My GM 3.4L has an overheat problem. When I have it running with the radiator cap off (I also put a spill free funnel on it), tiny bubbles come up. If I rev the engine, the coolant gets sucked back into the radiator. When I put the throttle back to idle, the coolant come shooting up about 2 or 3 inches and white foam builds up on the coolant surface. If I remove the funnel, I see the radiator full of white foam and it oozes out. I did a pressure test of the cooling system. It doesn't hold pressure. Has anyone here experienced this and know what the problem is? I'm suspecting a blown LIM gasket but don't know. How can I be sure it's not external to the engine? How can I be sure if it's the Lim gasket or the head gasket?

 

Update: I found 2 leaks. A small hose was leaking. It was cracked. One of the plastic quick-connects was cracked. I replaced these parts and got the system to not leak during the cooling system pressure test. I figured out a way of using the block tester (with the blue liquid) so the foam doesn't rise up into the tester. I connected a hose to the base of my spill free funnel and used some parts from the store to connect that hose to a wider diameter hose. I worked the tester for about 5 minutes before the white foam started going into the tester. The blue liquid turned green and was just starting to turn yellow when the foam entered the tester.

 

I used my inspection camera to view inside the cylinders. Cylinder 5 has water droplets from the water in my cooling system. I think it got sprayed there during the intake stroke, while I was cranking during the compression test. During the compression test, I had all spark plugs out and the fuse for the fuel pump was removed so no gas was sprayed in the cylinders. If the engine was running, that little bit of water would have been burnt off. Cylinder 4 has what appears to be some of the brown dexcool sludge on it. This is the only cylinder to have this. The other 4 cylinders look normal and clean.
I also found that there was brown stuff bridging gap on the spark plug for cylinder 4 and a build up of brown stuff around the circumference of the plug. This cylinder also has the lowest compression, even though it didn't get water sprayed in it during the test. The compression for the cylinders was from 165 to 185. That was a dry test.