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Old 02-29-2024, 04:21 AM
Join Date: Apr 2014
Location: Oahu
Posts: 38
victell is on a distinguished road
SOLVED: Need help with bogging and misfires, persistent P0171 & P0174 lean DTCs

SOLVED: MAF was at fault

Conclusion: even if there are no codes for MAF and the live data seems to be responding correctly, the MAF may still be at fault. Disconnect the MAF before starting the engine, then drive the vehicle. If there is a change then the MAF is likely at fault. Thanks 80stech for the tip.


My 2003 3.0 X5 has a bogging and misfire condition that I cannot seem to find the root cause. I have reached the limits of my knowledge and am asking for help before I waste more money replacing good parts.

Check engine light after miles of highway cruising (sustained high vacuum driving). Codes:
- P0171 & P0174 System too lean banks 1 and 2. No drivability issues.
Performed smoke test using DIY tester. Didnít notice any leaks but test area had a slight breeze and may have blown away any light wisps. I'd say test did not reveal any major leaks but because of conditions there may have been small leaks(s) I didnít see. Valve cover gasket was leaking oil and could be a source of vacuum leak so replaced:
- New valve cover gasket
- New OEM NGK BKR6EQUP spark plugs
Cleared codes and drove vehicle only periodically to keep battery charged. CEL did not return at this time.

Drove 5 miles to friendís house to replace rusted rear subframe and install rear muffler sections from LCI V8 model. Vehicle sat without being started while I slowly did work on the weekends. After about 3 months started engine to maintain system voltage during the many, many, many rounds of brake bleeding and ABS pump activations to get air out of the ABS system. This is when the drivability problems began. Engine would start, idle ok, then slowly misfire more and more until finally stalling. Upon restarts misfires began immediately. The only way to keep engine running was to continually feather the throttle. Thinking something might have crawled into the MAF and fouled the heating element:
- Sprayed MAF with CRC Mass Air Flow Sensor Cleaner.

Cleared codes (did not record codes), restarted and misfires went away for a few minutes then slowly came back. Finished subframe job, installed new rear mufflers, cleared codes, and drove home. Reduced power while driving. When almost home misfires began again and engine started bogging badly. Barely got it home. Removed new spark plugs to inspect:
- New spark plugs now look darker than the old ones I pulled out (possibly rich and fouled from the misfires?)
Retrieved codes:
- P0171 & P0174 - System too lean banks 1 and 2
- P1343 - Misfire cylinder 1 with fuel cut off
- P1345 - Misfire cylinder 2 with fuel cut off
- P1346 - Misfire during start cylinder 3
- P1350 - Misfire during start cylinder 5
- P1348 - Misfire during start cylinder 4
- P03XX - Misfires on all 6 cylinders

Rented fuel pressure tester. Started engine:
- Fuel pressure slowly rose to 52 psi after about 30 seconds (took too long to rise? and psi too high at idle, spec 36 - 46 psi)
- No pressure change when disconnecting vacuum hose to fuel pressure regulator
- Under wide open throttle fuel pressure drops to 50 psi

Suspecting soft-fail fuel pump:
- Replaced fuel pump
No change.

Performed compression test to check for blown head gasket:
- 200psi average all cylinder compression pressures = OK
Suspected a faulty rented fuel pressure tester, rented another one, this one new. Same results, fuel pressure very slowly rose to 52 psi, dropped to 50 psi under WOT. Tried tester on another X5, fuel pressure rose more rapidly to spec. Determined fuel tester was not faulty.

Removed intake boot to check as source of vacuum leak
- No cracks or damage to intake boot
Per an Xoutpost thread for checking the CCV system:
- Removed oil filler cap to feel for excessive vacuum, moderate vacuum found indicating CCV likely ok
Removed DISA to inspect, found flap door stuck in one position. Took apart and found hex cam slipped one position
- Repaired DISA and re-used old DISA O-ring.
No change

Suspected possible clogged exhaust due to mufflers sitting outside for months, maybe something crawled in and blocked it. Following another Xoutpost thread on clogged/plugged exhaust observed O2 upstream sensor live data with Foxwell 510:
- O2 voltage at idle hovers around 0.030v
- O2 voltage at part throttle jumps to about 0.800-0.600v then immediately drops to 0.030v
- O2 voltage at wide open throttle jumps to about 0.800-0.600v then immediately drops to 0.015v
O2 results indicate no clogged or blocked exhaust, but instead lean condition. Is this the correct interpretation? While monitoring live data it doesnít look like there's any upstream vs downstream rapid sensor switching going on, but I'm not sure how responsive the Foxwell is.

