[Anker]

I owned a Buick Park Avenue that started misfiring at mid-throttle. If I lifted the throttle the engine wouldn't fire at all. It got so bad that I had to give it full throttle on the highway to keep it going (I was on my way to Chicago from Boston). When I got to Chicago I had a workshop try to diagnose the misfires. They kept focusing on the injectors and I kept telling them to change the spark plug wires first. After several hours of them mucking around they finally followed my instructions and bingo, misfires gone.

The problem was cracked insulation on the wires. At low throttle settings and at full throttle the compression high enough to allow the plugs to spark, but at mid-throttle the compression was low enough that the spark went through the cracks instead of through the plugs. If you have ever owned a car with a vacuum gauge you will know when the compression is low. High vacuum = low compression.