Hi,

The symptoms you are relating, backfiring and dry plugs, are all pointing to fuel starvation or a gross vacuum leak. The 121 code seems anomalous, given the AMM response you reported. Both of those codes you saw are supposed to set the CEL; can you lift the 25A blade fuse (by the battery & PS res.) out for a moment to reset the codes and trim on the ECU and see if those codes return?

If fuel starvation, the most common trouble is cracks in solder on the circuit board in the fuel injection relay assembly. The easy fix is reflowing the solder on them. The relay is clipped to the snap that holds the ignition computer to the firewall just above the passenger's feet. Well, it was.

To check whether that all-too-common relay problem is what is preventing fuel pressure, connect a jumper wire between the left sides of fuse 4 and 6 to bypass the relay and operate the fuel pumps directly from the battery.

A used fuel rail can be a lot easier to find than adapters for your pressure gauge. Here is an example of one Randy built shown in the 7/9 FAQ click here or you can just braze in an A/C test fitting like I did on mine. I hope you don't need to do this for one car.

The regulator's failure is most commonly to dump gas in, not withhold it. The diaphragm rips, sending the fuel into its vacuum line, or the outlet to the return line sticks. But I had one stuck open once, resulting in less than 10 lb. pressure. The spring inside rusted and I could actually blow through it from rail connection to return line. I think that might be less common.