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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
    Commented Jun 27, 2017 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 ♦
    Commented Jun 27, 2017 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
    Commented Jun 28, 2017 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
    Commented Jun 28, 2017 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
    Commented Jun 28, 2017 at 21:39

2 Answers 2

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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

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  • Yes, that is what it was. The lifter shifts to shut it off, but then can't unshift or vice versa. Then the engine beats the thing to pieces as it clickety clacks away. The advantage of the AFM is so small that it is really a dumb feature on a GREAT engine. By reverting it to the older version with the cam and rewrite of the PCM code, it works like a champ.
    – JDubya
    Commented Feb 17, 2021 at 13:46
  • However, my experience has been that 'the problem is not solved' by replacing them - they just run for a short time and faila again because the real cause is not overcome - I do not know what the real cause is, but the real fix is to get rid of that kind of lifter. The engine is very efficient and gets almost as good mileage without ever going into the 4 cylinder mode. I have a suspicion that more problems are caused by the idiocy of the limp mode than anything else.
    – JDubya
    Commented Feb 17, 2021 at 14:35
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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.

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  • How can there be excessive fuel pressure on a single injector when all injectors are fed by a common rail?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented 2 days ago

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