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Discussion Starter · #1 ·
As I understand, the MAF can go bad, and eventually cause engine damage.

Is it because the MAF gets dirty, and so it underestimates the amount of air going into the engine? So the mixture gets leaned out excessively?

And so with this situation, with a failing MAF, what readings would you see in terms of long-term fuel trim on the Tech 2?

Would you see negative or positive readings?

Thanks
 

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Typically bad/worn MAF reads too low, so fuel trim goes lean.
 

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T7 is air volume controlled, so air volume determines amount of fuel.
 

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Fuel trim describes the correction the ECM applies based on the O2 sensor's readings. When the MAF gets old, it tends to read lean, and the ECM will correct with a positive fuel trim to make up the difference.

T7 is smarter than many fuel injection systems, and uses a MAP sensor plus a mathematical calculation to help determine if the MAF is providing accurate information or not. This calculation shows up as "MAF Deviation," a value that shows the difference between what the MAF said and the ECM calculated.

If either of these values - fuel trims or deviation - get too high, the CEL will be triggered.
 

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Discussion Starter · #7 ·
Ah, OK!

I have no CEL but I read somewhere you can use the Tech 2 for clues as to when the MAF sensor might be getting tired.

So therefore I should look for an increase in the long-term fuel trim, and/or an increase of the "MAF Deviation", correct?

Any ballpark figures on when I might want to get a new MAF? Engine runs great but I wouldn't want a tired MAF to grenade it!

Thanks for the insights!
 

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Any ballpark figures on when I might want to get a new MAF? Engine runs great but I wouldn't want a tired MAF to grenade it!
I think you answered your own question there.

I don't know what your car is, but I presume it's a T7 9-3. Check the parts listings and see what years the same MAF might have been used in 9-5s. A used MAF from a junkyard car might be a decent enough short, and maybe long, term solution.
 

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I don't think relying on fuel trim is a useful way to examine the MAF... Lots of variables affect fuel trim, and neither "long term" nor "short term" mean exactly what you think. Assuming no vacuum leaks, MAF Deviation is the way to guestimate MAF health.

The problem is course is that the MAF will grow more erroneous the higher the air flow, so constructing a plan to generate useful MAF deviation values is not as straightforward as it may seem.
 

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I don't know what the "window" for MAF-D is, how that value is calculated. I'm sure the information exists somewhere, but knowing what it is would be key to developing rules rather than its intended use as a troubleshooting aid. I'd assume. IDK.
 

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Curious now:
ok so generally speaking based on the above a potentially bad MAF results in lean fuel trim, so what causes the opposite ie having a fuel-rich environment - is it bad O2 sensors or something associated with air intake/vacuum leak?
 

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Curious now:
ok so generally speaking based on the above a potentially bad MAF results in lean fuel trim, so what causes the opposite ie having a fuel-rich environment - is it bad O2 sensors or something associated with air intake/vacuum leak?
That's one possible scenario, yes.
 

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@EdT It's a 2002 with 132K miles, not sure if the MAF is original.
@jvanabra Thanks, I'll see what the MAF deviation reading is on mine under different circumstances.
I've never had reason to change a MAF, either in my NG900 which made it almost to 250,000 miles, or my 9-3 which is aroun 150,000 miles. Also, I always use OEM quality air filters and the stock airbox. No K&N, open air cone filters, whatever. Also, my engine is stock, not tuned.

Other people consider MAF a maintenance item. Their operating environment may be different.
 
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