I only have two suspects going forward. One is a faulty fuel filter/pressure regulator combo due to the incorrect fuel pressure readings. But after wasting money replacing a good fuel pump I am hesitant to replace it without being more sure. The other thought is vacuum leak(s). If I remember correctly under wide open throttle the DME should operate under open loop and ignore O2 sensor readings. My thinking is that wide open throttle should mask all but the largest of vacuum leaks. I might be wrong on DME operation though. I may need to buy a real smoke test machine and try to find a windproof garage to test in. Garages are not common here in Hawaii.

My other issue is I'm not sure how to use some of the live data on the Foxwell 510. It has options like 'multiplicative mixture adaptation', but its value is 0.00 with unit %. There's 'additive bank mixture adaptation', but its 0.00 with unit ms. It also has 'adaptation 1 engine throttle' with a value of 0.48 but the unit is blank. There is more but I'm not sure what they are, how to use them, or if they're even reading accurately.

OK! That's all I've got so far. I would like to use diagnosis to determine failed parts instead of replacing good ones, but I donít know where to go from here. Any ideas or help is appreciated.
2003 E53 X5 3.0i 5MT
2003 E53 X5 4.4i

Last edited by victell; 04-22-2024 at 11:47 PM. Reason: solved
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Old 02-29-2024, 10:08 AM
Join Date: Mar 2016
Location: Saskatchewan Canada
Posts: 2,111
80stech is on a distinguished road
The fuel pressure should rise instantly, so if the gauge is hooked up properly (depressing the schrader valve) the problem is the fuel filter or maybe a pinched line or restriction.
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)
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Old 02-29-2024, 02:44 PM
Join Date: Apr 2014
Location: Oahu
Posts: 38
victell is on a distinguished road
Originally Posted by 80stech View Post
maybe a pinched line or restriction.
I thought about a pinched line from this poor guy who found the same thing after a bunch of part replacements. In his scenario the fuel pressure would go down to 30-40 psi. Mine drops to 50, but no further.

Originally Posted by 80stech View Post
the problem is the fuel filter or
Possibly, might have to replace it as process of elimination.

Originally Posted by 80stech View Post
The fuel pressure should rise instantly, so if the gauge is hooked up properly (depressing the schrader valve)
I hadn't thought of this. Will do another fuel pressure test.
2003 E53 X5 3.0i 5MT
2003 E53 X5 4.4i
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Old 02-29-2024, 04:25 PM
workingonit's Avatar
Join Date: May 2021
Location: DFW Texas
Posts: 756
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similar, but not, problems with mine; extended searching for ???

victell- my '01 X5 3.0i has had a similar problem with lean codes and also showed a misfire code P1345 - Misfire cylinder 2 with fuel cut off, IIRC, just once, in the 2.5 years I've owned it, but I've not yet located the source of the problem, yet. Fortunately, I've used a work-around to keep the CEL/SES/MIL boogeyman away, since the problem started.

The difference between your X5's problem and mine is that mine has never had any driveability problems, with the engine having had just about everything renewed in the three years before I traded for it (by the PO, specifically as the conditions that our car-for-car swap demanded), including plugs, coils, O2 sensors, vcg, Vanos, chain & guides. The only components I've had to change are the exhaust cam position sensor (two codes thrown concerning that was enough warning for me!), a new oil cap & gasket (some blow-by on top of the beauty cover), converting the noisy & cracking viscous fan assembly for a hi-power electric fan, and lastly, swapping the OE MAF sensor for a $20 Amazon unit (more details later).

I followed much the same path you've tread, testing with a homemade smoke tester, out in the open, with the same results you had, for the same reasons, but I've also used carb cleaner spray (which I've used previously on other cars to find vacuum leaks) with similar zero finds. Visual inspections also negative, as was the fuel pressure test (the regulator was brand new, I found out later, so I wasted $75 for the tester and attachments, I guess), and I rebuilt the DISA valve with a G.A.S. kit, so there would be no question concerning it.

