I've spent considerable time fighting with my 740T's EGR setup.
I'm assuming it's the same system.

The valve on the fender is a one-way, electrically actuated valve that will allow a variable amount of vacuum to the egr valve. The amount of vacuum to the EGR valve determines how much exhaust is fed into the intake. My EGR is under the intake manifold and attached with the same bolts that hold the oil breather box.

To check if the EGR valve operates (not the actuator), apply vacuum to its vacuum line at idle. If the idle drops as vacuum increases, the EGR valve is good.
Other parts that can go bad are the temperature sensor, the actuator (vacuum valve), the vacuum lines, and the wiring to and from the sensor and actuator.

I think the self-test mode (mode 3) for the Bosch LH2.4 system has a test that will move the flapper in the actuator for you. If it moves, the actuator is probably all right. You can turn on test mode 3 and touch the actuator. If it's working it will click and you should be able to feel it.

There are two types of temp sensors with different threads, different locations, and one is Positive Temperature Coefficient, one it Negative TC. PTC means the electrical resistance increases with temperature, NTC means it drops with temp increase.

It wouldn't hurt to replace the vacuum lines.

If you need parts, Sweden Auto Warehouse in Sacramento, CA had everything I could have needed for my 90's setup. The problem with mine was a bad sensor, but the system from a 92 has been put into my 90 car. The 90 vs 92 sensors are different Temp Coefficient types (PTC/NTC) but I forget which is which. I'm not sure about the 91.

Best of luck!
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1990 740 Turbo, on its way to stock specs, maybe beyond