MAF symptom

I meant to post this a while back in hopes it could be helpful to others.
My 03 alh was having this issue where occasionally it would stall hard on takeoff as though I had dumped the clutch. That became a little more clearly a total lack of very low end/ rpm power, then it faltered a few times on upshift. unplugged the maf and it was better. put in another MAF and it's been fine for miles.

Was there a CEL?

The easiest way to diagnose is to unplug to see if the engine runs better.

The best way to diagnose is to log the requested vs actual MAF and see if the actual keeps up to requested.

That isn't every detail, but in general gets you started, if not all the way there.

Cheers,

PH

JB05 said:

Was there a CEL?

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no never got CEL. It got bad enough one day that i nearly couldn't get in motion without stalling - unplugged the MAF and it was all better on a short drive so I put in a spare used MAF and it's been good since

Classic failed MAF is the tachometer will not go past 3000 rpm. This happens because with no MAF the default value is 500 at full throttle instead of 850. No CEL is unusual for a MAF failure

OP does your car still have a functional EGR ? A stuck open EGR will cause severe driveability problems

Powder Hound said:

The easiest way to diagnose is to unplug to see if the engine runs better.

The best way to diagnose is to log the requested vs actual MAF and see if the actual keeps up to requested.

That isn't every detail, but in general gets you started, if not all the way there.

Cheers,

PH

Click to expand...

How can you log the requested vs the actual MAF?

With a VCDS, you can start logging then select the data values. I'd have to look at the interface for a while to remember. I don't use mine often enough and every time it is a 'new' experience for some features.

IIRC, there is a small 'log' button on the lower right when you're looking at measuring blocks. Find the MAF items, then log them while the engine is running. Driving is best. You can do the drive, then check the log. Never while driving, of course. Having a friend check the log while you're driving is great.

What you would look for in a bad MAF is to drive, punching it then letting off the go-pedal. The requested MAF will spike up, but the actual will never catch up unless you go down to idle. The bigger the gap between the two, the worse your MAF has failed. A good MAF won't lag much and will catch the requested very quickly.

Cheers,

PH

Curious Chris said:

Classic failed MAF is the tachometer will not go past 3000 rpm. This happens because with no MAF the default value is 500 at full throttle instead of 850. No CEL is unusual for a MAF failure

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HI,

I have not had the low RPM issues BUT if I try to push past about 3300 RPM if feels like power is cut back so I let off ....

Often times there is not a check engine light but possibly once or twice there was BUT its usually an over boost code (I cant remember but maybe a po 234 ?).

Anyways, is that still a result of a bad MAF sensor ?

Should I check it by unplugging the MAF and trying to rev past 3200 ?

Or if the default value is 500 VS 850 would tha not allow me to rev high anyways?

Thanks

Andrew

I'm driving with one unplugged on one of mine right now due to another issue. With all else being normal, unplugging the MAF leaves me with a car that is low power until I go past 3k rpms, after which it seems to wake up. It is better with a fully functioning MAF, but it is driveable in its current state.

Best bet for answering your questions is to get a VCDS and watch the requested vs actual on the air mass sensor (group 3?). If your MAF is unplugged, the actual will display 550 or so and never move. Plugged in, if everything else is working well, the actual should be very close and never far behind requested.

If the MAF is failing, the actual will probably lag, and it might not ever catch up when the revs are high. But that could also be explained by, say, a boost leak.

So you can have a lot of fun when trying to sort this problem out. However, if you know the other parts of the air charging system (turbocharger, N75, vacuum pump, vacuum lines, turbo actuator, intercooler, intake manifold, EGR, and anything I haven't thought of) are working perfectly (or close to it) then your problem boils down to comparing actual vs requested on the air mass sensor. A lot of variance, then replace the MAF.

Cheers,

PH