Its common for the egr passage to plug up inside the intake manifold on those. That's just one possible reason for the code 51. The way the system works is during a deceleration the PCM monitors the map sensor signal, which would be reporting a very high vacuum (low manifold pressure), and while its doing that, the PCM commands the egr valve to open. By opening the egr valve the manifold pressure should increase (lose vacuum). The greater the change in the readings, the more egr flow that is capable of occurring. Little to no change indicates the egr system isn't working and the cause needs to be found.

BTW parts stores sell thousands of PCV valves every year simply because people buy them and replace them. I don't think I average finding more than two bad valves a year. Now that could be in part because people like to throw parts at the cars instead of taking it to a pro in attempt to save money. The two or so that I find bad are actually situations where someone has installed the wrong calibration valve, and it ends up causing a problem.

Better rethink this one, john_g. A '91 Metro has OBD1; no monitoring of the EGR performance is done by the PCM.
The OP needs to test the resistance of the EGR vacuum control solenoid, and then the rest of the circuit if that checks ok.

California emissions does require active monitoring of the egr system. There has been some issues with the wording of the trouble code, and the potential causes because of less than accurate service information. A solenoid electrical problem could indeed set a code 51, so can a bad vacuum hose, clogged intake manifold (the most common cause) defective egr valve, even a map sensor problem is potentially in play for this code. Accurate diagnostics for the specific vehicle always come into play.

There are reports from techs all over the country who attempted to diagnose these cars for the code 51, and the egr solenoid was replaced but the light still came on. The problem was incorrect service information.

Real shops, don't use universal sensors. We learned about that in the late 80's. The real question isn't why do some shops still occasionally try to use them, its why do parts stores still sell them?

The link you provided can never be am "answer all" for the automobile owner, none of those types of services can be. One of the greatest myths about auto repair there is, is that mechanics know whats wrong with the car based on the symptoms alone. The reported trouble can be quite specific enough to allow confirmation of the source of a concern to be made easily, and rapidly with little more than the mechanics experience. However in order to prove that the car is indeed repaired, testing before hand must be done to have a statistical measurement of the problem, and then subsequent testing must be done to confirm that the repair has made a difference.

One thing I will say for the site and its repair shop search, my shop came right up and had the highest number of stars in my area at 4.5 out of 5. Even the dealers only had 3. VBG....