* Insufficient Fuel Delivery at High Load: While the fuel pump might be fine at lower RPMs (lower demand), it could be struggling to keep up at high RPMs and under the higher load present in the higher gears. This could be due to a weak fuel pump, clogged fuel filter, or issues within the fuel injectors (though injector issues usually manifest differently).
* Ignition System Problems Under Load: Similar to fuel delivery, the ignition system (coils, wires, distributor cap and rotor if applicable) might be functioning adequately at lower RPMs but breaking down under the stress of higher RPMs and load. A weak coil, worn distributor cap/rotor (if equipped), or faulty ignition wires are prime suspects.
* Intake Air Restriction at High Load: A partially blocked air filter, a problem with the mass airflow sensor (MAF), or even a restrictive exhaust system can lead to lean running conditions (not enough air/fuel mixture) at higher RPMs and loads, causing sputtering and backfiring. The higher engine speeds exacerbate the issue.
* Vacuum Leak (Gear Specific): This is less likely, but a vacuum leak that only manifests at higher loads might be related to a specific component associated with higher gear operation. This is harder to diagnose.
* Transmission Related (Unlikely): While less probable, a problem within the transmission itself (though very unlikely to cause backfiring) could subtly affect engine load and RPM in a gear-specific way.
* Exhaust Restriction: A clogged catalytic converter or muffler can restrict exhaust flow more severely at higher RPMs and loads, causing backpressure that leads to misfires and backfires.
Troubleshooting Steps:
1. Check the basics: Start with the easiest things – inspect the air filter, check for any obvious vacuum leaks, and visually inspect spark plug wires and distributor cap/rotor (if applicable).
2. Fuel System Diagnostics: Test the fuel pressure at high RPMs in the affected gears. A low fuel pressure reading would point to a fuel delivery problem.
3. Ignition System Diagnostics: Check the spark plugs for wear and fouling. You might need a professional to test the ignition coils and wires for proper function under load.
4. Diagnostic Scan: A scan tool can read trouble codes (DTCs) from the car's computer, which might pinpoint the problem area. This is crucial for identifying potential issues.
It's important to diagnose this carefully. Backfiring can be a symptom of several issues, some of which can cause engine damage if not addressed. If you are not comfortable working on your car, it's best to take it to a qualified mechanic for diagnosis and repair.
