Engine-Related Issues:
* Spark Plugs/Ignition Coils: Worn, fouled, or failing spark plugs or ignition coils can cause misfires, especially under load (like when accelerating at higher RPMs). This is a common cause of top-end power loss.
* Air Filter: A clogged air filter restricts airflow to the engine, reducing power. This is a cheap and easy thing to check first.
* Fuel Filter: A clogged fuel filter restricts fuel flow, similarly reducing power, especially under load.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A faulty MAF sensor can provide incorrect readings, leading to a lean or rich fuel mixture and power loss.
* Throttle Position Sensor (TPS): A faulty TPS can prevent the engine from receiving the correct fuel/air mixture, causing power loss.
* Fuel Pump: A weak fuel pump might not deliver enough fuel at higher RPMs, causing power loss.
* Dirty Fuel Injectors: Clogged fuel injectors can prevent proper fuel delivery.
* Engine Compression: Low engine compression in one or more cylinders can significantly reduce power. This requires a compression test to diagnose.
Transmission-Related Issues (Less Likely, but Possible):
* Clutch (Manual Transmission Only): A slipping clutch will cause power loss, particularly in higher gears under acceleration.
* Transmission Problems (Automatic Transmission Only): Issues within the automatic transmission itself are less likely to manifest as *only* top-end power loss, but it's a possibility. This would likely involve more symptoms, such as slipping or rough shifting.
Other Possible Issues:
* Exhaust System Restriction: A severely clogged catalytic converter or muffler can restrict exhaust flow and reduce power.
Troubleshooting Steps:
1. Start with the easy and cheap checks: Inspect the air filter. If it's dirty, replace it.
2. Check for engine trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the car's computer. This will often pinpoint the problem.
3. Listen for unusual noises: Unusual engine noises (knocking, ticking, etc.) can indicate other problems.
4. Consider the age and mileage: Older vehicles are more prone to issues like worn spark plugs and ignition coils.
When to Seek Professional Help:
If you're uncomfortable performing these checks or if the problem persists after checking the simple things, take your Fiesta to a qualified mechanic. They have the tools and expertise to diagnose and repair more complex issues. Providing them with any trouble codes you've found will help them greatly.
