The best cleaner I've used comes in a black spray can and says "steam" up the side. I think it's a Gumout product, but i'm not positive. You can actually spray it on a pretty warm motor and it does a killer job eating through the grime, yet i never say any adverse effects. Maybe a little white-ish residue here or there. Fumes are horrid, however. I haven't been able to find it for quite some time in my one horse town, but when I lived in the twin cities it was readily available.
Good luck, man