[Dappa47]

Originally Posted by kenlena46

I need some help here and need to pick all your brains. My 2011 535i N55 had a check engine light come on so I took it to the mechanic to see what was going on. He said the coolant temperature sensor isn’t working correctly and he thinks I need a new sensor or it has some sort of electrical problem/short going on. He also said the car is giving a code for the low fuel pressure sensor so he advised me to take it to the BMW dealer to have them go through it because he thought it had to do with an electrical problem.

I took it into the dealership and they said the coolant temperature sensor was bad and needed to be replaced and the low fuel pressure sensor was bad and needed to be replaced as well. Because they wanted to charge me over $800 to do the job I decided it all looked fairly easy and I could do it myself. I got the parts from BMW and went to do the work. I noticed as I was looking for the low fuel pressure sensor that it was nowhere to be found. I did some research and found out that my vehicle wasn’t produced with a low fuel pressure sensor (N55 motor). So I went back to BMW and told them what I found and asked for them to find the reason my vehicle was putting out this code if the vehicle was made without that sensor. They ended up telling me that they have no clue why it does not have the low fuel pressure sensor and that they think I had my motor swapped at some point and during that process they deleted the sensor which is all BS! I asked them to provide me with pictures of the different vin number they found on the engine and they wouldn’t provide me with them.

So their solution to the problem was we can take the vehicle in on trade and give you a great deal on a new BMW and that’s all they could do.

I’m just looking to see if anyone else has experienced this whole no low fuel pressure sensor thing???

Hi there , I have the same issue you are having, did you ever find out where and if that model N55 has a low pressure fuel sensor.

My scanner shows a static 100 psi , which from my understanding isn't correct , it should be lower within the range of 50 to 72 psi .

Because the value is static, there is now way to determine if the LPFP is working properly unless you place a pressure gauge on it.

I have been trying to figure this out for the last 4 weeks, the truck runs but hesitates intermittently .


The HPFP when I logged my driving data shows that the pump PSi falls below 700 Psi at times , but I am not sure if that is due to a faulty LPFP , a failing one.

I replaced the HPFP when first got the truck, fuel injectors, spark plugs, coil packs , but I get these low key misfires when monitoring live data.

The HPFP is aftermarket and has been working fine as far as I can see, I am not sure if the problem lays with the LPFP , because I don't know what that lp fuel value should actually be.

If it's just some number made up by the DME, that doesn't help with diagnoses because you don't know what it actually needs to be , as a normal measurable value that you can compare ; to determine if the pump is weak or failing.

The car shows absolutely no codes , no check engine light , nada. So I don't even have a beacon to point me in th eright direction.


I have been scratching my head with this for a while , no one seems to be able to answer the question how is this DME on this N55 able to calculate the lPFP pressure in the line before the HPFP.

I called several dealerships and spoke to the parts guys and I got all different answers, either they don't know , bring it in for service or there telling me the sensor is in the tank ; which I haven't seen anything that suggest that and even if it is, none of them could tell me where it is or what it's attached too.

Most of them don't know anything, they simple look up and sell parts to you , one guy told me that the steel line where the sensor is supposed to be is the actually sensor, it's integrated into the line, I laughed hard!

I said so "where would the harness connection be sir , where is the computer measuring this value from", I said I am not a mechanic by trade but I studied electrical engineering, I know my way around a car well enough; and I told him that could never be.

"There's only one visible fuel rail sensor and it's after the HPFP on the rail before the injectors, that could only be measuring high fuel pressure so where is the low side harness connection".

"The computer uses that as it's measurement to actuate the solenoid in the HPFP to lower the pressure so it doesn't get too high , the pressure on that model pump is driven by the rotation of crank via a chain".

"That's how it builds pressure and that pressure is controlled by the DME via a solenoid and the High pressure fuel rail sensor".



He went silent and tried to get me off the phone.


Granted he may not have been the right person to ask but I would think these dudes would know something.

Any updates anyone has on this issue would be greatly appreciated , for I don't want to throw parts at it again.

[ZeBoost]

Hey there, not sure if you read through all of the posts but there is some good info there. Anyhow, I am not OP, but I ended up with OP's car. 2011 535i with PWG, currently clocking in at about 180k. The car has been running good enough and I drive it so little (50-100 miles a week) that I decided I was done chasing this issue.

