Who knew the MAF had such power?
 

Had an interesting fault over the last couple of weeks which lead to a new appreciation for what my MAF does:

It started one day driving home, it was just after I had discovered the kickdown switch on the auto and had been busy playing with that!
I pulled out of a side road onto the local 'highway' (Burwood Highway is just a glorified main road - 6 lanes in this particular location with 80kmh (50mph) speed limit) and booted it a bit at about 3/4 throttle.
At about 75kmh I was anticipating the change into 4th when "Thump"...

I backed off immediately and assessed the situation.
Car was running Ok. I saw a rev drop indicating change to (probably) 5th. 'D' was still illuminated normally in the bottom of the DIS (so no limp mode). Then the ABS and ESP lights came on and stayed on.

This all happened in a couple of seconds and I was still rolling along somewhere between 70 and 75 kmh, so I gingerly pressed the throttle and it seemed Ok, so I continued driving and other than feeling a bit 'doughy' it performed Ok for the rest of the journey home.

So I scan the car and look at the ABS module and get the fault: 18265 (P1857) Error Message from ECU.
The TCU gives the same code.
Rosstech tells me to look at faults in the ECU (der!).
The ECU gives the fault: 16486 (P0102) Mass Air Flow Sensor (G70) Signal too low

It turns out the signal from the MAF is not only used for fueling, the ECU uses the data to estimate engine loading (and therefore torque) and feeds this data to the TCU to optimise transmission shifts and to the ABS module for traction control.

I had no idea the MAF had such functionality!

So I drive the car for two weeks with the faults only using it for local journeys (mainly for fueling, couldn't care less about the ESP or ABS! ...However did we cope before :rolleyes:).

The new MAF takes about 20 mins to fit once it arrives, start the car and the ABS/ESP lights finally extinguish!
On driving since, you can certainly feel the difference in how the transmission functions - with the faulty MAF the transmission was softer or doughy in its shifts. With the MAF replaced, the shifts are crisper. Only someone in tune with the car could tell though.
The car also starts much easier - it was stumbling into life with the faulty MAF, taking more turns to fire than normal and only achieving a slow idle. If I tried to engage gear straight away, it would stumble and try to stall.
With the new MAF, it starts much quicker and revs through to about 1200rpm when it fires before settling at about 900. Doesn't stall if gear is engaged immediately either!

So there you go, seems the MAF is important after all!

Yeah it can be surprising how dependent the ECUs are on the MAF for things to work properly. It doesn't always log a fault code either - mine had a lumpy idle because of the MAF giving bad data but it was quite tricky to track down because at a glance the figures looked plausible. In the end, turning off the O2 sensors and forcing it to run open-loop proved the point. Interestingly, unplugging it and triggering a code will cause it to ignore the MAP until the code is cleared, even if it's plugged back in.

Changing the MAF to cure bad shifting was a trick amongst Audi master techs back in the days when D2s were still going through the dealerships.

That reminds me, I haven't cleared the codes yet - job for Friday (day off)!

CRC make a MAF cleaner spray, which I had good results with +++

Yeah, got some!

Didn't work in this case!

Yeah maf is very important, on some of theprevious cars i've had on rolling road days, power figures are down, change the maf & boom! back into the land of responsiveness!

Isopropyl alcohol is said to work. Back when we were all on Tyresmoke forums we used to fill a zip bag with some Isopropyl, dip just the MAF element in it overnight, let it dry out for a few hours. I don't know how much that helped.

Ideally I would want to scrub the metal element with a tiny glass fibre brush to remove any surface oxidation/deposits, but there is not much room to get in there.
Of course if some other part of the MAF fails then the above is not going to help.

I tend to view the MAF like the battery; once it starts to go wrong you "might" have some luck with reinvigorating it, but best to just fit a good-quality new one.

My understanding is that a bad MAF can knacker the gearbox

It certainly contributed to mine, although my gearbox failed at 118k which is within the levels they tend to go at anyway, but I do believe that the MAF fault hastened it as it feeds duff information to the TCU which then seems to cause failure sooner, its one of the reasons I went out and got VAGcom so that I could check it regularly +++