Yes- Pending Fault- P0174 (System too lean bank 2). Is there something super obvious I could have missed here?

I would just drive the X until you have a SES light come on. Read the fault code for the SES light. Your long term fuel trims needs to readjust to any changes you might have made. By reseating the maf intake tube, you might have eliminated the vacuum leak. The 02 sensor will readjust the mixture and eventually change the long term fuel trim.

Obvious? Like an error code that relates to a V8 on an I6? That kind of obvious...? :confused:

It shouldn't be terrible performance (limp mode) until the MIL kicks in. Are you still getting horrible power problems? I would try to get some miles on the engine to get the fuel trim to catch up with reality. If you are able to drive with light throttle and it will go through all gears and you can get up to highway speed (use manual shift to make sure all gears working).

If the engine starts behaving after 10-15 miles you may have gotten it fixed. If it seems to work ok not great and if full throttle kills power I would suspect fuel delivery.


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So I just drove 5kms (as I had limited time).

It didn't improve.

What I noticed is:

The accelerator/pedal putters/stops responding after about 20% of the way down.

Rev'ing in park works fine.

I did an odb2 check, and it now gives P0171 & P0174 (system too lean bank 1 & 2)

What does this indicate?

Several problems can cause lean on both banks but I don't think we have to go back to potential causes of those codes.

I strongly suggest you go back to the Indy to run diagnostics again. Regardless of the quality of input here we are not standing over your engine and reading the diagnostic screens. Even though you don't have trans code you still have the same symptoms. Indy conclusion was MAF. You purchased a used MAF. Never a good idea IMO.

My guess is that the MAF is still at least part of the problem. Should never have to drive it to get back normal performance. If the Indy diagnosis was correct it is logical the MAF replacement is not performing as it should. I would take the MAF you took off. Did the Indy try to clean the MAF? Sometimes that works or improves performance enough to verify the MAF is bad.

ok so this is interesting feedback; it will rev through high revs with no load but can't push the car. (stutters when under load). It sounds like fuel supply to me. if it can rev through range without hiccups, then it's getting enough gas when no torque.

read the page here: Problem 4: Engine Hesitates, Stumbles, Lacks Normal Power

Your mention of 'past 20%' throttle it could be the throttle position sensor (which in fly-by-wire throttle of x5 is also the throttle position; there are dual potentiometers in the throttle so it won't just completely stop when one fails but will cause all sort of horrible throttle response when one does).

That said if you can rev through throttle no-load and you can't get power under load, and are getting lean codes both banks, the engine is not getting enough gas. Either the MAF isn't measuring the air properly and is telling the computer to add too little gas, or a problem with the fuel delivery; dirty injectors, or low fuel pressure.

Since it's probably the easiest way to test, a fuel pressure gauge on the rail under load (car in reverse with brakes with a separate person to rev the car slightly) (reverse to not drive over the guy under the hood). If the pressure dips under load it's most likely the fuel pump; it can supply pressure but not volume; works when there is no load but when fuel quantity demand rises the pressure will drop.

I can't remember if you ever tested this car down to zero DTE; if the fuel pump can't supply pressure @ volume, i will be very surprised to see a Distance to Empty lower than 10-20 miles. I noticed a slight issue of hesitation with WOT on my wife's car when her fuel pump failed (soft fail) but it would 'run out of gas' at 70 miles to empty when the fuel pump was weak.

So; the type of hesitation you are describing is a HUGE mismatch in air/fuel mix it is NOT a vacuum leak level of mismatch where you get a code but don't feel it in the throttle response. If your car will run without MAF unplugged i would try that to see if the default program for mix will allow power under load. If that still shows hesitation as I will expect, then i would check fuel pressure under load. (and/or clean injectors). Once you've done that it's time to have somebody with more experience take a look. Even if you have dealer run a diagnosis. Your time is worth something too, they may know exactly the problem, give you a check-list of stuff you can DIY and get right to the exact problem.

If engine performance changes under load it is not conclusive the problem is fuel delivery, nor is driving with MAF disconnected pinpoint the problem. OP has not been considering a vacuum leak since Indy did a smoke test. A problem with throttle position sensor should have shown up in diagnostics as should have a fuel delivery problem.

mcfee03 has already gone to someone with more experience who diagnosed a bad MAF. IMO rather than do any troubleshooting that's what he should do again to verify the used MAF is working properly and if necessary I expect Indy will check further for any other problems such as fuel delivery.

Normally if it revs fine sitting still and dies under load at high rpms but low rpms under load seems fine its fuel related.

I have had a dirty fuel filter, bad filter in tank (the sock) and weak fuel pump all cause these EXACT symptoms including the lean codes across multiple vehicles not just bmw's.

Fuel pressure and volume are not mutually inclusive. It will seem fine at idle revving with good psi but poor volume (flow). Flow is fuel filter related normally. PSI is pump or regulator related.

Flow issues will cause starvation when driving but idle fine and rev ok.

You can spot flow issues with a pressure gauge but it has to be active while your driving when the problem occurs. Sitting still and revving isn't going to work. So you have to figure out how to rig up the pressure gauge to monitor it while driving.

I use a Bluetooth fuel pressure gauge for this. I don't know where I bought it as I have had it for years. Its a cheap Chinese gauge and probably isn't even accurate psi wise but it works well enough to spot starvation problems while driving when working on stuff.

Fuel delivery problem would have shown up but only if the mechanic connected a fuel pressure gauge at the fuel rail. An OBD II diagnostic will not show a fuel delivery problem such as a weak or failing fuel pump. The OBD II diagnostic may have shown a lean condition, but there are many reasons for a lean condition, such as vacuum leaks or a weak fuel pump.

Since the trans fail trouble code have disappeared after the installation of the used MAF, I would just leave the new MAF in place. You can look at live data from the MAF. You can review the flow rate of air through the MAF to see how many g/s of air is flowing through the MAF. At idle the g/s should be around 3-4 g/s.

I would connect a fuel pressure gauge at the fuel rail see what the pressure is with the key on engine not running, key on engine running, and engine running with the revs at 3000 rpm. Does the pressure goes down when you rev the engine? Does it go down and then back up?

And last but not least, was the fuel filter ever changed? People have been known to get the return line on the fuel filter confused with supply line. This has mess up diyers on this forum as well as other BMW forums. The engine will not run right if the two line are switched.