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Old 10-08-2012, 10:18 AM   #1
nola mike
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Another P0171 code...

Been reading up on this one. Haven't delved too deeply into it yet, and haven't replaced the vacuum hoses etc, but a few things that are a bit unusual (it seems), wanted to see if it was something that isn't as simple as a vacuum leak.

1) Only got P0171--shouldn't it throw a P0174 as well if a vacuum leak (or MAF for that matter?)
2) Came on at highway speed--seen suggestions that this may be a soft MAF failure/fuel pump issue? I've had the car 1.5 years, 20k miles, can't recall if I changed the fuel filter or not. Yeah, it's due if I haven't.
3) Car runs fine
4) Fuel trims seem ok--short/long term on both banks between -5/5%. Maybe LTFT hasn't had time to adjust, but STFT should be well off to trigger that code, right? And should continue to be off? I'm wondering if I had some weird short term problem (debris covering intake?) that's resolved (fingers crossed).

Reset the light, we'll see if it comes back, then off to troubleshoot!

Edit: I realized that this was at idle. Point 4 was particularly strange to me. I'm going to look at STFT at highway speed and see if that changes. I guess that would point more towards a fuel/MAF problem rather than vacuum, right?

Last edited by nola mike; 10-08-2012 at 10:20 AM.
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Old 10-08-2012, 10:21 AM   #2
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It won't always throw both banks if there's a vacuum leak. VCG and Disa vacuum leaks can only trigger one bank depending on the leak. Go to your car and try to move the Disa around. It shouldn't move very much, usually none at all. If it does, pull it out and reseal it where it meets the intake mani.
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Old 10-08-2012, 10:29 AM   #3
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I had the same code and chased everything vacuum related until I noticed the rubber elbow that goes from the throttle body to the air box was dry rotted and cracked. Picked one up from my local BMW dealer for about $40. Replaced it and turned out the light and no more code!
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Old 10-08-2012, 10:33 AM   #4
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Did a search, mult references to VCG, but can't find out what the hell it is?

@amhauke: Yeah, that's listed as one of the common vacuum leaks.

Last edited by nola mike; 10-08-2012 at 10:34 AM.
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Old 10-08-2012, 11:56 AM   #5
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VCG is valve cover gasket.
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Old 10-08-2012, 12:10 PM   #6
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VCG is valve cover gasket.
Didn't realize it was such an exotic part
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Old 10-08-2012, 12:29 PM   #7
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Huh?
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Old 10-08-2012, 01:09 PM   #8
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No, I was just searching around thinking that a VCG was some fancy part like a DISA or MAF or VANOS or something.
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Old 10-08-2012, 01:47 PM   #9
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Quote:
Originally Posted by nola mike View Post
Been reading up on this one. Haven't delved too deeply into it yet, and haven't replaced the vacuum hoses etc, but a few things that are a bit unusual (it seems), wanted to see if it was something that isn't as simple as a vacuum leak.

1) Only got P0171--shouldn't it throw a P0174 as well if a vacuum leak (or MAF for that matter?)
2) Came on at highway speed--seen suggestions that this may be a soft MAF failure/fuel pump issue? I've had the car 1.5 years, 20k miles, can't recall if I changed the fuel filter or not. Yeah, it's due if I haven't.
3) Car runs fine
4) Fuel trims seem ok--short/long term on both banks between -5/5%. Maybe LTFT hasn't had time to adjust, but STFT should be well off to trigger that code, right? And should continue to be off? I'm wondering if I had some weird short term problem (debris covering intake?) that's resolved (fingers crossed).

Reset the light, we'll see if it comes back, then off to troubleshoot!

Edit: I realized that this was at idle. Point 4 was particularly strange to me. I'm going to look at STFT at highway speed and see if that changes. I guess that would point more towards a fuel/MAF problem rather than vacuum, right?
I think you are on the right track -- reset the light and wait.

One
This is not a fatal fault, there is no danger of the engine melting down or spontaneously combusting. You have time to watch and see what develops.

Two
If this is a leak, it will progress to throw a P0171 and P0174. It still will not be fatal.

Three
If the problem returns as a single-bank error, and it's the same bank, my money is on the sensor failing and not on a vacuum leak.
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Old 10-08-2012, 02:16 PM   #10
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Suggest you check the first 2 links below in my signature.

You can have a single bank trip lean codes depending on the situation, especially if the lean condition is right on the edge of the lean condition.

Fuel trims are a good way to look at the condition, usually anything under +/- 5% is not going to trigger any codes. I recall you have to be +/- 10% or greater on the fuel trims.

Clearing codes is a the way to go and watch what happens.

Usually Lean or Rich codes do not indicate a O2 sensor is bad, it usually indicates the sensor is working, albeit it could be lazy.

