How do you test a MAF?

[NZ00Z3 187]

I'm chasing a rich (negative fuel trim) issue on my 2.0L Z3 with a M52TUB20 engine. The long term trims sit around -5.4% to -6.5% after being out for a drive.  Let the car sit and idle for 15 minutes and the long term trims drop back to near zero.  So, something changes when you are out driving it.

Original airbox with new filter.  Original O2 sensors. car has done 119,000 miles.  Drives fine, no perceived lost of power.

Thinking it could be the MAF.   It has the common 3 wire MAF, Siemens 5WK9605 used on all of the M52TUB's and many M54's.  A new VDO MAF is $350, so really want to know if it is the MAF before buying one.

So, how do you test a MAF?

Remember we are looking at a 5-6% error at 3,000 rpm when doing 100 kmph.

[BM WORLD 1283]

vac leaks ? hoses , pcv , seals etc  lots of places to look  

[NZ00Z3 187]

1 hour ago, BM WORLD said:

vac leaks ? hoses , pcv , seals etc  lots of places to look  

If it was a lean condition, I would agree with you Brent.

Its a rich condition, so not enough air, too much fuel or a stuffed sensor fooling the DME into thinking its one of the first two.

As the problem is present on both banks, fuel trims come to near zero when the car is idled, I looking at sensors more than leaking injectors etc.

[BM WORLD 1283]

17 minutes ago, NZ00Z3 said:

If it was a lean condition, I would agree with you Brent.

Its a rich condition, so not enough air, too much fuel or a stuffed sensor fooling the DME into thinking its one of the first two.

As the problem is present on both banks, fuel trims come to near zero when the car is idled, I looking at sensors more than leaking injectors etc.

ok 

also check fuel reg and vac hose going to it 

[dirtydoogle 383]

Are you able to view and plot the maf readings? That's usually the "easy" way 

Should be reasonably linear under low load (in nuetral) 

I would expect to see above 3.5 gm/s and below 4.5 at idle

If you have a non linear deviation then the fun starts 

[NZ00Z3 187]

MAF testing is difficult to understand

 

 

 

Edited March 25, 2021 by NZ00Z3

[BreakMyWindow 1874]

If you have inpa you can check the maf’s performance in terms of how many kg’s of air is registered by it being drawn in to the cylinders. I have no clue how much that is for a b20tu engine. If the maf is bad I would think you’d get some noticeable symptoms and not just slight fuel enrichment. The engine would run better with the MAF unplugged than it were plugged in if it’s bad as well.

[dirtydoogle 383]

That manual plot looks pretty decent, airflow deviations in the midrange like  that would be consistent with vanos position 

Are you able to graph the o2 voltage and injector duty? 

I would certainly be doing a smoke test on the car, purely due to its age