Make sure the plug wires are correctly placed on the distributor cap (coil pack). In most cases, (when these are changed) two are accidentally switched.
Simply take two at a time and switch them. (refer to a repair maunal for firing order first). You can look this up online as well at http://www.google.com.
If these are correctly placed, then pull the wires (one at a time) from the plugs, and pull the plugs out and check for oil on the plug. If any one of these has oil, then you may have found your miss fire.
If all there is good (clean these and run truck); pull the distrubutor cap off (if equipped) and make sure the rotor (just under cap in center) is not cracked or warn (check end where small metal end is located). This sparks at each plug point on the distrubutor cap.
That is the next thing to check. On the inside of the cap. look to see that the prongs are not white, grooved, or broken. If any of these are true, replace both cap and rotor. Some rotors pull off, others have a small screw that hold them in palce which is located on the side.
If you're truck has coil pack(s) then have them all (if more than one) tested. These sit on a unit called a modular. Look at the modular after removing the packs and see that it is not bubbling, white, or melted. If any of these are true, it needs replaced as these cannot be tested.
Contact your local automotive parts store for correct parts and pricing. NOTE: some parts (seldom) may need to be purchased from dealer. This is rare, but sometimes an only option.
Also, if you have a distributor, check to make sure the coil wire (center of cap) is pushed on completely on the cap as well as the coil. (coil may need replaced as well) which also cannot be tested.
If need be, and you have not already, replace all of the following to ensure all is good:
plugs, plug wires, cap and rotor, coil, (the coil wire will come with the plug wires, but make sure when purchasing).
If coil packs are present instead of distributor, replace the following:
plugs, plug wires, any coil packs that are tested back, modular if it looks anything like that described above. Also remember to check for oil residue, or blackened powder on the spark plug ends.
If any, replace as necessary. When you replace spark plugs and wires do the following to ensure the firing order remains:
Pull one plug at a time. Take plug wire off the plug only, until new plug is inserted; then replace the appropriate length plug wire. (compare old to new; some may be close in length, but not exact as these are aftermarket plug wires).
Do this for each plug and wire. Save the coil wire for last to know you are finished with all: coil wire is the shortest of all (usually) and only goes on once way at each end: typically. One end will be straight - this goes to the coil. (not coil pack) the other end will be a 90 degree; this goes on the center port on the distributor.
If coil packs are present, there will not be a coil wire, as the coil packs replaced this unit.
My apologies for the lengthy reply, however this is to ensure you save time, money, and are aware of the different options and possibilities.
Please let me know if you need any further assistance.
carsandcomps
Guru, Premium Expert @ Fixya.com
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