What will cause a gasoline engine to run fine at idle but miss under load?

When a gasoline engine runs smoothly at idle but experiences a miss under load, high resistance plug wires can be a primary suspect. At idle, the engine operates at lower RPMs and less load, requiring less current from the ignition system. However, as the load increases—such as when accelerating or climbing a hill—the demand for a strong spark to ignite the air-fuel mixture also increases.

High resistance in the plug wires means that there’s a loss of current flow from the ignition coil to the spark plugs. This can lead to weak or inconsistent sparks when the engine is under load. As a result, the combustion process becomes compromised, and the engine might miss or hesitate during acceleration or under heavy load conditions.

In contrast, other options do not specifically align with the symptoms described. For instance, a plugged air cleaner affects air intake and can cause overall performance issues, but it would generally cause problems at both idle and under load. A stuck open EGR valve introduces excessive exhaust gases into the intake, which can lead to rough running and poor performance under various conditions. A defective knock sensor primarily concerns timing adjustments to prevent engine knock, but it does not directly cause a miss under load in the way high-resistance plug wires do.