[Katshot]

Damn Wes, you were doing great 'til the last sentence. I know of no engine that applies EGR "constantly".
Wes is correct right up until the last sentence Sal. By "diluting" the intake charge it stops detonation and cuts emissions among other things.

 

[Katshot]

The EGR system depends on not only the VALVE but the exhaust gas PASSAGES. Even if the valve functions properly, the "system" may not work if the passages are clogged with carbon. This is a VERY common issue on MANY cars. In this case, you may need to clear the passages before the problem can be fixed.
Also one side note: Even though the EGR valve on the Fleetwood is an old style vacuum operated type, many newer EGR valves are "digital" and are operated via an electrical solenoid. These obviously CANNOT be diagnosed in the manor described above, and are easily identified by their wiring connection instead of a vacuum line connection. Also, EGR valve replacement can be a tricky issue on many vehicles due to the fact that there are several different types of vacuum-operated valves, and they all look alike. So I would agree that the safest path is to use the FACTORY replacement part.
BTW, there is also a possibility that EGR operation can be inhibited by the installation of aftermarket exhaust systems since they change the amount of system back-pressure.