My wife's '02 Windstar had an intermittent preignition problem. No codes, so the service tech told her to use mid-grade (89 octane). I Googled 'ford windstar 3.8 preignition' and a lot of results pointed to the Intake Manifold Runner Control bushings. Sure enough, the rear rod was laying down on the engine with a broken bushing. $5 later, no more preignition. Dorman #47099, 5 pack was $5 at a local parts store.