ReallySatisfies

Hey all,

I've got a '99 Sierra z71 5.3 that I've been trying to track down a gas mileage issue with. I used to average about 17, let it sit for about a year, started it once or twice per month. Just recently put it back into service and now get 12 mpg. Short of a slight stumble at idle it runs great. Truck has 140k miles.

I've replaced:
plugs with NGK-TR55
New barrel type ignition coils
MSD plug wires
EGR valve cleaned and solenoid checked, no issues
Air filter cleaned and oiled
Throttle body removed and cleaned
Intake cleaned
PCV replaced
O2 sensors replaced with NGK
New 28lb fuel injectors
Fuel filter replaced
Fuel Pressure regulator checked, no leaking diaghram
MAF cleaned
All fluids replaced with synthetic

I don't drive with a heavy foot. Short of new cats, I don't know what's left to check. Any ideas would be much appreciated.

How can the cats be checked?

Also, how to check for misfire?

Anyone seen a significant drop with the "up to 10% ethanol" put in the regular gas?

Thanks for any help

Chevy_King1500

iTrader: (7)

did you have it tuned for the bigger injectors? you can pressure test the cats to see if they are good.??

1Topgun

iTrader: (3)

Stock 5.3 injectors are ~24.5, so if you didn't have your PCM injector flow rate table revised for the new 28 lb'ers, it's running rich and mpg has dropped.

04Burban

Learn something new everyday!

ReallySatisfies

Forgot to mention those injectors just went in this morning, so not the source, but when I see the next drop I'll know why. They did clean up the idle though.

How is the pressure test on the cats done?

Thanks!

04Burban

Not to thread-jack, but how does that 4# help? it seems almost pointless to go up 4#
(asking in a teach me kinda way not a smart a$$ kinda way)

ReallySatisfies

I didn't really buy them for the upgrade, got them because they were 1/3 the cost of having my old ones cleaned, gotta love ebay.

04Burban

Sounds good.


So you're still having major issues AFTER replacing the gunky injectors??

ReallySatisfies

That's my latest test, I just put them in this morning, won't know how they're doing until I get another tank through. Looks promising though, really smoothed out the idle, if it weren't for a random slight stumble you wouldn't know it was running inside.

04Burban

Well run that and tell us how it goes. I would also pick up some seafoam or Z-Max and run it through one good time to clean all the other stuff...