MISSFIRE AT SLIGHT ACCILERATION

[george w]

I have a 2004 gmc 1500 with a 5.3 and under light acceleration around 1500 and going up a slight incline in high gear it will start to misfiring. If I give it more fuel it will stop misfiring and take off and pull right up the incline. When i start away from a stop and give it lots of fuel it will pull hard. It will not misfire unless its at low rpm. Only on a slight incline or when its in high gear and pulling up a hill around 1500 will it miss fire, i have replaced plugs with new A/C PLUGS, NEW WIRES, CHANGED ALL COILS , CLEANED MASS AIR FLOW AND I HAVE CHECKED FOR VACOUM LEAKS AS WELL. I have run a scan tool on it and the fuel pressure is good, 02's are switching good short and long fuel trim are good, no engine codes at all. When monitoring all 8 cylinder's up a hill and it starts to misfire I can't get It to show a glitch on any cyclinder. It seems to be getting worse. 

[Jsdirt]

That's a classic indicator of intake manifold gasket leaks. They can be tough to find. They're usually worse when the engine's cold. I take a water bottle with a hole drilled through the cap, and squirt water along the manifold gasket and listen for RPM changes, or excessive moisture out the tailpipe. Sometimes you can hear the water getting sucked right in.

 

Usually you'll have lean trims at idle that go immediately normal with throttle when this happens. Keep in mind when the engine is cold and in open loop, the o2's are out of the mixture equation.