98 k2500 bad fuel economy no power

[turbotrix2004]

ok so I have a 1998 gmc k2500 with a new vortec 5.7 and rebuilt 4l80. Problem I’m having is it has no pulling power, constantly downshifting, and terrible on fuel. I just blew through a full tank of gas on a 176 mile trip. Tires are 265/65/17, fuel pump is new along with filter and all sensors on engine and upgraded the spider injection as well as well as gutted cats. No codes, but exhaust smells a bit rich. Any help would be greatly appreciated

 

[turbotrix2004]

No ideas??

 

[big_mike]

What brand pump and spider? I have heard some are no better than failing stock units.

With fresh motor, pump, MPFI spider and all new sensors it should be strong.

Have you checked the fuel pressure just to triple check? What about ignition components? Gutting or removing cats will give off a stronger fuel smell so that shouldn't be a issue. I'm guessing the front O2's are working since you have no codes which means A/F should be good too.

Was that 4L80 rebuild done by a credible shop? Those have a little more drag and run a little higher RPM than a 4L60 which would use more gas. If you towed something that entire tank that will also lower the MPG per tank quote a bit depending on roads and load. Also what gear ratio do you have stock as that can really effect MPG and pulling capability when towing.

 

[Supercharged111]

Is it slower than before all the work? These trucks need a few mods to run worth a damn.

 

[turbotrix2004]

I got this truck with a bad motor so I don’t know what it was like before, but I had a 96 k1500 with a 5.7 that would run circles around this truck with a load. All parts were replaced with AC Delco or equivalent. As far as the gearing I don’t know, the glove box door is not the original. I’ve taken it to the shop several times and they tell me fuel pressure is good. I don’t get I dumped lots of money into this thing and it’s still not right

 

[randomguy]

Did you put O2 simulators in or tune out the cats? If not and you’re not getting a code that’s the first clue there may be an issue with the PCM.

Something like an Ultra Gauge would allow you to see sensor voltages, timing, and closed/open loop operation while driving. For example if the truck never enters closed loop that could explain your gas mileage.

 

[Supercharged111]

Closed loop isn't that significant for mileage and rear O2s have no impact on performance or fueling.

 

[Anthony Hawes]

would love to here more I am having same issue with new GM crate motor all the bells and whistles as far as stock goes. Added headers and did a 411 pcm swap and its still a dog getting about 11-12 mpg with no power, I mean I can barely parking brake the damn thing. also had a 97 1/2 ton 5.7 and that thing ran like a ***** ape! Please someone help us ouy

 

[Iowan]

Sure you don't have a Ford engine in there?

 

[RichLo]

I mean I can barely parking brake the damn thing.

What do you mean by this?