I'm getting a P0306, engine misfire. on 2003 Chevrolet S10

This is a new truck to me, service unknown. Starts up fine, idles with a slight roughness. Most people wouldn't notice anything. Drives fine until about 2000 RPM, going up a hill, or during acceleration, the check engine light flashes. The check engine does not come during slow driving or during idle. I've given it a complete tune up. Oil, Oil filter, gas filter, spark plugs, wires, rotor, cap, coil. Next I was going to check the compression of cylinder #6. Oh yes, prior to changing the oil I added Seafoam to the crankcase, ran it for a few minutes then changed it.

by in Barkhamsted, CT on December 21, 2011
2 answers 2 comments
ANSWER by on December 21, 2011
Check Engine Light flashing shows a serious misfire is happening in number 6 under load. Usually when its under load like that its ignition related.Inspect the ignition again, with the engine running spray water from a spray bottle on the distributor cap wires and spark plug area, if the insulation of the ignition system is good the engine note should not change. Try swapping the spark plug and wire from number 6 to another cylinder and see if the problem follows to another cylinder. After that you are looking for something specific to that cylinder, an injector or vacuum leak at that runner of the intake manifold.
ANSWER by on December 21, 2011
Customer Concern: Engine has a misfire and setting code P0306. Tests/Procedures: 1. Check the distributor cap for a problem. 2. Check for a defective Crankshaft Position (CKP) Sensor.
COMMENT by on December 23, 2011
Picked up a compression tester yesterday. Cylinder #6 had only 35PSI. Looks like its time for a new head. Thanks Mark L. Dec. 29: Took both heads off the engine and brought them to a local machine shop that was highly recommended. His analysis is as follows. Two to three exhaust valve seats have been damaged. This was evident when the valves were removed. New seats need to be installed and ground. The head gasket appears to be compromised possibly causing the P0306 code. The heads will also be cut to be made flat again. Thanks for the suggestions. Mark L.
COMMENT by on December 23, 2011
You can't assume it is valves until you perform a wet compression test. It good be a ring problem.