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book question: page 14 (MAF Engine With Vacuum Leak)

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01 Apr 2021 20:38 #47913 by edbi
I have a question about the book: on page 14 (MAF Engine With Vacuum Leak), when you create the vacuum leak I understand why the O2 flat-lines (lean due to unmetered air) and the STFT starts climbing and the LTFT eventually climbs and the RPM goes crazy but shouldn't the MAF signal go low rather than high (as less air would be getting sucked through it)?
Thanks in advance,
Ed

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10 Apr 2021 20:25 #48130 by juergen.scholl
Replied by juergen.scholl on topic book question: page 14 (MAF Engine With Vacuum Leak)
In theory your assumption should hold true....

Looking carefully at the graphed data you'll find that the IAC counts rise at the point in time when the vacuum leak is introduced. The graph of the air volume measured by the MAF is almost a mirror image of the IAC counts and this may explain the elevated air mass reading.

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13 Apr 2021 05:48 #48190 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic book question: page 14 (MAF Engine With Vacuum Leak)
That is pretty well what should happen with a MAF but immediately after you introduce an air leak the IACV will initially be trying to compensate by letting more in to increase RPM to prevent stalling.

The engine computer can get really confused with large vacuum leaks causeing the ide RPM to "hunt" up and down at least until the LTF kicks in. Some engine computers are much better at compensating than others.

On MAP engines a large vacuum leak can cause severe hunting, if the normal idle RPM is say 800 the idle speed with a vacuum leak can jump can continuously jump quite violently between say 400 rpm and 1500 rpm and back

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