Hummers are built for rough and tough road conditions - and so are the Hummer spark plug wires. But eventually, the forces of time will deteriorate the Hummer spark plug wires. The insulation jacket can deteriorate under the pressure of heat. Or, the jacket will become corroded by oil, moisture and gases. The insulation jacket may also be damaged by abrasion that stems from a vibrating engine. Spilled fluids or battery acid can eat away at the jacket materials. And every once in a while, wild animals will gnaw on the spark plug wires when they make a nest in your car engine. Often, the issue can be traced to loose terminals, which can happen when the plug wires are mishandled during other kinds of car maintenance operations. A mechanic or DIYer will tug on the wires, disconnecting or loosening them at their boots. So when you need to disconnect your Hummer spark plug wires, be careful! Do not pull on the wires, but grasp the plug wire and gently remove it. Wait until you engine is cold, since the connectors will become looser in hot conditions. Often, a visual inspection will reveal that the Hummer spark plug wire exterior has been damaged. Replace any wires that show damage such as a cut, grooves or marks from chafing or scorch marks from burning. The latter is another sign that arcing is occurring because of leaking electricity. Replace the entire Hummer spark plug wire set immediately as a matter of safety.