Randomly the accord will backfire upon start up. It seems to be completely random. Spark plugs and leads are pretty new and I looked at the plugs and they look fine. Fuel filter is around 30,000km old and I believe is an internal fuel filter (I only just bought the car and went to perform a service on it and bought a fuel filter and couldn't find it, looked it up and I believe it's inside the fuel tank). Vac lines checked and all good. Just wondering if you have any other suggestions as to what causes a backfire upon start up? It has never backfired while driving, drives reallyyyy smooth actually.