Replaced my sparks plugs in August and had trouble with the removal of plug # 3.I could not get to it to remove it and so I left it in and replaced all others. It ran OK like that but you could feel a little roughness. In December I went on vacation and the truck was not driven. When I returned the truck ran much more rough. I replaced the spark plugs again and this time removed # 3 (pain in the a>>). I also replaced the wires and distributor and rotor. I am still getting a code on cyl. #3. What might the issue be?

 

Make sure that your #3 plug wire isn't arcing to anything. The wires should absolutely never be allowed to rest against metal engine components, but be held away from the engine by wire looks, ties, etc. Plug wire insulation is not thick enough to stop an arc from happening if the wire is allowed to rest against the surface of the engine or other ground-carrying engine parts/accessories.

If the problem isn't in the plug, wire, or distributor cap, then the areas left to check are the ignition module, fuel injector, head gasket, and valves. I would recommend checking your compression in cyl 3. A small head gasket leak might not be noticable in your exhaust.

It's a longshot, but if the fuel injector is just dirty, try Chevron with Techron concentrate. Dump the whole bottle in and fill up with 89 octane gasoline. That always does the trick for dirty injectors on all of my vehicles. If the problem is the injector and the Techron doesn't help, you're probably looking at having to replace the whole fuel injection "spider" assembly. Not cheap, and not a quick job on the 4.3L engine. Hopefully your problem isn't that complicated.

Good luck!