Getting cylinder misfire codes, again.

[bugeyed]

04 Trailblazer 4.2 L
A few weeks ago TB started to run very rough & throwing P0302 code, so I stopped & purchased a coil pack. I reset the code so it would throw it again & I would be sure it was Cyl 2. It wouldn't throw a code, but still ran the same. It cleared up after a few days of driving. When changing the oil I decided to swap out Cyl 2 coil pack & cleaned the throttle body. Didn't fix it completely, but it was running a bit better & within a few days it was back to normal. Fast forward 2 weeks & it started over. Rough acceleration & idle. Code was P0301 this time. Cleared code & then it coded again with P0300 & that is consistent now. Since it's random of general misfire I wanted another approach. It dawned on me that the first time I was at about 1/4 tank of gas & the same now, so I filled it up & put in a can of Sea Foam. Thinking there may be water in the gas. I have driven about 45 minutes with no improvement. Started it this morning, cleared code & still running rough, but no new code. BTW Plugs were changed around 66K ago & fuel filter around then too. Any ideas are appreciated.
Thanks,
Kev

 

[Mooseman]

Don't clear the codes. It will give p0300 until it determines which cylinder is misfiring. Maybe #1 now.

 

[bugeyed]

OK, thanks, I am leaving it alone until it gives me more specific info.
Thanks,
Kev

 

[mrrsm]

This Erratic Firing and Running Rough sounds symptomatic of a Vacuum Leak around your Intake Manifold (10 Captured 10 mm Bolts) You might try snugging them all down to ONLY 87 Inch Pounds and then see if things settle down. If the rough idle improved afterwards... it may be that the Loose IM had newly cleared out spaces around its gasket(s) allowing for un-metered air to enter after the Sea-Foam job... making things seem worse as a result. Your vehicle has no Mass Air Flow Sensor...which would be another place to check and clean on later Model TBs and Envoys.