Changed the coolant on my F150 lately. There was a coolant drain plug on the engine.

Is there one the GM 3800? I did flush and change the coolant when I changed my water pump, but leaving some gallons of that chemistry in the engine musn't be very good.

If there is one, where is it

I believe you remove the 2 knock sensors if you want to fully drain the engine of coolant. Personally, I flush with water until it comes out clear. That doesn't leave gallons of anything but water in the engine.

Roy is correct, removing the 2 knock sensors is the best way to drain the block. But the rear sensor is a real pain to get to because of the transmission. Other alternative is to remove the upper hose from the engine, grab a garden hose and dump water into the radiator while the engine is running. The water pump will circulate the water through the engine and everything else until it comes out clear.

I use one of those tees you get with a Prestone flush kit and install it on the heater inlet hose. Hook a garden hose up to it, remove the rad cap, start the water, start the engine, and keep it going until clear water comes out where the rad cap attaches.

Garden hose works the best!!! And remember use only DISTILLED WATER when flushing or filling! hah