This is an extensive history, so I'll provide a simple and straightforward summary, noting specific OBDII readings throughout.

I purchased a 2004 Chevy Silverado 1500 Z71 (LM7 engine) a few years ago. It had 216k miles, and although well-used, it ran smoothly. Before two long-distance trips, I replaced various parts, including the AC accumulator, AC compressor, battery, radiator, rotors, brakes, struts, and transmission accumulator.

After the trips, the truck began to run rough, with poor gas mileage and difficulty accelerating past 60 mph. I replaced several parts, such as the water pump, fan clutch, and thermostat.

A few diagnostic codes led me to replace the catalytic converter on bank 2 and the evaporative purge on the intake manifold. However, the poor performance persisted.

[OBDII: STFT's varied as normal, LTFT's consistently around 10; B1S1 Varied as Normal, B1S2 Consistently around 600MV, B2S1 Varies as Normal, B2S2 Consistently around 100MV to 200MV]

I resolved a ticking noise by installing exhaust manifold clamps, but cylinder 7 remains clampless due to space constraints. I used various resources, like a Xtool D9 computer reader and Identifix Vehicle Repair, to replace all grounds and install two oil catch cans.

Replacing the PCM did not fix the issue. I removed and cleaned the intake manifold, head covers, and gaskets. I also replaced knock sensors, coils, spark plugs, wires, battery, throttle body, camshaft position sensor, and oil pressure sensor. Still, the problem persisted.

[OBDII: STFT's varied as normal, LTFT's consistently around 10; B1S1 Varied as Normal, B1S2 Consistently around 600MV, B2S1 Varies as Normal, B2S2 Consistently around 100MV to 200MV]

I discovered a grounding issue and fixed it by replacing and moving two ground wires. I also corrected a backward relay. Despite these fixes, the main issue remained unresolved.

An exhaust shop checked the Bank 1 catalytic converter for clogs and found it clean. Numerous parts were replaced, some multiple times. The OBDII readings remain the same.

The truck occasionally doesn't start until the second crank, which led me to replace the crankshaft, fuel pump, and fuel pressure sensor. Fuel pressure is at 60 psi, but I haven't tested it under load. The coolant is steady and bright orange. Oil pressure varies between 30 and 40.

The timing chain has minimal slack, and cylinder compression readings are as follows:

1-170, 2-165, 3-175, 4-175 5-170, 6-160, 7-170, 8-170

The engine is smooth and hums at idle under load. The transmission shifts hard occasionally. There's a slight whistling noise from the rear, possibly due to the rear differential.

The front differential leaks slowly, and the transfer case is clean. Front 4-wheel drive grinds while turning when activated.

I completed a test for the pedal and throttle body and found them in agreement, although I haven't tested them under load.

 

What brand of ignition coils and spark plugs did you use? These engines are picky as to what parts they like.

Also, where did you purchase the ignition coils and spark plugs?

 

Didn't see that you checked or replaced the fuel filter. Possibly restricting flow at higher load, just a thought.