The 84 500SEL doesn't use an EHA valve.  It uses a control pressure regular
(block with 2 fuel lines and an electrical connection located near front of
engine).  Fine tuning of fuel mixture is done with a frequency valve located
on left side of the fuel distributor.  It looks like an injector but has
fuel lines on both ends.
First, remove the two screws holding the cold start valve to the engine.
Carefully pull it up (keep the fuel line hooked up.) and place it in a small
container to catch any fuel that comes out.  Have someone start the engine
while you watch for fuel comming out.  There may be a spray while the
starter is enguaged but none after the engine starts.  If it leaks you have
found your problem.
If that wasn't it, remove the air cleaner assembly and make sure the mass
air flow plate (thats what I call it) is completly closed.  If it is
partially open, that will be your problem.
If the above fail to show the problem then you will have to check the
control pressure.  This takes a special fuel pressure guage.  This is not
the system pressure but fuel pressure used to control the fuel distributor.
I have seen quite a few control pressure regulators go bad and cause the
problem you have.
By the way, I have an 84 500SEL and I am speaking from experience.
Paul