The '02 Aerostar has had a nasty spark type knock for a while. Am running premium. Have run Neutra 1oz/gallon for six months (maybe 3500 miles) and still see heavy carbon in the tailpipe. The F150 ('95 4.9L) has traveled about the same distance (more highway miles though) and on same Neutra regime and it's dirty tailpipe is looking pretty clean now. So, I surmise the Aerostar was far worse and the Neutra eventually will get it cleaned up, but why wait. Maybe it needs an intake fog? Water, MMO, SeaFoam, Intake Cleaner (have an aerosol can of this on hand), what???? Any advice?