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Old 12-05-2012, 02:05 PM   #22
Scooby Specialist
Member#: 245218
Join Date: Apr 2010
Chapter/Region: South East
Location: FL
2013 STI Limited


Originally Posted by cmiovino View Post
Positive numbers indicate fuel is being added, aka a lean condition, aka leaks or a poor MAF.

Negative amounts indicate fuel is being taken away... which would mean what mechanically?

Think about this for a moment.... If the MAF is measuring ~20gps but there is an after MAF leak then the trims would go rich because the intake manifold is in vacuum and its sucking in ambient air. The O2's see the fuel mixture as being lean since extra air is getting into the combustion chamber that was not measured by the MAF.

Now lets go into the higher MAF gps values that you would see while in boost. So lets say you start to get into boost and the MAF sees 100gps of air. Since you are in boost and the intake manifold is now pressurized above atmosphere it is not sucking in any extra air, its actually blowing air out of where the leak is. So the MAF reads 100gps, but only 80gps of that metered air makes it into the combustion chamber. The O2 sensor now thinks the car is running rich and starts to pull out fuel.

So in this example the ECU thinks you're lean when in vacuum and rich when getting into boost.

OP needs to check more thoroughly for a boost leak.
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