Can vacuum leak affect fuel mileage? -

4.6L (1996-2004 Modular) Mustang Technical discussions on 1996-2004 4.6 Liter Modular Motors (2V and 4V) within.

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Old 10-18-2011, 09:25 AM   #1
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Location: Texas
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Default Can vacuum leak affect fuel mileage?

I ask this because I am pretty sure that I have a vacuum leak (can hear slight whistle around 1300-1600rpm), given that unsensored air would cause the motor to more than likely run lean would a vacuum leak let in enough air to this?
Then my next question, I have an '02 GT manual with 128,000 miles, just have muffler delete, cold air intake (why I have vacuum leak) and I have gotten 23 and 24 MPG out of my last two fuel tanks. I have no CEL.

I do drive pretty conservatively and a decent amount of highway, but that almost sounds like too good of fuel mileage out of 4.6?
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Old 10-18-2011, 10:46 AM   #2
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The engine will want to run lean but within reason the short term fuel trims will compensate for the unmetered air (assuming the leak is after the MAF)--in any event it needs to be fixed as it is also unfiltered air.

23 to 24 MPG (or better) is about right with conservative highway driving, I get 21.5 routinely with a mix of 10% around town, 20% rural, and 70% on the interstates.

What is a "muffler delete"? You have no mufflers at all?
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Old 10-18-2011, 10:57 AM   #3
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The previous owner had the mufflers replaced with "straight pipes", cheap way to make it loud. Stock manifold, cats, no mufflers and stock tubing/tips.
I plan on side exit soon with shorty headers, cats and mufflers (technically required on all cars in my county)
I plan on adjusting the intake again, since it only starting doing it after I took it off and put it back on, trying to make it not rest on my oil cap. Had no such luck, bad design on this intake.

I'm going to get a K&N ram air in the hopes that the hoses will no longer rest on my oil cap, plus I'm not a fan of the filter being stuffed away in the fender out of sight.
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