Thank you in advance.

Have an '03 Ford F150 4.2L with a rough idle once vehicle warms up. Did have a vacuum leak and fixed that but I still have a misfire in cylinder 6. Fault codes are P0306, P0316, & P0174. Read a lot of articles in regards to a vacuum leak but only for codes with both P0174 & P0171 faults codes but I only have one.

Checked if it was a spark plug. Pulled it out and tested. Good spark. Now stumped. Any help is greatly appreciated.

 

The misfire could be a plug wire or the coil pack, if you have more than 60k or so on the spark plugs, change them. The lean code may be the need for new plenum gaskets and isolator bolts if you can't find a vacuum leak anywhere else. Could also be an exhaust leak on that side before the O2 sensor.

 

Shouldn't the misfire continue to get faster with acceleration if it was a plenum seal and/or isolator bolts. Little new to solving misfires. It also misfires when coasting but seems to disappear when accelerating no matter how slow or how fast I accelerate. Don't want to start changing stuff until I can isolate the actual problem.

 

The plenum gaskets and isolator bolts don't cause a miss per se, but when they went on mine the idle was a bit rough and it tripped a lean code. Your actual misfire is something else, most likely ignition related. I think you have a combination of issues - a vacuum leak and an ignition breakdown. I've had to replace the coil pack twice, it was causing a misfire.

Like I said, over 60k on the spark plugs, change them - and if the plug wires are original they are due too. Recommend Motorcraft, rockauto.com has them at a fraction of the price of the dealer or a parts store.