just to add to this thread.. seafoam is a good product & not snake oil, however it is not magic & wont perform miracles. i have tested it for cleaning carbon on valves of heads removed from the engine & piston tops, it will quickly desolve a pretty thick layer of carbon build up compared to other chemicals like paint thinner, lacquer thinner, carb cleaner, brake cleaner etc. it does work for cleaning injectors/carbs when added to fuel but again not a miracle in a can.
as for the guys that said it blew head gaskets etc... i question their mechanical knowledge or ability, when adding it through the vac ports with engine running you have to be very carefull how much you introduce to the engine & how fast its done... adding any liquid to a cylinder has a big risk of hydro-locking the cylinder & breaking something or blowing out a head gasket in marginal condition, since most of the engines that people seafoam are high miles, who knows what the gaskets are like.
you are supposed to slowly add it to the vac port & not let the engine rpm drop too much, indicating too much raw liquid going into the cylinders. the idea is to just saturate/wet the valves & tops of pistons, not soak them with too much liquid. then after lettin it sit for ~15 minutes & starting the engine drive it with medium-heavy load on the engine & the rpms will blow out some of the broken down carbon & it will clean the internals somewhat. repaeat the process after a few tank fulls & maybe add some to the tank to help clean injectors etc. you likely wont see a big MPG improvement or fix any major problems, but if done right it will clean carbon deposits to a certain extent.
same for oil, it cleans sludge & dirt very well, just add it to clean oil on a higher mileage engine & watch how dirty the oil becomes in a short time. best bet is to add the right amount to the oil about 50-100 miles before a oil/filter change.
seafoam is a good product in general & will not blow head gaskets or cause any other damage... if done the right way.
