What causes an engine to backfire at high idle?

An engine may backfire at high idle due to a variety of factors, including an incorrect air-fuel mixture, which can result from a malfunctioning fuel injector or clogged fuel filter. Additionally, ignition timing issues, such as a faulty spark plug or ignition coil, can cause unburned fuel to ignite in the exhaust system. Vacuum leaks and exhaust system blockages can also contribute to this problem. Addressing these issues typically requires a thorough inspection of the fuel and ignition systems.