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post #1 of 20 (permalink) Old 01-10-2010, 08:44 AM Thread Starter
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High Mpg Vs. Fuel Cut Off

I don't know if anyone else has had this issue or would benefit from this experience, but here it goes anyway.

There is a stretch of road on my way to work about 1/2 a mile long. It's a gentle downhill. I can often take this stretch at 55 mph pulling 65-70 mpg (remember I said downhill). Every once in a while the car would feel like it was turning off. The mpg would peg at 99, and the car just seemed to die. If I gave it just a little bit of gas nothing would happen, but jut a little more it would cut back on and be about 60 mpg.

I spoke with the one ASE Certified Master Mechanic at our local dealership last week. He had no idea what it was, but was interested in finding out. I do not know who he talked to at GM about it, but he called me Friday and told me the answer.

Simply stated, I was getting too good mpg along that stretch. He said that the ultra high mpg was triggering the computer to do a fuel shut off. He said this is a rare issue, and only seems to happen in the 5 speed with the XFE "treatment". It's not a problem with the car, and is harmless.

His suggestion was that I pull it out of gear and roll, or just take the 60 mpg and be happy. A Cobalt is simply not ment to cruse at 65-70 mpg.

I just thought this was a little funny.

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post #2 of 20 (permalink) Old 01-10-2010, 09:53 AM
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Haha! yea man ive never seen my MPG's go past 35, but I do alot of city driving.

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post #3 of 20 (permalink) Old 01-10-2010, 11:28 AM
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I dont know what your talking about the car feeling like its "dying," but when your going downhill the inertia keeps the engine going and therefore does not need fuel. That means you aren't using any gas, which will explain the 99mpg's you see in the DIC. If you have a wideband in the car you will notice it go completely lean when your foot is off the gas going downhill.
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post #4 of 20 (permalink) Old 01-10-2010, 11:30 AM
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Quote:
Originally Posted by exiged View Post
I dont know what your talking about the car feeling like its "dying," but when your going downhill the inertia keeps the engine going and therefore does not need fuel. That means you aren't using any gas, which will explain the 99mpg's you see in the DIC. If you have a wideband in the car you will notice it go completely lean when your foot is off the gas going downhill.
x2 exactly what i was going to say

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post #5 of 20 (permalink) Old 01-10-2010, 11:37 AM Thread Starter
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Quote:
Originally Posted by exiged View Post
I dont know what your talking about the car feeling like its "dying," but when your going downhill the inertia keeps the engine going and therefore does not need fuel. That means you aren't using any gas, which will explain the 99mpg's you see in the DIC. If you have a wideband in the car you will notice it go completely lean when your foot is off the gas going downhill.
Welcome to the forum.

Nah, I do not have a wide band. What was going on was that the downward slope is a VERY small grade, so if I let off completely the car slows down almost as much as if you were driving on a level road. It does take some power to keep going, but not much at all. I was on the gas so lightly that the computer was switching the fuel off like I was not on the gas at all even though I was. This is why I could feel the fuel cut off and it seemed like the car was dying.

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post #6 of 20 (permalink) Old 01-10-2010, 11:48 AM
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you have an intake, correct?

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post #7 of 20 (permalink) Old 01-10-2010, 11:50 AM Thread Starter
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Air intake? Well yeah, but it's bone-stock.

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post #8 of 20 (permalink) Old 01-10-2010, 12:06 PM
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your car is bone stock? ported or oversized tb by any chance?

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post #9 of 20 (permalink) Old 01-10-2010, 01:30 PM Thread Starter
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Nope.... BONE-STOCK under hood. Nothing...

I was just bumping the point where the fuel cuts off is all.


Last edited by Will mpg; 01-10-2010 at 02:21 PM.
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post #10 of 20 (permalink) Old 01-10-2010, 04:58 PM
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i made a note about this on my myths about mpg thread



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