2003 Buick Regal poor idling. Error code PO300.

The test you need is called a "cylinder leakage test". Once the compression test shows low compression, this test will tell you why it's low.

The tester uses compressed air that goes through the tester and is regulated. The hose looks like the hose for a compression tester but it doesn't have a check valve in it. You pump in compressed air through the spark plug hole. The gauge on the tester will show percent leakage, but then you can listen at various places to find out where the leak is. If an exhaust valve is leaking, you'll hear a hiss at the tail pipe. For a leaking intake valve, you'll hear the hiss at the throttle body. With leaking piston rings you'll hear the hiss at the oil cap or dip stick tube. With a leaking head gasket or cracked head, you'll see a stream of tiny bubbles in the radiator.

The piston for the cylinder being tested has to be at top dead center. That is easy to find if you use a whistle screwed into the spark plug hole. It will stop whistling when the piston reaches top dead center on the compression stroke. Check at an auto parts store that rents or borrows tools to see if they have the tester and whistle.