The types of GMC spark plug wires have changed over the years. The older style of spark plug wires that came standard on your GMC truck had a copper wire core. This was a very durable wire that transferred the electrical impulse in an efficient manner. In time the copper would begin to degrade and the resistance in the wire would increase. This would cause the amount of the electrical charge it was transferring to decrease. The newer style of GMC spark plug wire has a carbon impregnated fiber core. This has a reduced level of resistance than the older cooper style of spark plug wire, however it has a tendency to become brittle over timed due to the heat of the engine. As the spark plug wire becomes brittle, the strands begin to crack increasing the resistance which in turn decreases the amount of electrical impulse that is being sent through the wire. For those reasons the periodical changing of the spark plug wires on your GMC truck's engine will be required for the engine to receive the appropriate amount of electrical impulse to explode the fuel. The wires can be tested with an ohm meter to determine the amount of resistance they currently have. Remember the lower the resistance, the better it can transfer the electrical impulse to the spark plug. Care should be taken when removing the wire from the spark plug. The boot that covers the plug has a tendency to stick to the plug over time. A little twist before the boot is pulled off should break this seal. If the wire is just pulled, its connection at the end which attaches it to the plug could be pulled off.