As Minnet stated, the EHA controls the mixture. This is done through the fuel distributor which rapidly switches between high and low pressure chambers to get the proper flow.

It can malfunction in several ways. Mine was corroded in my 91 300TE and after replacement with a factory refurb the problem was solved. Our dealseship tried first replacing the oxygen sensor and ECU. They had recently replaced the OVP and several other related parts for problems with this component which were causing the engine to die or to run way too rich..

I have been told that the piston that allows the fuel to switch between high and low pressure in the distributor can get gummed up and stick.If nothing else works, try opening it up and cleaning it. The MB refurbs are a lot cheaper than the new part and probably just as good if you have to replace it. Possibly injector cleaner could help, but on mine it just made it worse. Hopefully it is not the fuel distributor.