Hi guys, I've just about exhausted the internet trying to find out what the problem is. Some help would be much appreciated!

Truck: 2000 GMC Sierra 1500 5.3L z71 Standard cab 107,000 miles(Stock all around)

Issue: Motor misses at idle in park and in gear, but smooths out once truck is moving. Truck feels slow for a 5.3L in a standard cab.

What I've done so far:
Cleaned IAC, TB, MAF, EGR, K&N air filter
Changed motor oil (Mobil1 0w20), tranny filter & fluids, spark plugs (autolite) gapped at 0.05, fuel filter, PCV
FPR works (no fuel in vacuum line, fuel pressure change when vacuum is pulled)
Seafoamed tank & through brake booster vacuum line

Fuel Pressure: (this is a 2000, return style)
52psi with motor off
48psi at idle
45psi revving at idle
can't check at WOT as the gauge cable is not long enough to tape on windshield.

LTFT:
5%~8% at idle
7%~11% at WOT

O2 sensor 2:
0.1v~0.3v in open loop
0.4v~0.8v at idle
0.7v~0.85v at WOT

pre-cat O2 sensors seem to variate uniformly between 0.1v~0.8v

What's you guys' take on this?
Any help would be much appreciated!!

 

does your scantool have misfire graph or something like that. i would try to find the cyl that are missifreering first. you might have a colapesed lifter very comon on these engines. usally takes out the cam causes missfie. find witch cyl if possible go from there it looks like you have check all of the electrical so far it might be mehanical prob. find out witch cyl. then pull that side valve cover check for broken springs.

 

how many miles? i would guess a dirty injector or dead coil. No codes?

 

Thanks for the reply. I don't have a scan tool that can read misfire but I'll try to get hold of one. There's no lifter noise when the motor is warm (knock on wood), and it's stock cam.

I'll have it scanned and lets see what turns out.
Thanks~

 

I've sea foamed the tank as well as through the brake booster vacuum twice last month.

The truck has 107,000 miles on it so far.

No codes.

Thanks~

 

there might be no codes. does thes scanner say no missfires when the truck is runnung and your on misfire graph. the graph usually shows count of missfires on certail cylders. did it show any cly in perticular. it ticks cold for a couple of minutes when cold and then goes away right. you got to see wich cly misfirering or you stuck.

 

Yes I do have the famous piston slap on cold start ups. I haven't been able to locate anyone here that have scanner with misfire graph/log capabilities. Anyone around Raleigh/NC area have one? I've got a pack of cold beers waiting for ya!!

 

check to see if you have leaking intake gaskets. pour water or spray carb cleaner at the base of the intake. If they are leaking you will notice a definite change in the idle.

 

Yea I've tried that, sprayed starter fluid at intake, TB, vacuum hoses, EVAP valves...etc, no vacuum leak.

MAP sensor reads 5~6psi at idle, 9~11 WOT, 3~4 on deceleration, and 14.7 when motor's off.