Perform a cooling system pressure test and a dry and wet compression test. Minimum Ecotec V6 compression is 100psi.
Bubbles in the filler neck of the radiator can indicate a blown head gasket. White smoke out of the exhaust also. Dropping coolant level too. Coolant in oil, or oil in coolant as well. Check the condition of the oil on the dipstick, the oil would appear milky and cloudy. Also the condition of the coolant in the radiator filler neck, the coolant would have small deposits of oil floating around.
Compression Test Procedure.
1. Make sure you have a fully charged battery.
2. Run the engine until operating temperature is reached.
3. Turn the engine off and disconnect the spark plugs leads for the coil packs and spark plugs.
4. Remove the engine and fuel pump fuses.
5. Remove a spark plug and install the compression tester into whichever cylinder you want to start in. 1-6, 6-1, 1-3-5, 2-4-6. Whatever way you want.
6. You need another person to fully depress the throttle pedal and crank the engine over. Only crank the engine a maximum of four compression strokes (thumps) at a time, while observing the compression tester gauge and note down the highest reading.
7. Test the following other cylinders and compare all the readings at the end. The lowest reading on any given cylinder shouldn't be lower than 70% of the highest reading cylinder.
If a low compression reading is noted on one or more cylinders, you can 'wet test' the engine to isolate the issue even further. Add a small amount of engine oil in the combustion chamber of each cylinder and repeat the compression test again.
An large increase reading of compression pressure indicates that the piston rings, piston and/or cylinder bore are worn. A low increase reading of compression pressure can indicate burnt, obstructed or sticking valves or a leaking cylinder head gasket.