Nightmare P0306 To A P0300 Code

Discussion in 'SN95 4.6L Mustang Tech' started by DHASKJR, Jun 8, 2014.

  1. I just bought a 1996 gt 4.6 good shape body but need mechanical work which I thought was going to be easy. I did all the brakes brake lines master cylinder power steering pump and rack. Get it running pretty good with a slight misfire. Check the codes and I get a ton. Bank 1 rich bank 1 lean cyln 6 misfire.I noticed old owner changed out gas tank and left old fuel filter so I changed it finding the gas lines were a mess. The return was bent in half closing off the flow. I fix that mess and the rich code went away. I cleaned the mass air flow sensor and the lean code went away. Only thing left was the p0306 cyln 6 misfire. I changed all plugs and wires didn't go away. Swapped coils in front to see if the cylinder code would change and it didn't. I put in a new fuel injector in 6 fire it up and now Ive got a p0300. Multiple misfire. Now I am at a loss.
  2. First DOUBLE check the spark plug wire routing. Correct wires run to the correct coil pack and spark plug wires. Ensure that all factory stand off and looms have been used.

    Next check to see that the cam sensor is connected. Then have the alternator tested for a bad diode and correct output.

    Next check the CKP sensor to be sure there isn't a loose bolt holding it. Perform a visual inspection of the crank damper. Look for any cracks or signs of excessive "run out".

    Finally, give yourself a pat on the back for finding the cause of the rich DTC in the pinched return line. Great work!

    Once every thing has been checked and the P0300 remains some consideration has to be given to a "base engine" problem. For example excessively worn timing chains or a crack crank damper.
  3. I had similar symptoms and ran through all of the electrical tests (which you should) and ended up diagnosing a cylinder 2 exhaust valve seal leak which was allowing oil to coke up carbon on the valve seat, thereby preventing full compression on that cylinder and throwing the engine out of balance.

    Run a compression *and* cylinder leak down test before replacing any parts that are not confirmed to be bad. The leak down test found my problem, the compression test did not. I pulled the cylinder head off and poured gasoline into the exhaust manifold. It leaked only from cylinder 2 exhaust valve.

    Note: if you end up having to do a valve job on one head you absolutely must do it on the other head as well. I got my engine back together after doing only one head and chased a misfire code to the other bank before pulling that head and doing it all over. The newly refreshed cylinder head needs to be matched on the other side with one that isn't worn.
    trombonedemon likes this.