Mass or Volume Air flow A Circuit error - 30i shakes and cuts out

ajguk · Post by **ajguk** » Mon Jun 05, 2023 8:56 pm

Hi all

I've an issue with my 30i Z4 (2009, ~84K) - started her up a few weeks ago, revs dropped, car shook, engine cut out. The EML came on - I left it, started it up again a few days later and the car started, revs dropped and I thought she was going to stall, they then held under 1k rpm and it seemed fine. I got hold of an OBD2 sensor (Amazon special so I'm aware this may not be fully accurate) and pulled a P0171 and P0174 codes which I believe are lean codes on both banks. I took her in to my local BMW indy and they couldn't immediately diagnose the issue (the codes came back) - and an odd thing is that once the revs have stabilised after starting, the car is fine, it's only when I started the engine the issue appeared. He recommended I spirited drive with a tank of vpower and see how it goes. Unfortunately, on that drive the problem got worse, the EML came back on and I drove home once that cleared.

This weekend I've had the intake, air box etc out right back to the throttle body and I can't see any splits that would cause a vacuum leak. I disconnected the MAF sensor and the car seemed to be fine....for about three startups, then it died again and is now throwing 'P0100 Mass or volume air flow A circuit'. What's next to try? Is it a new MAF sensor (or should I try cleaning it first) or could it be a cam position sensor as another thread has mentioned, or something else? Any advice much appreciated!

mkinternet · Post by **mkinternet** » Mon Jun 05, 2023 9:03 pm

Hi

Sounds like an air leak after the MAF sensor so maybe follow the hoses and look for a split. That would be my first thought. Also check clips around the hose sections - they can fail.

Steve

ajguk · Post by **ajguk** » Mon Jun 05, 2023 9:10 pm

I've had that section off behind the MAF sensor to the manifold and like I say, no cracks or gaps that I can see. The jubilee clips are a bit rusty but they tighten up nicely. If there was a vacuum leak, is there an easy way to test the pipe between the MAF sensor and the throttle body pipe?

mkinternet · Post by **mkinternet** » Mon Jun 05, 2023 9:17 pm

There seems to be a discrepancy between what the MAF is reading an what is entering the engine. Could be a faulty MAF, maybe order one from amazon and if not the probelm send back (not exactly in the spirit of things I know but !!!!) or find someone local to you who has the same type and size engine and do a swap.

ajguk · Post by **ajguk** » Mon Jun 05, 2023 9:26 pm

There are plenty of MAF sensors on Autodoc, I'll check the part numbers and get one ordered. Any brands to go for or avoid? Also wondering if it's worth getting the cam sensor at the same time.

ajguk · Post by **ajguk** » Mon Jun 05, 2023 10:09 pm

Delphi MAF sensor ordered from Autodoc. Didn't fancy completely cheaping out, and if it makes no difference I can send it back. Will update!

flybobbie · Post by **flybobbie** » Tue Jun 06, 2023 8:22 am

I had problem with my 23i.
Start and revs were a bit erratic for 2-3 seconds then stabilised.
Turned out to be oil in spark plug hole no.4.
Fitted new cam cover all sorted.
The revs would stabilise once the start up voltage hit 14 volts, run erratic on initial 12volts before alternator kicks in.

ajguk · Post by **ajguk** » Tue Jun 06, 2023 3:05 pm

flybobbie wrote: ↑Tue Jun 06, 2023 8:22 am I had problem with my 23i.
Start and revs were a bit erratic for 2-3 seconds then stabilised.
Turned out to be oil in spark plug hole no.4.
Fitted new cam cover all sorted.
The revs would stabilise once the start up voltage hit 14 volts, run erratic on initial 12volts before alternator kicks in.

Well I have a MAF and MAP sensor on the way, so will try those first then have a look at the plugs if I need to. Was it pretty visible once you took the coil packs out or did you have to pull the plugs too? I've recently replaced the eccentric motor gasket because of a small oil leak and am wondering if that has played a part as well.

edit: having said that I did have the coils out when I did the gasket and the plugs were dry (no oil). Unless it's fouled a sensor on that side of the engine?

ajguk · Post by **ajguk** » Mon Jun 12, 2023 7:38 pm

OK so new MAP and MAF sensors fitted, no difference, still does the same thing.

