The OVP provides conditioned electricity to the electronics (engine controls, climate control) to prevent a voltage surge from frying them. If it is bad or intermittant, it will interrupt current supply to the engine computer, ignition module, and so forth, giving you erratic running. You have to measure the output, not whether or not it has current supply.

On a 91 you probably have electronic injection, not KE-jet (definitely if you have a MASS unit instead of individual relays). I don't know if the fuel pump is operated via the "big box" or via a standard fuel pump relay.

You need to pull the trouble codes before getting all bent out of shape, that system has on-board diagnostics and will tell you what isn't working.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!