2008 300hp overheating

[Drew Gray]

Hi
My 2008 E300DPX has started overheating.
I've replaced the complete water pump (impeller and housing), both thermostats and the poppet valve with no change.
There does not appear to any sort of blockage in the system.
My dealer ***ures me that all service bulletins/updates have been carried out.
I've tried a reduced tension spring in the poppet valve also, once again no change.
I have noticed that the 2 small holes on the poppet valve diaphram cover puff air when the engine is idling, is this normal? Could this be exhaust g***es or compression g***es causing this?
Is there a way to check the cylinder head gaskets and lower power head gaskets without dismantling the engine? Will a simple compression test do this?

Thanks in advance for any and all ***istance.

[boats.net]

Run engine to get warm then run a compression test to see if all cylinders are even( within about 15% of each other) look for water on spark plugs, there was some cases of water tube corrosion, need an water pressure installed to see if have at least 4 psi of water pressure at idle.