MAF Flow Rate - dying MAF sensor?

[NPM108]

2.0 TFSI K03 - Stage 2+ with a VWR CAI and Miltek TBE

I've been doing some logging lately and have been noticing my MAF flow rates dropping over time. I apreciate that ambient temperatures play a role so I've been trying to log with consistent air temps.

A Stage 2+ K03 should run at around 270-280bhp so putting that into MAF g/s flow rates should be around 220g/s

I've seen around 205g/s in the past but lately haven't been getting past 188-190g/s under full load 3rd gear close to WOT. Filter in the VWR intake is clean. Coil packs are new as are he plugs, and DV is a new Rev.G that just went in.

No codes with VCDS and logs available. Unplugging the MAF at idle didn't change anything but I didn't try it on the road with the MAF unplugged.

Is it possible to have a dying MAF that is reading slow and slowly reporting less and less flow to the ECU. Both short term and long term fuel trims are -3.5% or there abouts so ECU is removing fuel presumably as airflow is less than expected from the ecu map.

My understanding was that the VWR intake was pretty good and should flow above 200g/s.

Any ideas?

 

[Flexiblemouse]

Can you post the log? Sounds like a bad maf or boost leak but the log should make diagnosis easier

 

[NPM108]

I didn't think a boost leak could affect the MAF readings, so that's good to know as to be honest I've been chasing a suspectd boost leak. As lately I've been unable to hit boost requested, and starting to think my K03 with 80,000 miles on it is giving up! In these logs, it shows around 1.74PSI below requested as the peak but consistently is below requested. I also normally boost to around 20.5PSI but on this run requested didn't go above

 

[Flexiblemouse]

Having a hard time reading the log properly, the formatting is confusing (I guess because I'm on a phone). A boost leak will not affect the MAF except that you will see inconsistent airflow, but that will also mess up the AFR. The air fuel ratio looks OK, and from the boost the car is making your mass readings should be higher. This could indicate the MAF is reading low, but i'd bet that's more to do with your intake than with the sensor itself. If the MAF scaling is not correct then the sensor will not read properly, and in cold weather you will be making more power, not less.

My diagnosis:

Incorrect MAF scaling on new intake causing MAF to read low. Not a big problem as you will still be making the same power, it's just that you can't read it from the MAF any more. Refitting the standard intake should make the readings return to normal.

 

[NPM108]

Thanks flexiblemouse for the analysis. Good to know that by the sounds of things my MAF is fine and it's just a scaling issue. I thought VWR prided themselves in having a correctly scaled MAF housing. The negative fuel trims was starting to make me wonder a bit, but having looked through some old logs my K03 used to be able to hit and exceed boost requested.

So I'm thinking I need to pursue a boost leak issue, possibly from one of the O rings in the charge piping or coming off the turbo. Whilst I am at it I will also check the Turbo downpipe is securely bolted, exhaust manifold, turbo etc. Can a slightly clogged CAT rob me of some boost at all?

Also checked my PCV and that is closing correctly so don't have a boost leak from there.

 

[Flexiblemouse]

The cat should not affect boost but if it was clogged you would lose some power. Regarding the boost leak it does not like a big one, so might be hard to find. Check the N75 and its hoses, especially the one from the turbo. Anything after the turbo won't affect boost, will just be an exhaust leak.

 

[NPM108]

Thanks for the reply . So the VAC hoses on the N75....are there are any other vac hoses that tend to give problems that you know of? I know it's only 1.5PSI or there about's but it's annoying to see the car isn't hitting the requested boost.

 

[Flexiblemouse]

The only hoses I have had problems with are the 3 to and from the N75. The turbo inlet pipe might be worth checking, make sure it is seated properly and not damaged at all. Other than that it's DV, intercooler piping etc

 

[NPM108]

I bought a boost leak test kit that uses compressed air. I'll do a proper test from the turbo discharge pipe, but on a quick low pressure 5psi test connected on my CAI inlet instead of the filter, I found air escaping where the MAF is installed. The rubber seal around the MAF is allowing air out (its screwed down properly), is this normal? When the car is running, does the turbo inlet suck air in or does it flow based on air entering the engine bay??

I still have to check my N75 pipes.

 

[Rick @ Unicorn Motor Dev.]

MAF is never pressurized, so a leak under pressure is often the case when pressurized.

Rick