* Fuel System Problems:
* Clogged Fuel Filter: A dirty fuel filter restricts fuel flow, preventing the engine from getting enough fuel for higher speeds.
* Weak Fuel Pump: The pump might not be delivering sufficient fuel pressure at higher RPMs.
* Clogged Fuel Injectors: Clogged injectors deliver less fuel than needed.
* Low Fuel Pressure: A leak in the fuel line or a faulty fuel pressure regulator can cause this.
* Ignition System Problems:
* Spark Plug Issues: Worn, fouled, or improperly gapped spark plugs can cause misfires, especially under load.
* Ignition Coil Problems: A failing ignition coil may not provide sufficient spark to ignite the fuel-air mixture consistently.
* Distributor Cap and Rotor (if applicable): Cracks or corrosion can interrupt the spark.
* Air Intake System Problems:
* Clogged Air Filter: Restricts airflow, leading to a lean fuel mixture and poor performance.
* Mass Airflow Sensor (MAF) Issues: A faulty MAF sensor provides inaccurate air flow readings to the engine computer, leading to improper fuel delivery.
* Vacuum Leaks: Leaks in the intake system disrupt the proper air/fuel mixture.
* Transmission Problems:
* Transmission Fluid Issues: Low, dirty, or incorrect transmission fluid can cause slippage and poor acceleration.
* Transmission Problems (Internal): More serious internal issues within the transmission itself (clutch packs, bands, etc.) are possible, but less likely to only show above 15mph.
* Exhaust System Problems:
* Clogged Catalytic Converter: A blocked catalytic converter dramatically restricts exhaust flow, limiting engine power.
* Exhaust Leak Before Catalytic Converter: While less common to limit acceleration only above 15 mph, it's possible.
* Other Potential Issues:
* Oxygen Sensor Problems: Incorrect readings from the oxygen sensor will lead to a poor air/fuel mixture.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate throttle position information to the engine computer.
Troubleshooting Steps:
1. Check the obvious: Start with the easiest checks: fuel level, air filter condition.
2. Listen for unusual noises: Any unusual sounds from the engine, transmission, or exhaust could point to a specific problem area.
3. Check engine codes: Use an OBD-II scanner to retrieve any diagnostic trouble codes (DTCs) stored in the car's computer. These codes provide valuable clues.
4. Visual inspection: Carefully inspect all components mentioned above, looking for any obvious signs of damage or wear.
5. Professional Diagnosis: If you're not comfortable working on your car yourself, take it to a qualified mechanic for a proper diagnosis. They have the tools and expertise to pinpoint the problem effectively.
The fact that the issue arises only above 15 mph suggests something related to load, fuel delivery at higher RPMs, or the transmission. However, a thorough check of all the above systems is necessary for accurate diagnosis.