There are a few bits of info I know from posts here, conversations with techs and other BMW enthusiasts, as well as my own research.

There are indeed early N55 engines that did not have a LP sensor. The LP pump, as best I have been able to determine, runs constantly and should average about 72 PSI. It will drop somewhat during hard acceleration or when the car is off/first started, but it's trying to run a constant 72. I have connected a manual gauge to the fuel line to test.

The HPFP should run about 700-800 PSI at idle, 1300-1500 under throttle and see occasional blips to 1800-2000 PSI under hard acceleration. I replaced my HPFP with a BMW OE part from FCP Euro, at the time I paid about 1k for the kit. My HPFP readings are inline with the examples I listed. Mine did throw an occasional code for low pressure in the high pressure system, also, I had long starts before the HPFP swap.

I don't have the code in front of me but mine had a code for low pressure fuel sensor shorted to ground, I believe. I chased it for quite a while, and threw lots of parts at it. Some to chase the issue, some to stay on top of maintenance items. Coils, plugs, injectors, new battery, valve cover, ISTA to read data and reset adaptations, smoke tests, pressure tests, etc. The car ran fairly well, but did have a misfire at idle and what felt like boost breakup under throttle in certain conditions (medium acceleration, holding throttle steady).

I decided to replace some of the turbo piping, charge pipe, etc. because I got a good deal on it. When I did that the idle got better, not perfect, but better. However, I still had the boost breakup issue. One day while driving, I got into boost and the engine went into limp mode. Turned the car off, turned it back on and all was well. No codes or lingering issues. Then the boost into limp mode happened again, but this time I scanned the car while still running and in limp mode, it turned out to be a failed waste gate solenoid. After replacing the solenoid and vacuum lines, the car ran better than ever and I got rid of the boost breakup.

At that point I called it "good enough." I stay on top of oil changes, run chevron fuel system cleaner every few months and try to fill up with good gas. I kicked around the idea of dropping the oil pan to do the rod bearings, but I may just drive it til the wheels fall off as is and see how high a well cared for n55 can go. I've seen some hit 250k, so I have a ways to go.

Good luck and feel free to reach out with any questions.

[Dappa47]

Originally Posted by ZeBoost

Hey there, not sure if you read through all of the posts but there is some good info there. Anyhow, I am not OP, but I ended up with OP's car. 2011 535i with PWG, currently clocking in at about 180k. The car has been running good enough and I drive it so little (50-100 miles a week) that I decided I was done chasing this issue.

There are a few bits of info I know from posts here, conversations with techs and other BMW enthusiasts, as well as my own research.

There are indeed early N55 engines that did not have a LP sensor. The LP pump, as best I have been able to determine, runs constantly and should average about 72 PSI. It will drop somewhat during hard acceleration or when the car is off/first started, but it's trying to run a constant 72. I have connected a manual gauge to the fuel line to test.

The HPFP should run about 700-800 PSI at idle, 1300-1500 under throttle and see occasional blips to 1800-2000 PSI under hard acceleration. I replaced my HPFP with a BMW OE part from FCP Euro, at the time I paid about 1k for the kit. My HPFP readings are inline with the examples I listed. Mine did throw an occasional code for low pressure in the high pressure system, also, I had long starts before the HPFP swap.

I don't have the code in front of me but mine had a code for low pressure fuel sensor shorted to ground, I believe. I chased it for quite a while, and threw lots of parts at it. Some to chase the issue, some to stay on top of maintenance items. Coils, plugs, injectors, new battery, valve cover, ISTA to read data and reset adaptations, smoke tests, pressure tests, etc. The car ran fairly well, but did have a misfire at idle and what felt like boost breakup under throttle in certain conditions (medium acceleration, holding throttle steady).

I decided to replace some of the turbo piping, charge pipe, etc. because I got a good deal on it. When I did that the idle got better, not perfect, but better. However, I still had the boost breakup issue. One day while driving, I got into boost and the engine went into limp mode. Turned the car off, turned it back on and all was well. No codes or lingering issues. Then the boost into limp mode happened again, but this time I scanned the car while still running and in limp mode, it turned out to be a failed waste gate solenoid. After replacing the solenoid and vacuum lines, the car ran better than ever and I got rid of the boost breakup.

