Got a sump full of sludge on my 2000 S 10 V6 Vortec Blazer. It has had a couple of overheat episodes with radiator and water pump issues. Got it all sorted but now the cooling system is pressurizing it is obviously forcing water into the oil side of the engine somewhere. If I leave the radiator cap loose no water gets into the oil.
I have flushed it out and got the oil clean and seems to be running fine otherwise.
I see the intake manifold can leak on these. Where exactly do they leak. Also what are the chances of a blown head gasket, cracked head or leaking oil cooler. At the moment I am looking at pulling the V6 out and dropping a V8 into it.

 

VERY common for the lower intake gaskets to leak at the corners, (need to raise the AC compressor to spot an external leak there). The gaskets can leak internally and or externally. The front coolant passage goes through the intake manifold all the way across to the opposite head. The rear coolant jackets in the heads are corked off by the intake, but they can still leak. All of the OEM style gaskets eventually leak, DON'T use them. Felpro MS98002T gasket set is a permanent fix if clean up, prep, and assembly is done correctly using torque sequence and specs, (very important). Use Dexcool coolant only, do not mix with any other type of coolant, and it needs to be flushed every 3 years or 36K miles. Neglecting or improper service will cause brown "mud" to form in the entire cooling system ALL of the "mud" must be flushed from the engine, heater core, and radiator, or the problem will return.


Oil cooler can leak, but it's rare. Heads and head gaskets are extremely rare failure items on the 4.3L unless it has been severely overheated.