If the front bank is lean it could be DISA, fuel pressure, other vacuum hose/leak problem.

If you are getting the code only when driving it highway speeds, suggest you monitor fuel trims, MAF readings and if you cannot pinpoint the issue, if I were you, I would replace the fuel pump before the MAF as it is cheaper, the fuel pump WILL leave you stranded and the MAF is less likely the culprit and it usually WILL NOT leave you stranded.

At the end of the day, all these cars need fuel pumps, MAF's, cam position sensors, CCV, intake boots and vacuum hoses. Some need valve cover gaskets and VANOS seals as well. This assumes plugs and fuel and air filters are a given.
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Hidden OBC Menu - Check Voltage, Temps, Fuel Level - http://forum.e46fanatics.com/showthread.php?t=239619

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Old 10-08-2012, 02:49 PM   #11
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QUESTION
There is Freeze Frame data in my OBD II scan tool that indicates LTFT and STFT, and numbers are given. What do these numbers mean? What does it mean if the numbers are high or low, what is the normal range?

I think the data is Long Term Fuel Trim and Short Term Fuel Trim, but I don't know what these are and I don't know what the numbers mean when they appear.
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Old 10-08-2012, 03:19 PM   #12
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Quote:
Originally Posted by nola mike View Post
Been reading up on this one. Haven't delved too deeply into it yet, and haven't replaced the vacuum hoses etc, but a few things that are a bit unusual (it seems), wanted to see if it was something that isn't as simple as a vacuum leak.

1) Only got P0171--shouldn't it throw a P0174 as well if a vacuum leak (or MAF for that matter?)
2) Came on at highway speed--seen suggestions that this may be a soft MAF failure/fuel pump issue? I've had the car 1.5 years, 20k miles, can't recall if I changed the fuel filter or not. Yeah, it's due if I haven't.
3) Car runs fine
4) Fuel trims seem ok--short/long term on both banks between -5/5%. Maybe LTFT hasn't had time to adjust, but STFT should be well off to trigger that code, right? And should continue to be off? I'm wondering if I had some weird short term problem (debris covering intake?) that's resolved (fingers crossed).

Reset the light, we'll see if it comes back, then off to troubleshoot!

Edit: I realized that this was at idle. Point 4 was particularly strange to me. I'm going to look at STFT at highway speed and see if that changes. I guess that would point more towards a fuel/MAF problem rather than vacuum, right?
Swap the O2 sensors and see if the code switches to P0174. It's too early to worry about fuel trim levels, and your numbers seem to be ok.
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Old 10-08-2012, 06:45 PM   #13
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JD,

Some scan tools only show fuel trims after freeze frame, some display the fuel trims almost real time and some tools do not even show the fuel trim data.

The number indicate how much fuel is being added or taken away, where 0 is actually the expected baseline.

Trims with a + figures are adding fuel due to a lean mixture situation
Trims with a - figure are removing fuel due to a rich mixture situation

Usually anything within the +/- 3% range is tolerable, although this may be indicating something is up. At around 5% you should start to really take some action as something is not right and usually above 10% you will trigger either lean or rich codes.

Lean codes are far more common due to fuel delivery, vacuum leaks, carbon deposits and sometimes VANOS issues.

Rich codes are more often due to aftermarket/ebay MAF's or some other rather obvious problem that is often due to a incorrect or bad part.
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Fuel pump failures - http://forum.e46fanatics.com/showthread.php?t=929501

Temp Info - http://forum.e46fanatics.com/showthread.php?t=964491

Hidden OBC Menu - Check Voltage, Temps, Fuel Level - http://forum.e46fanatics.com/showthread.php?t=239619

E46/E39 GM5 Door Lock Info - www.bmwgm5.com

Lower hose fan switch O-ring - BMW #13621743299
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Old 10-09-2012, 03:18 PM   #14
nola mike
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Well, reset the light. No recurrence after about 50 miles mixed driving.
@jfoj: Yeah, everything will eventually need replaced, but I'd rather not mess with it if a) there isn't anything wrong (yet), b) it's something that gives warning before it breaks, and c) it won't leave me stranded. Out of all the possibilities, only fuel pump meets c). Interesting link on the FP--my tank had a bit over 1/4 when the SES came on. I'll check the hidden data.
Since my experience with this car (and BMW's) is somewhat limited, I'm basing this off of prior cars--a vacuum leak should trip a lean code at idle. If it's huge, it will have driveability issues. At speed the effect of a vacuum leak is much less pronounced. If it happens again, esp at speed, I'll replace the FP.
@SeanC: Why too early to check FT's? That's the root cause of what trips the lean code, and lets me know how far out of range I am and for how long. I don't see that the O2 sensor would be a problem, since it's what is reporting the lean/rich condition, and my voltage is fluctuating. At worst it's lazy. Either way, I should get an O2 sensor related code.
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Old 10-09-2012, 06:33 PM   #15
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The fuel trims are the ultimate consequence, not the root cause of a lean situation. There is a gazillion things that might cause a lean code, and trims will be affected the same way in every single one them, that is they will increase until the limit is reached, at which point a SES light will be triggered. If the fuel trims were the root cause, resetting the fuel adaptations would get rid of the SES light. Just doesn't work that way.