At that point I called it "good enough." I stay on top of oil changes, run chevron fuel system cleaner every few months and try to fill up with good gas. I kicked around the idea of dropping the oil pan to do the rod bearings, but I may just drive it til the wheels fall off as is and see how high a well cared for n55 can go. I've seen some hit 250k, so I have a ways to go.

Good luck and feel free to reach out with any questions.

So you never figured out if and where the low fuel pressure sensor was and what was causing that fault?

I read through the thread, no one seemed to have an answer to where the DMe is measuring the Low pressure fuel pump pressure from.

You say your reads around 76 or so, that is correct , mine reads on a scanner 100psi and it stays static , no fluctuations .

So that is what I am trying to figure out at the moment , I get hesitation and I can see that the engine misfires , but I have absolutely no codes for anything . No low fuel pressure , no misfires , nothing .

Just intermittent hesitation. So I am trying to pin point whether it’s my low pressure fuel pump or high pressure pump causing the hesitation .

Everything else has been changed , spark plugs, fuel injectors, coils .


Because my low pressure reading is inaccurate , I figure I would start there first , trying to change the sensor , but like your version N55 , there is no low pressure sensor to change . I have an early N55 , in an 2012 X6.

So that is what I am trying to figure out , where is the sensor and or what is the DME using to measure fuel pressure from the low pressure pump side.

If there are any other threads on here with respect to this topic, please point me to it if ya can , just drop a link in ya reply. Would greatly appreciate it.

[ZeBoost]

As best I can figure, the N55 does not have a low pressure sensor, anywhere. Not in line, not in the tank, not in the LPFP. The fuel pump is either on or off, and it appears the plumbing does allow for some bleed off of pressure (about 10 PSI or so). Based on my research, it seems the early N55 DME has a static, default value of 100 PSI for the LPFP, but doesn't actually affect the LPFP pressure in any way. I believe the DME shows this LP sensor shorted code, because it expects a sensor value and is instead not receiving any signal, so it assumes, the sensor is shorted (instead of showing high or low pressure warnings).

What I have noticed, and is consistent with my assumptions, is that if you monitor the rail fuel sensor, when the vehicle is off and has been sitting for a while, you'll see about 60 PSI. This is because when the LPFP is off, it holds most of the pressure in the line, except for a small amount of bleed off. Then if you push the start button without putting your foot on the break (so the engine doesn't fire), you'll see the rail fuel sensor show 72 ish PSI. This happens because the HPFP isn't boosting fuel pressure, so you see the LPFP readings. You can confirm this with a manual gauge connected to the fuel line as well.

If I were in your position, I would smoke test the boost side of the engine intake system. There are a few videos on YT that show the procedure, it's not too tough if you have access to a smoke machine. Just to make sure your issue isn't that you are loosing boost or sucking extra air into the system.

Another interesting bit on the early N55 is that they use vacuum to actuate the waste gate on the turbo. The engine doesn't generate vacuum, so a vacuum pump is added to the system and run off a chain (it's near the HPFP). This pump builds up vacuum in a chamber on top off the valve cover. Which is then used to move the actual waste gate. This is done via DME signal to the waste gate solenoid. If the vacuum (lines, pump, VC vacuum reservoir, etc) isn't holding and isn't sufficient to operate the waste gate (which by default is open, meaning no boost) you'll get odd surges/hesitation under acceleration.

With regard to the misfire at idle, mine seems to have gotten better each time I changed a part. I wouldn't say it's prefect at idle, but I can live it with it. Also, once the car is warmed up, it's hardly noticeable. One thing I have heard is that there is a DME update that turns your idle speed up a hundred or so RPM, which many people have said is the fix for the misfire at idle issue (we're not the only ones who have mentioned it).

Two things for you to try. One, when my idle was especially rough, before I started changing thins out, I could smooth it out about 50% but putting the car into sport mode. I'm not 100% clear on all of the parameters that are changed for idle in sport vs non sport but I did see a noticeable improvement. Two, whenever I worked on the car and reset adaptations, the idle got much better for the first few trips, and was about 150 RPM higher than usual. Then the rough idle would return. If you have access to INPA/ISTA, you can reset them there, if not. You can try the throttle method

Initially I believed my rough idle and boost breakup/hesitation were related, but in my case at least, they were being caused by different issues.

