SOLVED: Need help with bogging and misfires, persistent P0171 & P0174 lean DTCs

[victell]

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.

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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.

History:
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. Didn’t 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 didn’t 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 friend’s 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 doesn’t 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 don’t know where to go from here. Any ideas or help is appreciated.