Typically, with higher mileage vehicles it may do some harm. On higher mileage engines, sludge/carbon sometimes help 'seal' the piston rings and valves a bit better. So if you wipe all of that carbon out of the engine, you might now realize that you are burning a bit more oil than before.
However, I have used seafoam on higher mileage engines with great success, there's about a 10% chance of an ill-effect happening.
Saturday, June 13th, 2009 AT 6:24 PM