Are there any more sensors to look at that might cause that same issue? If I had oil round a spark plug could it still throw that mass air flow code? Running out of ideas a bit and taking it in to the garage might get expensive very quickly!

flybobbie · Post by **flybobbie** » Mon Jun 12, 2023 8:06 pm

ajguk wrote: ↑Tue Jun 06, 2023 3:05 pm
flybobbie wrote: ↑Tue Jun 06, 2023 8:22 am I had problem with my 23i.
Start and revs were a bit erratic for 2-3 seconds then stabilised.
Turned out to be oil in spark plug hole no.4.
Fitted new cam cover all sorted.
The revs would stabilise once the start up voltage hit 14 volts, run erratic on initial 12volts before alternator kicks in.
Well I have a MAF and MAP sensor on the way, so will try those first then have a look at the plugs if I need to. Was it pretty visible once you took the coil packs out or did you have to pull the plugs too? I've recently replaced the eccentric motor gasket because of a small oil leak and am wondering if that has played a part as well.

edit: having said that I did have the coils out when I did the gasket and the plugs were dry (no oil). Unless it's fouled a sensor on that side of the engine?

Pulled coil the end was coated in oil.

ajguk · Post by **ajguk** » Mon Jun 12, 2023 8:13 pm

flybobbie wrote: ↑Mon Jun 12, 2023 8:06 pm
ajguk wrote: ↑Tue Jun 06, 2023 3:05 pm
flybobbie wrote: ↑Tue Jun 06, 2023 8:22 am I had problem with my 23i.
Start and revs were a bit erratic for 2-3 seconds then stabilised.
Turned out to be oil in spark plug hole no.4.
Fitted new cam cover all sorted.
The revs would stabilise once the start up voltage hit 14 volts, run erratic on initial 12volts before alternator kicks in.
Well I have a MAF and MAP sensor on the way, so will try those first then have a look at the plugs if I need to. Was it pretty visible once you took the coil packs out or did you have to pull the plugs too? I've recently replaced the eccentric motor gasket because of a small oil leak and am wondering if that has played a part as well.

edit: having said that I did have the coils out when I did the gasket and the plugs were dry (no oil). Unless it's fouled a sensor on that side of the engine?
Pulled coil the end was coated in oil.

Did yours throw a code at all? It wouldn't surprise me if oil had got in because of the slight leak I had before the e-motor gasket got changed.

edit: checked all plugs and coils and they're bone dry. I've disconnected the battery as well for ten minutes and reinstalled the new maf sensor in the hope it would reset the codes but they're still there and it's exhibiting the same symptoms

ajguk · Post by **ajguk** » Mon Jun 12, 2023 9:34 pm

This post seems to suggest that pulling the maf sensor will void the lean running codes: https://www.bimmerforums.com/forum/show ... ight=p0100

I'm now thinking it's not the maf sensor at all and could be something else, but I've now no idea how to figure it out......my OBD2 sensor will not clear the codes and unplugging the battery doesn't seem to have helped.

Jon2215 · Post by **Jon2215** » Tue Jun 13, 2023 9:34 am

ajguk wrote: ↑Mon Jun 05, 2023 9:10 pm I've had that section off behind the MAF sensor to the manifold and like I say, no cracks or gaps that I can see. The jubilee clips are a bit rusty but they tighten up nicely. If there was a vacuum leak, is there an easy way to test the pipe between the MAF sensor and the throttle body pipe?

Find a garage that carry out a smoke test to 100% rule out a vacuum leak.

ajguk · Post by **ajguk** » Tue Jul 18, 2023 10:52 pm

Thought I’d update here with the fix. I’ve another post here re a leaky valve tropic gasket and it was this causing the codes and the engine to cut out. Replaced with a new genuine BMW part from the dealer and we’re all good again.

Just a note re the sensors I bought - after a bit of begging and persuasion AUTODOC agreed to a refund, even though they’d been opened and used (the seal said “no replacement if opened”). Result!

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