My dad recently got into the truck game. He got a 99, 4.8, 4x4 regular cab step side, 150xxx miles. Only mods are 285/76/16 buckshot mudders and a glass pack muffler. Redneck mods ftw. Im pretty sure this is a 4.10 differential truck as well.

Issues: 14.4 mpg and a very slight misfire around 45mph after the tc locks up.

Engine codes: Fuel system status lean(both banks), catalyist efficiency or something emissions related

Relevant parts replace: New plugs last week(ngk tr5), ac delco fuel pump (within the last year)

Diagnostics tests: Fuel pressure test showed good(58 with regulator unplugged and 45-60 driving under all conditions), Hptuners misfire history on current cyls. 1-8 showed nothing, hptuners showed good fuel trims on 30 minute log(12% or less), knock was detected when misfire was felt.

2 things I noticed: DFCO seemed late to kick in(maybe related to tires), and on a previous mod I think I remember the fuel trims going lean on a deep decel or coming to a stop. Truck also has a low idle. Maybe the fuel trim issue is related to the IAC needing to be repaired/cleaned. Either way, I cannot get it go lean driving and it shouldnt affect mileage.

Where to go from here? Would coil packs and wires be picked up by the scanners misfire? Could false knock be what is causing me to feel the hesitation?

 

False knock could be what you're feeling. What are the duke trims at idle? Could need intake gaskets.

 

Didn't think to check to idle trims. That's a good idea to check that and check for a vacuum leak.

 

wow 14.4 mpg is an issues for a 4x4 ?? lol i have a 2wd 6.0 stock just tune nd 4.10s and it gives me 11-12 mpg lol