DIAGNOSE – THE ENGINE BACKFIRES
by Lance Wright
The engine backfires. The symptom may be more frequent on deceleration when the accelerator pedal is released from high engine speeds.
Backfiring is usually the result of too much oxygen in the exhaust system. This causes a combustion in the exhaust system which results in backfire from the exhaust. Oxygen can enter the exhaust from leaks present in any component of the exhaust or from the air injection system if the vehicle is equipped with such a system.
The first step in the diagnosis is a careful visual inspection of the exhaust system. Inspect the exhaust system for leaks. Check the muffler, catalytic converter, and pipes for damage or leaks where they are connected. Inspect the exhaust manifold for cracks. Inspect the exhaust manifold gasket and the gasket that connects the exhaust pipe to the manifold for leaks. If the vehicle is equipped with an air injection system it should be inspected for proper operation. Refer to the “Air Injection” system in the emission control section of this website to gain a working knowledge of the system components. The Air injection system injections air into the exhaust under certain conditions to reduce emissions. If air is injected into the exhaust manifold when the engine is hot on deceleration, it can result in severe backfiring. Temporarily disconnect the air injection system to determine if the symptom is corrected. This can be done by removing the belt or the rubber hoses that connect to the metal check valves. If the symptom is corrected, the air injection system should be tested. Refer to the manufacturer specific repair manual for testing the system used on your vehicle. This will consist of checking for the proper operation of the diverter valves and ensuring the vacuum hose routing is correct and functioning normally.
PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine parts. Never disconnect any part of the vehicles emission control system other than for temporary testing. The system is designed to reduce the emissions of the vehicle and should be immediately repaired when defects are found. Disconnecting any part of the system is usually prohibited by federal and state laws.
owned his own auto repair shop for 30 years before retiring in 2006.)