I have 99 chevy silverado misfing on 6 cylinder plug was loose so tighten it down . STill rough idle what should i do? Should i replace plugs and wires? I had a computer put on it and code said missing on 6th cylinder.
You need to figure out which part is giving you issue. The 99 Silverado had two basic engine styles: V6 and V8.
The V6 has the older style "dizzy" ignition system, which involved a distributor, rotor, cap, wires, & plugs. To test this type of engine, you can pull plug wires individually to see if there is a change in how the engine runs. Pull the plug wire to the #6 cylinder at the plug. Place a Phillips head screwdriver into the end of the spark plug wire (should be a large enough screwdriver the plug wire will hold the screwdriver snugly). Lay the shank of the screwdriver across another metal part of the engine. Start the truck, then holding only the insulated the portion of the screw driver, pull it slightly off of the metal part. You're looking to see if there is a spark which forms off of the screwdriver. If there is, the problem is most likely the plug. Replace all of them. If there isn't a spark, the problem is upstream in either the plug wire or in the distributor cap/rotor. You can pull the distributor cap and look for carbon traces (a dark line along the inside of the cap). If there is, replace the cap & rotor. If you still don't find anything and you know you've got quite a few miles on these parts (cap/rotor/wires), go ahead and replace them. They probably needed it any way.
The V8 engine is an LSx variant which has a coil over ignition system. The coils are located on top of the valve cover. Each cylinder has it's own coil. Each coil has electronics in it which allow it to fire the spark plug. It requires power from the battery and a signal from the ECU to fire. There are short spark plug wires which go from the coil down to the spark plug. To check the #6 spark plug, do as was suggested in the V6 model with a screwdriver. If you get spark off of the screwdriver shank, replace the spark plug. If you don't get a spark, it's more than likely the coil, as these can go bad (either through electronics or internally). You'd need to replace the coil itself. You can test the coil, but changing it out with one of the other coils. If the misfire follows the coil, the issue is with the coil. If it stays at #6 cylinder, you'll have to dig deeper. At that point it could be a wiring issue or even an ECU, but that's highly unlikely.
While not unheard of, the spark plug wires don't usually go bad (though can stand to be replaced when the plugs are replaced at the 100k mile tune-up). They are less than a foot long, so not much to them, really.