that 72 psi to supply the injectors and get you home.

    I also know that I can really jack up how my car is running by playing with the LPFP input pressures. Of course most don't have the ability to do this and should theoretically be seeing 72psi no matter what if everything else is functioning properly.

    Here's a couple images of the fail LPFP sensor.

    Click here to enlarge

    Click here to enlarge
    Last edited by SteveAZ; 05-12-2014 at 10:25 PM. Reason: Added images

  14. #14
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    Its not something I do all the time. It just ended up that way a couple times because I wanted to test something and didn't feel like pulling things off to move the feeds around. The first time I did it I expected the car to limp right away. I was surprised it let things go that way, I agree its prob not the safest which is why I only did it when I needed to do a quick pull and test something.

  15. #15
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    @SteveAZ I believe i read somewhere that when the DME believes the LPFP sensor has gone bad it runs the LPFP pump at full throttle as a "fail safe."
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    Click here to enlarge Originally Posted by lulz_m3 Click here to enlarge
    @SteveAZ I believe i read somewhere that when the DME believes the LPFP sensor has gone bad it runs the LPFP pump at full throttle as a "fail safe."
    @lulz_m3

    My inclination is that you are correct although I have no first hand experience with an LPFP sensor failure. I do know there are a few conditions that will cause the LPFP to go 100% duty cycle though.

    Personally if I suspected this were your issue I would throw a meter on the LPFP at idle and see what the voltage is. At idle your PWM should be around 50% duty cycle and you should see approx. 6.5 volts in my experience. If it were running 100% DC I'd expect you'd see much higher pressures and it would read around 13.5 volts.

    The fact that you aren't dipping below 50psi is a good sign. I wouldn't necessarily get to caught up that you're not seeing the requested 72psi, especially if you're not getting any errors. Can you post some logs?

    Right now with my configuration, I'm running at about 63psi at idle and 65psi at cruise and the DME is perfectly happy to do so but I'm flowing more fuel and it doesn't get stressed on WOT runs.

    If I can help and you'd like me to perform a couple experiments, I'm happy to do so.

    Right now, my car obviously isn't completely stock, but much of it is and I have the ability to return it to virtually stock or at least simulate it. Even have a new drivers side assembly in route so that I can return that to stock as well for the up and coming stage 3 testing.

    Lemme know.

  17. #17
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    @SteveAZ, thanks man, i appreciate the help! This log was taken before I replaced the LPFP:
    http://www.datazap.me/u/lulzm3/low-p...9&zoom=234-302

    One thing that sticks out to me is that the HPFP is overshooting target quite a bit, which makes me think its possible that the LP fuel system pressure could be higher than reported by the Low pressure sensor, assuming (and that's a big assumption) the DME even references it...

    Here is another log the day after i replaced the LPFP:
    http://www.datazap.me/u/lulzm3/low-p...19&zoom=15-120

    I should have added LT & ST fuel trims but didn't think about it at the time i grabbed these. I remember glancing at them and not seeing anything out of the normal. I did have a P2A2D (Fuel low pressure system, fuel pressure low) code stored the other day, but no CEL or limp. I left the AP on live view for my drive into work to see the high and low pressures being monitored and saw it dip as low as 29psi, so something is up.

    I think i'm going to throw the new LP sensor in tonight, and if that doesn't resolve it i will start checking the voltage coming from the EKP. Do you happen to know which wires I need to check? Thanks for your help Steve.
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    I saw the 29psi in your pre-log...it corresponded with you letting off the throttle and was a momentary dip, that's not uncommon but is pretty low. Ultimately what we care about is that the HPFP (rail) is maintaining pressures and that doesn't seem to have been an issue. It is common for it to overshoot target pressures on WOT though...again not a major concern.

    It'll be interesting to see if the new LPFP sensor has any impact. I've had the DME target similar pressures...but it's usually because I'm doing some sort of experiment and manipulating it. However, the fact that you don't see 70+ psi on WOT when PWM should be 100%...that's a bit odd and has me a little bit concerned.

    Do the LPFP sensor and see what it gets you. Here is a link to the HPFP DIY I did, but it very clearly shows the LPFP sensor too and the steps to get to it.

    https://www.dropbox.com/s/hikw49qbf2...HPFP%20DIY.pdf

    If that doesn't get you anywhere, we'll talk about all the possible causes and you can decide how you want to proceed.

    As for measuring the voltage, I've always done it right at the LPFP...but again, my situation is "special"....Click here to enlarge

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    @SteveAZ, Just threw the new sensor in... appears to have resolved the issue. Now its registering 84-85 psi, so it looks like i'll need to load up INPA and do some adaptation resets. Here is a log: http://www.datazap.me/u/lulzm3/low-p...zoom=2597-2774

    I'm curious if resetting adaptations would have done anything to remedy the issue with it reading low, but I don't care to put the old sensor back in lol.

    Thanks to Tony @VargasTurboTech for the tip on replacing the sensor by only removing the TB, much easier than what i was planning to do. It was still sort of a pain in the ass because the electric junction box is blocking access to the sensor, but i ended up bending the bracket up which allowed me enough room to replace it. Click here to enlarge
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    Click here to enlarge

    Wanna send me that sensor?...I'll pay shipping and I'll test it Click here to enlarge

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    Click here to enlarge Originally Posted by SteveAZ Click here to enlarge
    Click here to enlarge

    Wanna send me that sensor?...I'll pay shipping and I'll test it Click here to enlarge
    Sure thing, i was planning on selling you my old bucket assembly for a core anyways, ill just toss it in the box. PM heading your way.
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    Click here to enlarge I'd rep ya if I could, but apparently I've already done that too many times Click here to enlarge

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