[Dappa47]

Originally Posted by ZeBoost

As best I can figure, the N55 does not have a low pressure sensor, anywhere. Not in line, not in the tank, not in the LPFP. The fuel pump is either on or off, and it appears the plumbing does allow for some bleed off of pressure (about 10 PSI or so). Based on my research, it seems the early N55 DME has a static, default value of 100 PSI for the LPFP, but doesn't actually affect the LPFP pressure in any way. I believe the DME shows this LP sensor shorted code, because it expects a sensor value and is instead not receiving any signal, so it assumes, the sensor is shorted (instead of showing high or low pressure warnings).

What I have noticed, and is consistent with my assumptions, is that if you monitor the rail fuel sensor, when the vehicle is off and has been sitting for a while, you'll see about 60 PSI. This is because when the LPFP is off, it holds most of the pressure in the line, except for a small amount of bleed off. Then if you push the start button without putting your foot on the break (so the engine doesn't fire), you'll see the rail fuel sensor show 72 ish PSI. This happens because the HPFP isn't boosting fuel pressure, so you see the LPFP readings. You can confirm this with a manual gauge connected to the fuel line as well.

If I were in your position, I would smoke test the boost side of the engine intake system. There are a few videos on YT that show the procedure, it's not too tough if you have access to a smoke machine. Just to make sure your issue isn't that you are loosing boost or sucking extra air into the system.

Another interesting bit on the early N55 is that they use vacuum to actuate the waste gate on the turbo. The engine doesn't generate vacuum, so a vacuum pump is added to the system and run off a chain (it's near the HPFP). This pump builds up vacuum in a chamber on top off the valve cover. Which is then used to move the actual waste gate. This is done via DME signal to the waste gate solenoid. If the vacuum (lines, pump, VC vacuum reservoir, etc) isn't holding and isn't sufficient to operate the waste gate (which by default is open, meaning no boost) you'll get odd surges/hesitation under acceleration.

With regard to the misfire at idle, mine seems to have gotten better each time I changed a part. I wouldn't say it's prefect at idle, but I can live it with it. Also, once the car is warmed up, it's hardly noticeable. One thing I have heard is that there is a DME update that turns your idle speed up a hundred or so RPM, which many people have said is the fix for the misfire at idle issue (we're not the only ones who have mentioned it).

Two things for you to try. One, when my idle was especially rough, before I started changing thins out, I could smooth it out about 50% but putting the car into sport mode. I'm not 100% clear on all of the parameters that are changed for idle in sport vs non sport but I did see a noticeable improvement. Two, whenever I worked on the car and reset adaptations, the idle got much better for the first few trips, and was about 150 RPM higher than usual. Then the rough idle would return. If you have access to INPA/ISTA, you can reset them there, if not. You can try the throttle method

Initially I believed my rough idle and boost breakup/hesitation were related, but in my case at least, they were being caused by different issues.

Brother , thank you for the feed back, appreciate it. You gave me a sense of direction . I will definitely look at rail pressure first , then smoke test

[ZeBoost]

Happy to help! Again, feel free to reach out with any questions. I have quite a bit of experience with the platform and have worked a ton on the car. I did the HPFP and rail sensor, alternator, water pump and thermostat, valve cover, waste gate adjustment, waste gate solenoid, vacuum line replacement, electrical troubleshooting, transmission service, turbo piping, belt and tensioner, boost pressure testing, smoke test, etc.

It's a solid car, and even though my list above makes it seem like it's a disaster, it's been dependable. I have a high mileage car and wanted to make it as reliable as possible while learning about the engine. It's never left me stranded, it still gets compliments and it's sporty for a 4 door sedan. When it comes time to replace it, I may grab an f10 m5 because I like the f10 so much.

