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After a few years, power declines sharply. Check and you will usually find heavy carbon in the engine, which raises fuel consumption, cuts performance, fails the emissions test and, in bad cases, causes knocking. What causes it, and what can be done?
Causes
Carbon comes from incomplete combustion. Short trips, long idling, poor fuel quality and a neglected intake all contribute. Deposits build on injectors, intake valves, the combustion chamber and piston crowns, upsetting the mixture and the airflow.
What to do
Mild deposits are dealt with by a fuel system cleaner added at the recommended dose, or a professional intake clean. Prevent it by driving longer distances occasionally, avoiding prolonged idling and servicing on schedule.
Severe carbon needs a workshop: walnut blasting on direct-injection engines, or a chemical clean.