I'm still searching for the reason my 350SDL is running hot under heavy load. I'm working my way towards the radiator testing/replacement but noticed the EGR valve this weekend. How is this tested for operation? Can I pinch the vacuum hose to it to disable it?

I see there is a tube that goes from what looks like the control switch to the cars interior. What is it? Door locks?

EGR valve is there to help with emissions, part of EDS system. The EGR system won't cause any overheating. Radiator & or other engine componets will cause overheating, head gasket leakage is common on this engine.

Is head gasket leakage common on the 300SDL as well?

What causes the head gasket failure?

Head gaskets on aluminum head diesels are subject to more stress than any other car. Any over heating will cause premature failure. The engines uses stretch bolts & they are designed to keep the head secure to block. Stress from combustion heat is a common reason for failure. Show me a car w/over 200k miles & most likely the h/gasket has been replaced.