I have heard many good things about TECHRON. I tend to use it in new cars to keep them clean, so I can't speak to any improvements made on older cars of its cleaning ability. I do happen to think it keeps a new fuel system clean for a longer period of time. TECHRON concentrate is available from Chevron at most auto parts stores. I've also read that its best to occasionally change up the fuel or additive you use. Its possible that using the same fuel brand and additive could start to create its own version of carbon build up and by simply switching to a new additive every 6,000-10,000 miles could help prevent this. I've heard good things about Lucas Oil and Redline fuel system cleaners as well and they tend to be less expensive than TECHRON. Again, I can't speak for the cleaning power of these products, since I think they are all more effective as preventative products.

it depends on the year of the car because fuel injection cleaners are ment to soften up carbon and most of the "in tank" fuel pumps have brushes in them that are made of carbon and some fuel injection cleaners can harm the fuel pump. any fuel system service I do I usually do it right at the engine instead of putting it through the tank.