97 F150 4.6 unsolved misfires, no more ideas

[dhwort]

This 97 4.6 150 has 150K miles and has been a great engine, no oil burning or leaks or other problems. All of a sudden it starts running rough and throws P0303 and P0308 (cylinder 3 and 8 misfires). Replace plugs & wires (Autolite plugs, Borg-Warner wires), no change. Plug looked worse on 8. Get spark on both. Can feel injectors fireing. Unplug each injector and rpm drops except on 3 and 8, no change when they are unplugged. Replaced #8 injector, no change. Swapped the two coil packs to see if bad cylinders move, no change. The computer was replaced about a year ago. Could cam position sensor do this? Any other suggestions?

[2.2 Straight six]

sounds pretty unusual, if everything's working but there's still a misfire. did you gap the plugs as per the truck's required spec? probably won't make a difference as there's still a spark.

maybe you should have the computer checked, as everything's working maybe the mapping's off, it might be injecting the fuel or sparking the injectors at the wrong time, the ignition could be off.

[reree]

sounds like a burnt valve to me.

[unclebubbles]

Same thought crossed my mind reree, burnt valves or a head gasket blown between two cyls. But i dont think that would set the trouble codes he`s getting. Wouldnt hurt to check the compression though to be sure. Also check the coil connections, plug socket...ect on those cyls for moisture. And to answer a question from post#1, ive never seen a cam pos sensor cause a miss on just 1 or 2 cyls and it stay in the same spot.

[dhwort]

Have checked compression in both cylinders giving trouble. Both are 144-145.
Tonight I changed the cam position sensor even though it seemed unlikely to affect only two cylinders. It made no difference.

[wolf9304]

yes it would make a difference if the plugs are gaped wrong even if you still get spark have you made sure the plug wires went in the right place just asking check firing order first also you said you replace the computer a year ago hopefully you got the right one put in as they have different codes for them and my experince if you have a burnt valve usually your truck will start smoking even if its a little might explain it if none of this helps take it some where so it can be hook up to a diag. machine some times codes dont tell the whole problem why not spend the money usually costs around 80.00 to 100.00 dollars or u can spend more time and money doing it the hard way of trial and error

[Ian Szgatti]

did you check for signal coming off the ignition modual?