Engine Backfires

ENGINE BACKFIRES

Description
The engine backfires through either the intake or exhaust. Backfire may occur during acceleration, cruise, deceleration, or engine shutoff.

Explanation

Exhaust Backfire
Engine backfire through the exhaust is caused by unburned fuel in the exhaust combining with oxygen. The fuel may be from a misfire or an overly rich fuel mixture. The oxygen may be from an exhaust leak or a malfunctioning air injection system (if equipped).

Backfire may cause severe damage to the exhaust system. Catalytic converters and mufflers may be damaged with just one backfire.

Intake Backfire
Engine backfire through the intake is caused by an intake valve opening while conditions inside the associated cylinder are sufficient to cause combustion. The causes of the combustion may be from ignition misfire, exhaust gas, or extremely hot carbon particles in the combustion chamber.

Causes

Exhaust Backfire

NOTE: Exhaust backfire requires the presence of both fuel and oxygen in the exhaust. These two necessary elements are often introduced from two separate problems. Repairing either problem and removing one element (air or fuel) from the exhaust will stop the backfire but the remaining problem will continue to cause other damage.

Exhaust Leaks
Any leak in the exhaust system will allow air to enter the exhaust stream. If unburned fuel is also present, backfire may occur.

Testing
- Visually inspect the entire exhaust system for leaks.

Air Injection Problems
The air injection system allows air to enter the exhaust during deceleration. The air injection system should divert air from the exhaust system during deceleration, to prevent combustion (backfire) with unburned fuel (normally present during deceleration).

Testing
- Inspect all air injection vacuum lines and electrical connections.
- Pinch off air injection hose leading into the exhaust system. If the exhaust backfire stops, see Emission Control Systems for further testing. Emission Control Systems

Ignition Misfire
When ignition misfire occurs, all of the unburned fuel is drawn directly into the exhaust. If any oxygen is present in the exhaust, backfire may occur.

Testing
- Use an Ignition scope to verify proper secondary voltages and burn times.
- Inspect all spark plugs for signs of misfire. Complete ignition misfire will result in a wet fuel fouled spark plug. Occasional or intermittent misfire may show no signs of problems on the spark plugs. Check porcelain insulators for evidence of cracking.
- Inspect ignition cables for signs for insulation damage or arcing.
- Inspect ignition cables for high resistance readings or corroded terminals.
- Inspect the distributor cap for signs of cracking or arcing.
- For additional testing see Ignitions System. Ignition System

Excessively Rich Air/Fuel Mixture
A rich air/fuel mixture results in unburned fuel remaining after combustion. If any oxygen is present in the exhaust, backfire may occur.

Testing
- Check air/fuel mixture and adjust if necessary.
- Check choke for proper operation. Carburetor
- Check fuel control system for proper operation, see Computers and Control Systems. Computers and Control Systems

Intake Backfire

NOTE: Intake backfire often causes additional damage. Check for blown off vacuum lines, ruptured vacuum diaphragms, and blown out gaskets.

Lean Air/Fuel Mixture or Lack of Enrichment on Acceleration
A lean air/fuel mixture during acceleration results in extremely high combustion temperatures. Heated carbon particles or remaining exhaust gases ignite the incoming fuel mixture resulting in backfire. Computers and Control Systems

Testing
- Verify accelerator pump operatation
- Remove the air cleaner assembly.
- With the engine "OFF", look down the throat of the carburetor while cycling the throttle lever.
- Each time the throttle is opened, a stream of fuel should be sprayed down the throat of the carburetor.

CAUTION:
- Do not perform this check with a hot engine.
- To prevent flooding the engine, minimize the number of times the throttle lever is cycled.

- Verify there are no vacuum leaks present.
- Check the Air/Fuel Mixture and adjust if necessary.

Ignition Misfire
Out of time/sequence ignition fire may ignite the incoming fuel mixture.

Testing
- Verify the firing order and ignition cable routing is correct.
- Verify the distributor cap has no cracks or carbon tracking.
- Check ignition timing and adjust if necessary.

Intake Valve Stuck Open or Broken Valve Spring
If an intake valve is stuck open when the spark plug ignites the mixture, fuel in the intake will also ignite.

Testing
- Perform Compression Check.
- If a low cylinder detected, perform a cylinder leakdown check.

NOTE: A cylinder leakdown check utilizes regulated, compressed air to pressurize each cylinder (at Top Dead Center Compression). While performing this check, the cause of the compression leak may be determined by listening for escaping air:
- Air heard leaking from the exhaust indicates an exhaust valve problem
- Air heard leaking from the intake or throttle body indicates an intake valve problem.
- Air heard leaking from the PCV valve or PCV breather hose indicates a piston, ring, or cylinder wall problem.
- Air heard leaking from an adjacent cylinder indicates a head gasket failure between the two cylinders (both adjacent cylinders will have identically low compression).
- Air seen bubbling out of the radiator indicates a head gasket failure (or a cracked head/block).