2007 Suburban 1500 4x4 with 5.3 L flex fuel LTZ 267,000+ miles. Having trouble with running rough and misfire. The ODB II indicates P0307.
Checked wires - replaced though nothing is wrong, no change Checked spark output with spark detector - solid steady spark Plug looked wet, but with oil Checked oil, 1 qt too high so removed 1 qt - no affect solid misfire.
Listened to all 8 injectors - all sound the same Pulled all 8 spark plugs and commenced compression check on #7 - over 300 psi Cylinder was overloaded with fuel and spewed gas out of pressure release of the tester.
Ran the engine starter with all 8 plugs out until gas quit coming out of #7 Compression check #7 = 145 psi, #1 = 150 psi, remainder were 185 - 192 psi Reinstalled plugs after burning oil off of #7 plug with #1 in #7 position. Restart has solid misfire on #7
Remove power plug from spark generator for #7 - no affect, tried several others and they all made obvious difference in operation, but no change for #7
Pulled rail and interchanged #6 and #7 injectors (inspection has nothing obviously wrong), put back together and started engine - solid misfire on #7 only - tested by pulling connectors on spark generator of each.
Wires are new, plugs are 8,000 miles old, #7 cylinder had AFM lifter stuck and was replaced 8,000 miles ago as stop-gap measure to keep daughter's car running.
Have 10W-30 on last change and someone added two quarts when it only needed 1/2 qt. I removed a quart so the dipstick now shows at the top of the crosshatch.
Any idea what could be wrong? I am planning to pay to have someone put a new type of testing of equipment on to see what is happening. What should they be looking for?