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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?

Thanks!

JDubya
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Yes i can tell you exactly what the problem is you have bad lifters in the cylinder that is misfiring. Flex Fuel engines in the Suburban Tahoes Yukons have dift lifters This 5.3 engine is designed to run on 4 cylinders mostly in the cruise mode.All so these lifters are notorious for collapsing or sticking you must replace them problem solved

John Shipman
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The main issue could be related to excessive fuel pressure in cylinder 7 or a malfunction in the injector system or intake/exhaust valves, especially given the history of the AFM lifter sticking in cylinder 7. The variation in compression pressures across the cylinders and the presence of excess fuel in 7 suggest a possible injector leak, over-fueling, or valve malfunction. Since swapping injectors and spark plugs did not resolve the problem, it is recommended to perform a cylinder leak-down test, inspect the valves more thoroughly, and analyze sensor data using advanced diagnostic equipment to identify the exact cause.

mostafa
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