Being a US spec engine,you also have a large crankcase breather pipe under the intake manifold with a air pump for the exhaust gas recirculation. A lot of problems such as you describe come from that pump not working or the valve on the exhaust manifold burnt out inside with actual exhaust gas pumping into the crankcase causing a lot of over pressure. In your case i would remove the intake manifold and check all the crankcase vent hoses and the EGR system.