I've used a Foxwell NT510 Elite to graph O2 sensors, also, and they were OK, looked at all the live data, compared readings before and after adaptations cleared, and have done hundreds of test sessions using it and once or twice using INPA (but prefer the Foxwell). Nothing much to see, except high fuel trim numbers on both banks.

Concerning fuel trims: mine were so high back in late '21 and early '22, that I'd get the "money light" on (as Eric O. on South Main Auto youtube channel likes to say), and as the next emissions inspection was coming up, I tried taking out my OE MAF sensor (Siemens), which I had previously cleaned with the proper cleaner several times, and replaced it with a $20 ($23? at the time) Chinesium "MostPlus" sensor from Amazon. X5chemist had suggested doing that, as he had much earlier (and still reports that his is performing fine, now 2+ years later).

Immediately, after the swap (and resetting the fuel trims to zero), my MAF readings were much better, and correspondingly the LTFT's (long-term fuel trims) have become lower (better) ever since, though they are still going as high as +9% long-term, in stop-and-go low-speed driving (which is 99% of the only 1500 miles...in the 2.5 years I've driven it), and the long-terms fall to as little as +4%, during the very few extended highway trips it's been on). Either the OE MAF was underreporting airflow, or the cheap replacement is overreporting it, but it's been a good work-around, that I can live with (the X5 passes inspections, the exhaust smells clean, and it is what it is). I had an Amazon refund coming awhile back, so I used it to get a second cheap MAF, as a spare!

Back in Oct.-Nov.'23, I was shopping for a factory-built professional (or semi-pro) smoke tester, after I figured my two previous home-built testers were insufficient. After much ado, after getting & sending back, two cheap-ish units that were broken or defective as built, I finally ordered a better one (which the two cheap others were clearly clones of), the AutoLine Pro EVAP Smoke Machine Leak Tester Ė Shop Series https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1
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from Amazon, to further test my X5 for the phantom vacuum leak I'm sure that MUST be out there, somewhere.

I've got vacuum caps, replacement silicone hoses, and clamps standing by, waiting for the day when I can take over the single garage bay that my wife's precious Lexus sits in, take out the obstructing plastic crap covering the rear of the engine, and smoke test the X5 again. We live on a semi-rural sandhill, with wind-blown sand & oak debris always present (plus feral cats, squirrels, rats, mice, scorpions, spiders, snakes,etc. always trying to get into the garage), so I usually can't open up dust & dirt sensitive components or areas, while outside on my plywood work platform, that I use for external component maintenance.
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I also have halogen & LED worklights, an endosope, inspection mirrors, Rhino Ramps (so, in case I suspect a vacuum leak near the fuel pump, I can get under there), etc., standing by, but things keep popping up to prevent my smoke test.
  • 1) holidays,
  • 2) cold or wet weather (N.Texas type),
  • 3) constant ongoing projects that my wife needs my help on,
  • 3) old age health (73 y.o.),
  • 4) just in the last two weeks: the precious Lexus started up smoking & sputtering, so I opened up the engine covers, found a rodent nest, inspected for damage (found none), tested (with Foxwell & Torque Pro app for codes, and found none), then ....see this thread https://www.clublexus.com/forums/es-...r-coolant.html

So, now I'm going to smoke test two vehicles (guess whose vehicle will get tested first, hahaha), to find what, if anything, is really amiss. victell- I wish you luck, I'm hoping you find something, anything, as a longtime mystery problem can drive one nuts, after awhile.
01 BMW X5 E53,3.0i-5L40E, 7/13/01
topas-blau,Leder-grau,"resto-project car"

14 Lexus ES350,3.5L-U660E
09 HHR Panel,2.2L-4T45E
04 Chevy 2500HD,6.0L-4L80E
98 GMC Sierra 1500,5.7L-4L60E

66 Chevelle Malibu 2dr ht.,327>441c.i.-TH350>PGlide/transbrake
08 Cobalt Coupe,2.2L-4T45E
69 & 75 C10s,350c.i.-TH350
86 S10,2.8L-700R4
73 Volvo 142,2.0L-MT4
72 & 73 VW SuperBeetles,1.6l-MT4
64 VW,1.2l-MT4
67 Dodge Monaco 500 2dr ht.,383c.i.-A727
56 Chevy 210 4dr,265c.i.-PGlide

Last edited by workingonit; 03-03-2024 at 01:56 PM. Reason: added info
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Old 02-29-2024, 08:39 PM
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Location: Racine, WI
Posts: 10,966
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This video is after replacing FPR.

