You are absoutely right; when the Service Engine Soon light indicates a problem with the EGR valve, it is usually just that the valve needs cleaning, rather than needing to be replaced. Sometimes the connecting passages in the manifolds which carry the exhaust to the EGR valve may be clogged; so it is also important to at least look at those passages when the valve is off. If they are relatively unobstructed at the point where you can see them; it is likely they'll be OK through their whole length. But if they are clogged; it sometimes may be necessary to do further disassembly, in order to access and thoroughly clean the entire system.
There is no electronic portion of the EGR valve in your car. It is a simple mechanical device, which is attached to the manifold by two bolts; in which a spring loaded actuating diaphragm opens and closes a metering rod in response to a vacuum signal. Here's a link to a photo of your valve: http://info.rockauto.com/SMP/SMPDetail3
.html?EGV894.html You can reach under the diaphragm with your hands, and move it upward against the spring by pressing on it with two thumbs. If this is done while the engine is idling; lifting the diaphragm should cause the engine to stall, or slow down dramatically. If lifting the diaphragm has little or no effect; then the EGR valve or its passages are clogged. Sometimes an EGR valve will stick. In many cases, it can be freed up by just working the diaphragm up and down; without even removing it. An aerosol can of Valvoline Syn Power fuel injection throttle body cleaner can also be a useful tool to help clean out deposits from the EGR valve; by spraying the solvent directly on the dirty area.
There is a separately mounted electronic vacuum control solenoid on this vehicle, which is very reliable, and requires no maintenance. However, if the vacuum hoses between the throttle body, the control solenoid, and the EGR valve are loose, cracked (even in tiny areas), damaged, or leaking; this can cause the computer to set trouble codes.
My experience with EGR valve trouble codes is that they are often misleading. I have repeatedly seen EGR troble codes generated in a car which had nothing wrong with the EGR system. The problem in those cases often turns out to be that the spark plugs are either worn, or are an unsuitable choice for this particular motor. Mitsubishi engines are unusually sensitive to differences in spark plug brands and configurations. Because of this situation, I would only use either a Champion # 4071 Truck plug (at .040" gap) or a Bosch Super Plus # 7956 plug (at .044" gap). The Champion Truck plug may not be stocked by most parts stores; but they usually can order it. The Bosch Super Plus is a new and greatly improved part, which supersedes the old Super line. This plug may also not yet be commonly stocked, and will have to be ordered. But it is well worth the trouble to get the best part.
The other thing that can cause EGR codes to set is an excessively lean fuel mixture. This can come from a clogged fuel filter, a throttle position sensor that is improperly adjusted, or a dirty throttle body or mass air flow sensor.
And the reason that gas caps set trouble codes is often not that they are worn out; but rather that they are not tightened at least four clicks past the point where they stop moving.
I hope this helps!!!