When the OVP relay is bad or missing the KE control unit is unpowered. With a warm car the feedback control system (Lambda) should be set to zero current correction. This normally means the car runs about the same warm with or without electrical control.
If your car runs good without electrical control it probably is running rich to cover up a problem. When the OVP is installed the system tries to adjust itself to its appropriate lean mixture. Your car either doesn't like it there or there is a problem with the control system. It will have to be monitored for the answer.
Bosch Master, ASE Master, L1
33 years MB technician