It's called GM Top Engine Cleaner. I've heard it's STRONG stuff. If you soak with a solvent, warm up the car, then turn it off, remove all plugs and soak. After soak, crank motor over with rags over open spark plug holes to blow out any remaining liquid (hydrolock prevention), then start your motor and let it idle. You will blow obnoxious amounts of smoke out the back, but it should clear after a little while. Then, change your oil as solvents may leak deposits or thin your oil.

Water cleaning: I've never seen dramatic improvements when I look down my spark plug holes. Yet, there are multiple sources that say it works, drawing analogies to blown head gaskets leaking coolant into the chamber and cleaning it like new.

 

Just for the record, we used it a little differently...

Get the engine hot, then stick a small vacuum line in the container, keep up the RPM while it sucks it in. Hopefully you get the whole can in before you kill it. Then wait 15 mins for the stuff to do its' magic, then start and enjoy the smoke you create...

Did this to two different VW motors, made them spotless inside the bores, even in the intakes...