The following is straight from a service manual, in regards to the TSB issued for a "cold knock" on the northstar, caused by excessive carbon.
Before considering major engine repairs, the technician should perform the following procedure to clean carbon from the engine combustion chambers.
Note: This procedure must be strictly followed. Hydrostatic lock may result causing severe engine damage if this procedure is not strictly adhered to. (that's important.)
1) Obtain one can of GM TOP engine cleaner (TEC) P/N 1052626.
2) Start the engine and bring the engine coolant temp to 200 degrees on the instrument panel gauge. (220F on a scan tool)
IMPORTANT: Do not raise the engine speed above idle
3) with the engine idling, disconnect the PCV hose from the PCV valve and slowly spray the TEC into the end of the hose.
4) Raise the engine speed to approximately 2000 rpm until you see a cloud of white smoke coming out of the exhaust pipe, then shut off the engine.
5) Let the vehicle sit for at least 20 minutes, preferably overnight. This allows time for the TEC to act on the carbon deposits.
6) Once the soak period has been completed, start the engine and proceed to remove the remainder of the TEC from the engine by increasing the engine speed to 2000rpm until the white smoke has diminished.
For engine-diy-novices, I recommend you follow that advice, and make SURE you get the aerosol can of TEC. It's much less likely to hydrolock the engine, and you can't pour it in too fast like you can with the bottle. After you finish the above, take it on a highway on-ramp, and with the shifter in 2nd, floor it. You can further exercise the rings by letting the car speed drop to 30mph, and flooring it back up to 60....
Oh, you're going to want to change your plugs after you do this. A lot of crap gets moved around, and some of it can collect in the plugs, shorting it out. Also some of the cleaning solution leaks past the rings, so if you leave it overnight (recommended), change the oil. It's not urgent, and it's not even mentioned in the article, but it probably does change the oil chemistry. So why chance it...