It won't hurt anything.

The most common method is to disconnect the hose to your brake booster (at the booster). Stick that end into a bowl/cup/etc of Seafoam, hovering it over the liquid so it's just barely touching and sucks the liquid up via vacuum. I use half the can typically. Shut the car off as soon as it sucks it up. Let it sit 5-10 mins, then idle it out.

I really don't think this does much on scaled carbon deposits, as I've done head work a few times and seen what it takes to physically remove the scaling. I've let Seafoam soak on head setting on a bench and it had no effect. But, I'm sure it's got to clean something or it wouldn't be so popular...right?! <img src="/forums/images/graemlins/confused.gif" alt="" /> <img src="/forums/images/graemlins/wink.gif" alt="" />

Nah, really, it's probably good at cleaning sludge.

As for cleaning the fuel system, use a couple gallons of E85. It's a lot cheaper.