In a predicament, please help

What year car is this?

On my '90 the EGR valve wasn't even hooked to any sensors aside from a vacuum line, I blocked it off when I put on my header and never had any problems. I don't really see why a faulty EGR valve would cause detonation either.

Have you checked your timing?

Timing would make it ping more than an egr.

Ahhh....I have no idea how you check the timing on a DIS car. Hopefully someone here can chime in.

However it might be worthwhile to check the timing belt. Any idea how long it's been since it was changed?

I hope this helps. I don't know the '91 that well; my maintenance book doesn't mention any difference in the EGR circuit over 89-93.
* The purpose of Exhaust Gas Recirculation is to lower combustion temperatures. Thus, less tendency to ping.
* Ignition timing on a DIS is just like most EEC-IV's: pull the SPOUT plug and look for 10* advance +/-2 on the #1 (front) cylinder. Please note there's no (simple) way to change it.
* Cam timing: Rotate the crank to TDC with a 7/8" socket (clockwise only!!!) and confirm the mark on the cam pulley lines up with the mark on the (belt shield, I believe. It's obvious). You may find the cam mark is 180* off, that's OK, rotate the crank one more revolution.
* You haven't mentioned what may be the key. The EGR Vacuum Regulator (EVR) controls vacuum to the EGR. Either a plugged vacuum line or a malfunctioning EVR will cause the EGR not to function. Again, my book doesn't say whether the '91 has an EVR. You'll need to trace the vacuum line from the EGR and confirm for yourself. The EVR is a small, black cylinder with two vacuum connections (in & out) and the electrical connector and is probably on the firewall.
You made a very good point when you asked Can a faulty sensor cause detonation. Not in this case, I don't reckon.
Hope this helps, good luck!

I pulled the vacuum lines off the EVR, and applied vacuum to the EGR line. The valve opened.

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Check. EGR valve functions OK.

I than reattached the lines to the EVR, and pulled the line off of the vacuum tree. I pulled a vacuum, and it did not hold. The EGR valve did not open. Thing is, the car was off and there was no power to the EVR. Should it hold vacuum anyways?

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Da Book says the EVR has a constant internal leak by design.
The reason why the EGR didn't open in this test is the EVR needs power to open. From the first note we know the vacuum line is OK.
Next test would be to check for voltage to the EVR, should be battery voltage KOEO on one of the two pins (the other pin goes to the PCM, switch-to-ground). The voltage should be 0 DCV during hot idle.
It doesn't mention imposing a voltage on the EVR to test it. Probably a good way to burn out the solenoid. Try this at your own discretion, and only for brief moments.