M50B25 NV engine dies on idle with MAF, idles without MAF

[Starrkey]

So recently my car started to completely die when idling. Immediately when I release the gas pedal the engine revs down and dies. While driving the car runs like normal. When I unplug the MAF sensor the car idles, rougly, but it has never idled perfectly as long as I've had it. I can drive it completely normally, it's maybe slightly heavier as I believe it runs pretty rich without the MAF sensor.

I sourced an original Bosch MAF sensor from a crashed car. switched them up and nothing changed. Still it didn't idle with the MAF plugged in. I cleaned both with designated MAF sensor cleaner before trying them too. I can't tell if they are broken or not, but what are the odds of my MAF dying on me all of a sudden, and me taking a spare one from an another car and it happening to be dead too? I heard those thigns are pretty sturdy.

I heard that the MAF might get confused if there is a vaccuum leak in the headers/rubber tubing, searched for a leak with break cleaner. The car did increase revs two times, but I couldn't tell if it was just the idle being rough, since I couldn't locate a specific place where it would suck it in.

So with this information do you people have any suggestions as to what could be bothering my car, or should I continue to look into the MAF being broken or there being a vaccuum leak?

[E34 Lives]

Check for spark, check for fuel is a good starting point. Semi clogged fuel filter or non performing ignition component can create rough idle or engine cut out.

Possible vacuum leaks, check the air intake boot for cracks or holes, check all the vacuum hoses for damage. Spray cleaner around to see if you can locate leak or do a smoke test to find leak.

If the car dies when you let your foot off accelerator then excess air is entering your system. If you let foot off of accelerator and you step on brake and then car runs rough and dies then you might have a brake booster vacuum leak! Brake booster vacuum leaks will not show up on smoke test. You can pump brakes rapidly and aggressively to see if this chokes out engine, if so then booster needs rebuilding.

[sevenfood]

Hi Starrkey.

Please also check these relays, I have once experienced this.

725614

[Starrkey]

I have an E34 525i non-vanos that runs very roughly on low rpm. It doesn't want to move at all at under 1300 rpm, starts to stutter very roughly and easily dies. If I put it on neutral and let go of the accelerator at speeds (40km/h+) it dies as soon as the RPM drop, as if there is nothing telling the engine to idle. However if I come to a calm stop without putting it in neutral at high RPM, it might not die and start idling again.

If I unplug the MAF sensor and the car goes to "limp mode" runs roughly but doesn't have those problems.

So far I have tried:
- Cleaning and changing the MAF sensor, no difference (I couldn't confirm the new (used) MAF sensor was working, but I've got no reason to believe it wasn't)
- Changing the O2 sensor, no difference
- Unplugging the throttle control valve, car idled at higher RPM but the problems didn't go away
- Searchin for vaccuum leaks, didn't find any BUT I didn't use smoke, rather sprayed the intake with carb. pilot

What I'm thinking might still be causing the problem
- Clogged Idle Control Valve
- Vaccuum leak that I've missed
- Faulty MAF (highly unlikely, like I said I did install an another MAF sensor from a crashed car, but alas I can't know if it isn't broken too)

I think I'm tackling the ICV next, but do you know if the ICV's behaviour is affected by the car going in to "limp mode"? As far as I know it means the car doesn't take readings from the MAF and O2 sensors, rather it runs on fixed values from the factory. The car works pretty much perfectly fine when I unplug the MAF sensor, maybe a little lazier than usual. I've put an embarrasing amount of kilometers on the car on "limp mode" (some few thousand I think) and I'd like to get the car running normally again, but I'm out of ideas.

And do you also know if the car still takes values from one or the other if you unplug the MAF or the O2 sensor? So far I haven't tried unplugging the O2 sensor but leaving the MAF on. If it does take the readings from the other, plugged sensor then this might be a way to tell if it is in fact the MAF that's giving too much air to my precious engine.

[fo3]

I tried everything, incl trying to understand how the system runs with one sensor disconnected like you are asking, ie does it ignore them all on default map or not? I couldn't get an answer. I did try unplugging the O2 and it made no difference BTW. I tested and/or replaced everything except MAF (due to it's rarity and cost). Even though the MAF looked and tested OK, my issue ended up being the MAF.

I bought a tested and known good one from rockauto (cardone tested) and it fixed the stalling rough and lean running issue.

https://www.bimmerforums.com/forum/showthread.php?2480158-Could-it-be-the-MAF-m50b25-NV


E: I have to add after replacing the MAF it did run rough and surge for a while. I felt I had done something bad (replacement TPS and ICV were not as good as the original? Or DOA/faulty?). But after 100km driving it started running and idling smooth again.

If you're sure the second MAF was good then start by checking the wiring to the MAF, one bad wire or connection and lose 0.2V then it will run bad. But unfortunately there's only one way to test the if it's your MAF: pull one from a running car to borrow, or just buy a tested/new one.

My bad one looked OK from resistance or volt tests so I wasted time on other components rather than just buying a known good one. If there's no one local to you with a 525NV that is willing to lend out a MAF for 15min then buy one from USA.


For anyone stuck looking for a NV MAF Rockauto sell the cardone reman/tested Bosch 0280213011 under Cardone part# 7410275 for ~US$95 plus core (which was $50), and shipping charges.
Of course I didn't bother posting the core back so including delivery it cost me ~US$200.
Good price considering new ones are $1500 and lots of time not wasted and lots of unnecessary parts could have not been bought.