[Dappa47]

Originally Posted by ZeBoost

Happy to help! Again, feel free to reach out with any questions. I have quite a bit of experience with the platform and have worked a ton on the car. I did the HPFP and rail sensor, alternator, water pump and thermostat, valve cover, waste gate adjustment, waste gate solenoid, vacuum line replacement, electrical troubleshooting, transmission service, turbo piping, belt and tensioner, boost pressure testing, smoke test, etc.

It's a solid car, and even though my list above makes it seem like it's a disaster, it's been dependable. I have a high mileage car and wanted to make it as reliable as possible while learning about the engine. It's never left me stranded, it still gets compliments and it's sporty for a 4 door sedan. When it comes time to replace it, I may grab an f10 m5 because I like the f10 so much.

Man thank you! You remind me of myself , I have been working on my truck ever since I got it , love it , love the look , I have some 22's on mine , upgraded my head unit to the andriod and upgraded my dash to digital. Truck looks beautiful inside and out, .

Like you I have done a ton of work, everything you mentioned other than the waste gate adjusting , dropping the sub frame and doing the rod bearings.

Which I plan on doing next year when the weather gets warm again.

My truck has around 130k miles on it, looks immaculate , I am going to do my suspension in a week or two before the weather gets colder.

Just finished upgrading my air lines on my compressor to 1/2" so I can fully power my new Thor G2 air impact so I can do the job. I have cordless impacts as well but I always wanted to own and use an air impact, call it weird.

Love the sound of an air impact , don't know why.

But I have saved myself a shit load of money, thousands so far, I did my valve cover, , just recently changed my eccentric shaft , the Motor and bearings for the shaft.

Belt Tensioner, chain tensioner , I did so much to that truck I forget, so yeah I know my way around the platform a little well.


But your help is greatly appreciated, the idea of checking the LPFP pressure by using the high pressure sensor when the engine is off.

I didn't even think that, I just recently purchased a fuel pressure gauge kit and I was going to do just that , check pressure with the gauge to make sure the low pressure side is working properly .

We will see .


The High pressure sensor is new so whatever reading I get off of it , I am sure will be accurate.

So I should be able to rule out that the Lpfp is causing my hesitation.

Hopefully it's not under load cause then I won't be able to pin it down , it idles fine and was idling rough , sounding like it wanted to shut off , but since I changed the valvetronic motor and my leaking oil filter housing pressure sensor that seemed to stop at idle.

So if my low pressure reads ok then I have to look for a boost leak .

It can be frustrating , when you have no engine codes , you have to start guessing and throwing parts at it, that's when it becomes expensive.


Last question, why did you have to adjust ya wastegate valve, what was the car doing and how did you pinpoint it was that?

[ZeBoost]

That's awesome man, glad you're enjoying the platform and learning about it just like myself!

I've done both the rail fuel sensor and manual gauge on the low pressure line, both were pretty consistent.

As for the waste gate adjustment, 2 reasons. First, I had a strange rattle near the turbo under certain conditions. That turned out to be an exhaust shroud. Second, the boost breakup/hesitation, made me think maybe there was an issue with the waste gate actuator arm binding on something. I never suspected the solenoid, which ended up being the problem. When I replaced the solenoid, I also replaced the vacuum lines.

[Dappa47]

Just wanted to give an update, to anyone who may have experienced the hesitation without any misfire codes or check engine light. I figured out my issue, it ended up being my transfer case . I had recently changed my tires in the rear to a different brand due to the rears having a bubble on one of them. I changed both .

Not sure how long after , but that is when the symptoms appeared . I didn’t think that would have caused any issue what so ever , for I never had that issue with my X5 when changing tires.


But that seemed to be the case. Doing my research lead me to the transfer case, I recalibrated it and the issue seemed to go away and appeared intermittently.

So I changed the fluid and did another recalibration. It has been working fine with intermittent slight hesitation, which isn’t very noticeable .

Before it was undriveable .

I will be changing out my front tires with the same brand to fully resolve any future issues.

I guess the Xdrive is very sensitive , at least on my x6 to tire differences.

I guess when changing tires you have o recalibrate the transfer case and give the computer time to adapt to the new torque differences being the tires and traction might be different than before. at least that’s what it appears to be , to me.

Hope this helps anyone who may have experienced the hesitation and couldn’t figure out where and what was causing it. Especially with no check engine light or codes to direct you to a specific area or component.