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GM truck running very rich

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Well, the CEL did appear after getting rid of the plug defoulers. So I had new cats welded in this week.

CEL is gone, horrible running rich smell is gone, but the bad MPG seems to be hanging around. Still on the first full tank of my regular route driving but seems to be sucking it down, much more than the 5.3 motor should. Will update again when I have an actual MPG from a full tank refill.
 
When you say 'much more than the 5.3 should'...what's your benchmark? Have you driven the same size truck with the same size motor?

Your numbers really aren't that far off the 'baseline' EPA numbers. A little weight in the back...a higher mileage engine...any number of things could explain the difference, up to and including driving habits (not even necessarily just the way you drive, but where you drive).
 
phucheneh said:
When you say 'much more than the 5.3 should'...what's your benchmark? Have you driven the same size truck with the same size motor?
Click to expand...

This will be my third full sized GM truck with the 5.3 and 4sp auto. I drive almost the exact same loop every day and am just kind of tuned into when it should be needing gas. This has a larger tank, when it is close to empty it takes about 27ish gallons to fill as opposed to 22 ish for my other two, yet doesn't not seem to be getting any more miles from a tank of gas.

I will have more concrete numbers after a tank or two of gas. I still have the MPG numbers on my phone from the previous truck to compare.
 
My money is still on some sort of tune that you've got on the ecu. Maybe contact a few dealers and see what they would charge to flash a stock tune. Or maybe some of the aftermarket tuners would do it cheaper?
 
Did we determine if a 'simple' check is possible? I can't remember...I wanna say so, but I don't see it on the previous page.

Meaning, can a GM-specific scan tool be hooked up to it and a software P/N retrieved?

Did you ever post your long term fuel trim numbers? Not a graph...just where they're at right now. It can also help to monitor them during a test drive to see how abrupt the changes are...after a few miles of driving, maybe 10-20 tops, they should be leveling out. Past that, deviation should be very small and gradual. STFT handles...well, short-term stuff. LGFT is more intended to compensate for gradual wear.
 
Its possible that someone out there has written some software to read the ecu parameters given the popularity of the drivetrain. Not sure your average scan tool can tell you anything useful.
 
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