Before I replaced it the pressure bounced with every injector pulse on the order of a psi.

Had very simulator errors suggesting vacuum leak. The thing is: e53 doesn't know the fuel pressure it does the matu assuming 50.0 psi and any time it isn't, it will send less or more fuel.

In the video above I was wot rev and it would pull the psi down like 1.5 psi but at idle it was rock steady.

I thought I took a measurement of the small o-ring but that was the only problem. Gas under pressure was getting past and throwing off the regulation.
2011 E70 ē N55 (me)
2012 E70 ē N63 (wife)
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Old 02-29-2024, 09:11 PM
Join Date: Jun 2021
Location: Austin, TX
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X5chemist is on a distinguished road
What are MAF readings? Do you see any oil residue on the intake?
'06 X5 3.0i - bought @143,123 miles (12/26/20)
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Old 03-01-2024, 12:50 AM
Join Date: Apr 2014
Location: Oahu
Posts: 38
victell is on a distinguished road
80stech, good call on the fuel rail shrader valve. Swapped with a different one and response from the tester is much better now.
andrewwynn, looks like my fuel pressure is now behaving similar to your video. Here's a video of mine:

workingonit, I also tried spraying different areas to find vacuum leaks but some places I cannot get to with the spray so I couldnít be confident. Iím coming to the conclusion that a well-built smoke machine with a still environment to work in is essential. Interesting that the chinese MAF worked well for you. I did not have such luck with them on my previous X5, but that was years ago and maybe the quality has gone up s
ince. Iíll keep that in mind.

X5chemist, video below of my MAF readings but Iím not sure how to interpret them. I noticed residue inside the intake at the DISA port, but nothing wet to the touch. Is that significant?

2003 E53 X5 3.0i 5MT
2003 E53 X5 4.4i
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Old 03-01-2024, 01:04 AM
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Join Date: Jun 2015
Location: Racine, WI
Posts: 10,966
andrewwynn will become famous soon enough
Thatís really slow pressure build!

Itís not that hard to get the FPR out.

I would pull the filter/FPR and check those o-rings. For about a buck you can likely resolve.

Query; ever have hard starts at random times? That was another symptom of wife's car when the FPR o-ring failed.
2011 E70 ē N55 (me)
2012 E70 ē N63 (wife)
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Old 03-01-2024, 02:25 AM
Join Date: Apr 2014
Location: Oahu
Posts: 38
victell is on a distinguished road
It looks like a slow build but I suspect the rental tester + adapter + swapped schrader valve probably didn't allow max volume flow to the gauge 100%. But at full throttle the fuel pressure stayed pretty solid.

I have not had hard starts at random. This X5 may run like crap most of the time, but it is eager to do so and the engine starts promptly every time.
2003 E53 X5 3.0i 5MT
2003 E53 X5 4.4i
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Old 03-01-2024, 10:20 AM
Join Date: Jan 2024
Location: United States
Posts: 59
Factory6speed is on a distinguished road
Engine would start, idle ok, then slowly misfire more and more until finally stalling. Upon restarts misfires began immediately. The only way to keep engine running was to continually feather the throttle.
I had this one morning with my e36. I went through a whole diag ordeal and panicked that the engine was shot. But all it was, was the circular plug for the ignition wire harness wasn't totally plugged in. It was half way in. It ran fine sometimes, I even drove it for 45 minutes the night before, and the next day it starts misfiring like that and would come and go.

I discovered it would change as I hit the valve cover, and that the harness was just moving slightly. Looked at that screwed the plug in properly and it's good. I doubt it but maybe that's all it is, check electrical connectors.
'05 E53 X5 3.0 6mt. Sterling gray metallic / black. 130k
'17 F30 340ix 6mt. Black sapphire metallic / black. 80k
'96 E36 328is 5mt. Alpine white / grey. 150k
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