LargestAl

2004 Suburban with the 5.3 flex fuel, but I only run gas. Just a daily driver.

It has been throwing a P0300 code on and off for about a year. All the basics have been changed - plugs, wires, O2 sensors, intake gasket (including a light sanding to flatten the mating surface), fuel filter, fuel pump. Swapping around ignition coils does not make a difference. A few new injectors, and moving them around - no difference.

The only thing that seems to be consistent, is that once it switches from open loop fueling to closed loop fueling, it idles horrible. When scanned, All cylinders show a little misfire but 2 and 5 are off the charts. 8 shows none.

The front O2 sensors seem to be OK, as their voltage is fluctuating within range, and the fuel trim is near 0. The rear O2 sensors are right on the specifications.

The plugs look to be running slightly lean, but not bad. No carbon buildup in heads, no dirt in manifold, Cylinder compression good.

I am at my wits end and am looking for any and all suggestions. HELP!

Thanks in advance.

repairman54

I'd look for harness issues to the coils and injectors. Electronic cleaner / lubricant in all the connectors. Check for poor grounds at the PCM also.
Do a compression test across all 8 just to be sure to see where the motors at.
Did you use AC plugs? Others can cause issues especially the ''trick'' ones.

LargestAl

Thanks for replying.
Plugs were changed/swapped/tested/gapped. No changes. I will do another compression test, as I lost the paper from a week or two ago.

The plastic on enough connectors has been degraded, so I'll build a new harness.

For some unknown reason, did I tighten the knock sensors when I replaced the valley pan gasket? Oh well, time to tear it apart again.