If the diesel was enough to soak through the material, take the cover off and get the foam out asap. Your favorite cleaner ought to clean the cushion (Simple Green to Ivory Snow...) and the foam, but Peggie Hall recommends [url]www.pureayre.com[/url].

I don't know if this would be at all helpful, but you might want to try some oil dry or kitty litter (same thing). That stuff will suck oil out of concrete like you wouldn't believe, so I'd think it would get it out of anything porous.