An engine stumbles during acceleration if it has a fouled spark plug, a vacuum leak, or a blocked airflow meter or airflow sensor. A mechanic has to conduct comprehensive tests to find the cause of the stumbling and to ascertain that the engine is not backfiring or missing. Most of these problems are easy to fix once they have been properly diagnosed.
An engine may stumble if its electrodes are worn out or fouled by oil. Mechanics fix this problem by cleaning spark plugs or replacing them if they are worn out. Dead spots on the airflow meter or airflow sensor may impair combustion and cause the engine to stumble. Electrical diagnostics that troubleshoot the sensors can identify this problem.
Large vacuum leaks may also cause the engine to hesitate when the driver begins accelerating. Mechanics diagnose vacuum leaks by switching off the engine, connecting compressed air to the system and listening for leaks. Vacuum leaks are hard to spot, but mechanics can fix them by repairing the leaking hoses or gaskets.
If none of these common problems is the cause of engine hesitation, the engine could be backfiring or missing, rather than stumbling. Backfiring is often the result of vacuum leaks or problems with the ignition system or the vehicle's computer.