your fuel thermostat.
MB# 601 070 02 82
#. 125 in the attached picture.
O-ring #. 149 is a common failure item.
If the O-ring fails = sucking air into the injection pump.
Some symptoms:
* Sputtering
* Poor performance
* Erratic idle
* Lack of power
* Stalling
* Difficult or impossible to start
To test this, you can join hoses #. 197 and #. 191 = bypass it, and take the car for a 1 - 3 mile drive.