I hope he means EHA!
You won't find much under HEA, but do a search for EHA or electro hydraulic actuator and see what you find. It's black in color and is "about half the size of a cigarette pack" on the driver side of your fuel distributor. Take off your air filter element to check it.
Also, have you had your fuel filter changed recently? The copper seals on the filter can be tricky and must be tightened down properly to not leak.
'91 300E, 215K miles