Why is my Ford Focus engine so loud?
A failing or dirty sensor can send incorrect data, resulting in too much or too little fuel going to the engine. This can result in a rough running engine that is louder than normal.
Bad or Dirty Spark Plugs: Bad spark plugs can cause the vehicle to misfire which will make it run louder.
An engine that is suddenly running louder than normal is usually crying for help. In the majority of cases an underlying problem is causing the engine to rumble. Problems can range from something simple like dirty spark plugs to the more serious failing muffler or catalytic converter.
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最近對15年車齡，16.5萬英裏(27萬公裏）的老車（3.5 V6 2GR-FE）做“大保養”，換下原裝火花塞，看它的狀況好像問題不大（可能一是DENSO原裝正品質量；二是長期好機油勤換+佛係駕駛）：
長期好機油勤換的結果，發動機內部貌似無油膩，很幹淨：
