Just do a search on Seafoam here...
I can attest that it significantly cleans-out the gunk and carbon from the engine's top-end. It's a yearly tradition to smoke-up the neighborhood with a good old-fashioned Seafoamin'
Also, my folks have a '93 Chevy Truck that overheated and cracked the rad. It wouldn't turn over, so it was figured to have a trashed engine. Upon recommendation of a certain family member
Seafoam was dumped directly into the cylinders by removing the spark plugs.
After sitting for a few hours, the plugs were replaced and it started up and ran smoothly. The stuff works great as a maintenance item or to free up sticky engine parts. Using it to improve FE as an additive, has yet to be proven as statistically significant.