I did some stuff one time to clean the engine and it did break loose all the carbon build up. It did such a great job it clogged the oil brain holes in the heads and block and run the oil pan dry and ruined the engine. Ever since then, I use NO additives.

 

I have seen it done several ways , the first being with water poured into the carb while the vehicle was running not huge amounts but an 8 oz cup is sufficient and yopu pour slowly so you don't choke the engine out. The other method was with dry gas (denatured alcohol ) and water mixed and sucked in through a vacuum line keeping the engine revving so it does not get choked out as well. The third method was an Edelbrock water injector which kept the valves clean and also acted as a octane booster might to cushion the ping from to much timing . I have also seen transmission fluid used in a similar fashion to clean carbon from the valves but that usualy ended with the plugs being cleaned or replaced. Just some old school ideas I don't know if there valid any more but they worked in the day.

 

I took a car in on trade from a guy I did some work for once. As I was checking it all out i found the oil was like Molasses and looked like it never had been changed.

I added some diesel fuel to the oil and let the engine run for a few and then drained it and the bottom end of the engine was super clean and looked so much better.

 

I have seen that done as well sometimes you get lucky and it helps clean it up sometimes the dirt that was keeping the rings sealed gets washed out and it smokes like a chimney....lol

 

Ricers that run rotary engines do the same with ATF to clean carbon from the crank case and rotor surfaces. its a ritual with them and they swear by it. they say ATF is very effective in softening and freeing carbon. im with weedahoe, no additives for me either. i have an inate fear of non-compressable fluids in the combustion chamber blowing head gaskets

 

1qt of atf in place of 1qt of oil at an oil change is an old mechanics trick. ATF is high in detergent. It will help clean an engine. But like Weed said. It can cause more problems then good when the junk breaks loose and clogs up oil returns and the oil pump pickup screen.