I've started doing some logging, new to this so forgive me if this is a daft question.

I took my Fabia vRS for a good burn, engine hot and logged the MAF requested over MAF actual during a 3rd gear WOT... I've noticed the MAF actual doesn't really line up that well with the specified, also, can someone explain why the MAF requested flat lines?

Any advice is greatly appreciated, thanks.

Phil.

Green = Specified (Requested)

Yellowy White = Actual

Edited 1 October, 201114 yr by pyromorph

The MAF requested flatlines because the amount requested is based on throttle position, pretty much. If you have your foot to the floor, that flatline sticks to the amount of air that's actually needed, but because the MAF doesn't control how much air is going in (it just measures, the control is done by the turbo spinning faster/slower) it never matches up perfectly. The ECU has to wait for the turbo to spin up, pulling in air, wait for the MAF to signal that there's enough air going in to meet the requested figure, then the ECU changes the turbo vanes to slow the turbo down but because it's a very fast spinning wheel there's some inertia and it keeps spinning briefly. You'll see the same if you run a boost log, boost requested will hit a flatline but actual boost will overshoot and then drop down to match.

The only parts of that graph I'd be very worried about is about halfway through your WOT run when suddenly the MAF Actual figure dies right off. MAF Requested is still topped out so I assume you still had your foot to the floor at that point, in which case your MAF should have been reading way more than it does. Also the bit at the end where the requested goes up, but actual stays virtually the same. The other parts where it doesn't match it actually looks like it might be pretty close, it's hard to tell with no scale on the graph how badly it's out. I reckon it'd be worth trying a different MAF if you can swap with someone though, it looks like yours might be slowly on its way out IMO.

The MAF requested flatlines because the amount requested is based on throttle position, pretty much. If you have your foot to the floor, that flatline sticks to the amount of air that's actually needed, but because the MAF doesn't control how much air is going in (it just measures, the control is done by the turbo spinning faster/slower) it never matches up perfectly. The ECU has to wait for the turbo to spin up, pulling in air, wait for the MAF to signal that there's enough air going in to meet the requested figure, then the ECU changes the turbo vanes to slow the turbo down but because it's a very fast spinning wheel there's some inertia and it keeps spinning briefly. You'll see the same if you run a boost log, boost requested will hit a flatline but actual boost will overshoot and then drop down to match.

Thanks for the info, that's very helpful.

The only parts of that graph I'd be very worried about is about halfway through your WOT run when suddenly the MAF Actual figure dies right off. MAF Requested is still topped out so I assume you still had your foot to the floor at that point, in which case your MAF should have been reading way more than it does. Also the bit at the end where the requested goes up, but actual stays virtually the same. The other parts where it doesn't match it actually looks like it might be pretty close, it's hard to tell with no scale on the graph how badly it's out. I reckon it'd be worth trying a different MAF if you can swap with someone though, it looks like yours might be slowly on its way out IMO.

The car has done 102k on it's original MAF. I suspect the MAF is a bit iffy, but I wanted to see if I could "prove" it by logging... I might try another, get the engine hot and try again. I'll post the graphs back once done.

Thanks for the help.

Phil.