Nightmare P0306 To A P0300 Code

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.

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.