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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!

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    have you tried swapping the coil on 7 to a different cylinder? a shorted coil can produce a good looking spark and still cause a misfire. – Ben Jun 27 '17 at 16:29
  • Agree with @Ben ... this could easily just be the coil which is bad. Changing it out with another cylinder will tell the tale for you. Also, that's a sizable difference in cylinder compression. This could be due to the #7 being washed down with gasoline. When all is said and done, you'll want to change your oil, because the gas will have infiltrated considering the amount you were saying was in the cylinder. – Pᴀᴜʟsᴛᴇʀ2 Jun 27 '17 at 21:47
  • Yes, in fact her friend has connections and got a good one to replace it with before I started working on it. He replaced all the wires and as I understood it, replaced each of the coils with the new one and tried it after each with no improvement. – JDubya Jun 28 '17 at 21:17
  • The tester that I have for the spark checker is supposed to be able to determine whether or not the spark is too weak. At least that was the claim. It looks like a spark plug with hollowed end and a big clip in the center. It sparked just fine. – JDubya Jun 28 '17 at 21:21
  • When the first problem occurred 8000 miles ago, I bought an AFM preventer that plugs into the ODB II. It took all of the Stabilitrak and other idiot failures out. Based on those problems, I think that the PCM is questionable. The overfill with oil got the PCM off its dime and started putting the engine into limp mode. I don't know for sure, but that is what it sounds like. The transmission is shifting hard again and P0641 occurred once. No one seems to know what that is about, but it is looking like PCM to me. – JDubya Jun 28 '17 at 21:39

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