WD40 is an excellent cleaner. It'll clean just about anything. If you filled the engine up and soaked it with WD40 for 24hrs, I'd imagine it would dislodge quite a bit of crud.
Personally, WD40 smells a little funky when it's powering an internal combustion engine. I ran my 125cc dirt bike on it when I first got it back together because I was inside the house and I didn't want it reeking like gas LOL.
Anyway, running a little water mist, or a small stream into a running engine will "steam clean" and bond with the carbon deposits removing them from the inside of your engine. Even so, I don't recommend driving the car off a bridge, or into any large body of standing water, or moving water, with the engine still running. I hear the piston's get awfully ornry about compressing liquid.
