Volvo S40 Engine is backfiring Inspection at your home or office.

Common reasons for this to happen:

• When moisture or resistance in the car’s electronic ignition system allows a crossover generation of current to the wrong cylinder.

• A lean mixture caused by either an engine vacuum leak or low fuel-pump pressure.

• A faulty air injection check valve (part of the car’s emission control system) allows too much air into the car’s exhaust system.

• A failed air intake boot can cause a backfire by altering the feedback voltage to the PCM.

• Faulty ignition components or spark plugs may also cause backfires if the ignition timing drifts off or the plugs fire intermittently.

• An exhaust leak such as broken welds or other small holes can take in cool air during deceleration and cause a popping or backfiring in the exhaust system.