Since you have a lean code affecting bank 1 only, the right way to start the diagnosis is to swap the O2 sensors and rule out any O2 sensor related issues (especially if you have over 100k miles). If they are in the soft fail mode, they won't trigger an O2 sensor code, but the ECU will still adjust for the lean condition.

If the code doesn't switch to P0174 after swapping sensors, then you can proceed to rule out a possible vacuum leak (downstream MAF), and the way to do this is a smoke test.

If no diagnosis after all this, check back with us. Don't start throwing parts at the car yet. Good luck.
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Old 10-09-2012, 07:01 PM   #16
nola mike
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Quote:
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The fuel trims are the ultimate consequence, not the root cause of a lean situation.
No, not the cause of a lean situation, but the reason why the lean code was triggered--if the FTs are out of whack, that's indicating a lean condition, and that's why the code is triggered. STFTs will tell me if everything is running correctly at that moment, whereas the the SES light/code just tells me that something was out of spec at some point. Much more info in the FT data.
Quote:
Originally Posted by SeanC View Post
If they are in the soft fail mode, they won't trigger an O2 sensor code, but the ECU will still adjust for the lean condition.
What do you mean by "soft fail mode"? The fact that my STFT fluctuates around 0, and the voltage in the sensors is also fluctuating points to a working sensor. It could be "lazy", ie not switching fast enough. Not sure that that would be enough to throw a p0171. Should throw a P0153/P0133 though.

Quote:
Originally Posted by SeanC View Post
If no diagnosis after all this, check back with us. Don't start throwing parts at the car yet. Good luck.
Don't worry, that's not my speed
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Old 10-12-2012, 08:59 PM   #17
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So drove about 200 miles before the light came on again. Again at the end of a 50 mile drive. This time I had my scanner with me and hooked it up--STFT were now >25% on both banks at idle. The car then ran like crap for a minute before I shut it off to go to work. On my way home, the code thrown was another P0171. Observed at idle that the STFTs were ~20% on both banks, dropping to 0-9% at highway speeds. At the end of this 50 mile trip, I had a P0174 as a "pending" code. Also had 3 separate codes for "cylinder misfire--fuel cut off" (P1343/5/7) on cylinders 1,2, and 3. Those disappeared by the time I got home though. The STFT was higher when I was part throttle, lower when steady/accelerating. All of this now goes along with a vacuum leak I think. (BTW, watching that live data on a trip is mesmerizing). And all of the numbers now make some sense I think, except that I'm not sure what to make of the misfire codes. I'm going to go through the vacuum leak troubleshooting and see what happens.
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Old 10-16-2012, 02:36 PM   #18
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Bit of an update. Tore a few things apart today. Aside from the DISA, I haven't seen any obvious leaks
The upper/lower intake boots, ICV grommets, lower CCV hose all looked good. I ran out of time to go further.
So can a broken flapper alone trigger the lean codes, or is it only if the o-ring is bad? The DISA was in there pretty tight.
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Old 10-16-2012, 02:37 PM   #19
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Bit of an update. Tore a few things apart today. Aside from the DISA, I haven't seen any obvious leaks
The upper/lower intake boots, ICV grommets, lower CCV hose all looked good. I ran out of time to go further.
So can a broken flapper alone trigger the lean codes, or is it only if the o-ring is bad? The DISA was in there pretty tight.
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Old 10-16-2012, 06:23 PM   #20
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Do you have just a broken flapper or do you also have a blown vacuum diaphragm as well?

The DISA can play some crazy games with the intake performance, cannot say for absolute certainty that the broken DISA flap can and will cause a lean code on only one bank, but you need to get everything sorted out to continue to rule out specific issues.

Depending on mileage, not a bad idea to replace O2 sensors, recommended at 100k miles anyway, fresh O2 sensors usually do not resolve lean codes, they usually help you see how bad things really are regarding fuel mixture issues.
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Fuel pump failures - http://forum.e46fanatics.com/showthread.php?t=929501

Temp Info - http://forum.e46fanatics.com/showthread.php?t=964491

Hidden OBC Menu - Check Voltage, Temps, Fuel Level - http://forum.e46fanatics.com/showthread.php?t=239619

E46/E39 GM5 Door Lock Info - www.bmwgm5.com

Lower hose fan switch O-ring - BMW #13621743299
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