2004 Nissan Sentra Owner's Manual - Page 216

Aftermarket fuel additives NISSAN does not recommend the use of any aftermarket fuel additives (for example, fuel injector cleaner, octane booster, intake valve deposit removers, etc.) which are sold commercially. Many of these additives intended for gum, varnish or deposit removal may contain active solvents or similar ingredients that can be harmful to the fuel system and engine. damage. If any of the above symptoms are encountered, have your vehicle checked at a NISSAN dealer. However, now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is not a cause for concern, because you get the greatest fuel benefit when there is light spark knock for a short time under heavy engine load. Octane rating tips Using unleaded gasoline with an octane rating lower than stated can cause persistent, heavy "spark knock." ("Spark knock" is a metallic rapping noise.) If severe, this can lead to engine damage. If you detect a persistent heavy spark knock even when using gasoline of the stated octane rating, or if you hear steady spark knock while holding a steady speed on level roads, have a NISSAN dealer correct the condition. Failure to correct the condition is misuse of the vehicle, for which NISSAN is not responsible. Incorrect ignition timing may result in spark knock, after-run and/or overheating, which may cause excessive fuel consumption or engine 9-4 Technical and consumer information