* Transmission Problems: This is the most common and serious possibility. Several transmission issues can cause over-revving:
* Clutch Problems (Manual Transmission): A worn clutch, a slipping clutch, or a problem with the clutch hydraulics (master or slave cylinder) will cause the engine to rev higher than expected while the transmission is trying to engage.
* Transmission Internal Problems (Automatic or Manual): Worn synchronizers (manual), faulty solenoids or valve body (automatic), low transmission fluid, or other internal damage can all lead to over-revving. This often manifests as a delay in engagement or a harsh shift.
* Transmission Mount Issues: A worn or broken transmission mount can allow excessive movement, leading to misalignment and over-revving, especially during gear changes.
* Engine Problems: While less likely to directly cause over-revving during shifting, engine issues can contribute:
* Throttle Body Issues: A sticking or dirty throttle body can cause the engine to rev higher than commanded.
* Vacuum Leaks: Vacuum leaks affect the engine's control system, leading to unpredictable behavior, including potential over-revving.
* Computer/Sensor Issues:
* Faulty Sensors: A malfunctioning sensor, such as the crankshaft position sensor (CKP) or the throttle position sensor (TPS), can send incorrect information to the engine control module (ECM), resulting in unusual engine behavior.
* ECM Problems: A faulty ECM can misinterpret sensor data and cause various issues, including over-revving.
What to do:
1. Check Transmission Fluid (Automatic): Low or dirty transmission fluid is a common cause of transmission problems. Check the fluid level and condition. If it's low, have it topped up by a mechanic and inspect for leaks. If it's dirty, it likely needs to be changed.
2. Listen for unusual noises: Pay close attention to any grinding, whining, or knocking sounds coming from the transmission during gear changes. These are strong indicators of internal problems.
3. Inspect for leaks: Look for any fluid leaks around the transmission, clutch (manual), or engine.
4. Professional Diagnosis: Given the potential seriousness and the number of possible causes, it's strongly recommended that you take your Ford Focus to a qualified mechanic for a proper diagnosis. They have the tools and expertise to pinpoint the problem and perform the necessary repairs. Ignoring the problem could lead to significant and costly damage.
Do not attempt to diagnose or repair this yourself unless you have significant mechanical experience. Incorrect diagnosis and repair can worsen the problem.
