New Replacement MAF faulty ?

[DH106]

Hi

I've a 98 A3 18T (AGU engine) that's normally a very smooth drive but recently developed irratic idling and poorer MPG. There was some very useful advice on this forum about disconnecting the MAF so the ECU reverts to inbuilt defaults - so I did this and the idling was noticably better. So I used VAG-Com and sure enough there was a MAF low/ intermittant signal fault code.

I bought a new MAF to replace the original one (original was Bosch part 0280 217 117). Unfortunately I didn't buy an OEM MAF but this one states it is compatible with that Bosch part no. and is pretty much identical to look at. The connector is the same 4 wire one.

On fitting the new MAF, the engine behaved terribly, idling up and down before stalling after 10secs or so. A little accelerator will keep it running - but boy, it's dog rough :-( Disconnecting it reverts immediately to the MUCH better but not ideal ECU defaults.

So I connected VAG-Com again and had a look at Measuring Block 002 and logged the mass flow at idle, then revved the engine (in neutral) to 2000 and 3000 RPM to log air flow at these RPM. Then repeated this logging with MAF disconnected. The results were that both were similar at idle (~900RPM) at about 3.3g/s but at 2000 and 3000RPM the values with the new MAF connected are MUCH higher: at 2000RPM No MAF=6.4g/s, MAF=~14g/s and at 3000RPM: No MAF=9.5g/s, MAF=~25g/s

Can anyone advise what good ball-park figures should be ? Seems to me the ECU's defaults (i.e. MAF disconnected) are much nearer the mark due to the engine being much smoother (almost 'normal') than with the MAF connected. It's pretty much undrivable with this MAF connected. Is this new MAF faulty - or is there some calibration procedure I need to do? If I'm reading things right the MAF seems to be greatly over-reading the air flow so presumably the engine is running way over rich with it connected.

Any advice appreciated !

[kenney]

The values of the sensor should be measured by driving in 3rd gear uphill with full throttle 870...1000 mg/H.You achieve nothing by disconnecting the sensor other than log a fault code in the ECU.A golden rule use only original VAG electronical parts and i fancy that is where your problem is

[DH106]

Thanks for the quick reply Kenny.
I suspect you're right about only using genuine VAG components - but I need to prove this MAF I have is faulty so I can get a refund.

I take your point about the VAG-Com MAF reading should be done in 3rd uphill etc. but the car is barely drivable with the MAF connected.

My logic for disconnecting the MAF and comparing was that I felt the ECU's default values should be at least 'ball-park' correct and should compare within a range to what you'd expect with the MAF connected. That's not what I get however - the values are way different above idle.

I'll have to try do this test as you say in 3rd uphil at the right RPM at WOT. But I'll have to borrow a laptop with a battery that works so I can ditch the extension lead !