Ford Repair: 89 Mustang EEC-IV, ford vehicle, egr valve

Question
My mustang is constantly displaying a code 31, which is EVP,EVR, or PFE circuit below minimum voltage. It's an 89 LX with the 302 that I'm restoring. I've already replaced the EGR valve, the EVP, EVR, and both thermactor air solenoids. The voltage is within range for the EVP, and the EGR valve is actuating. I'm trying to find out what the voltage range is on the EVR, and where is the PFE. Chilton's and Hayne's both mention it, but don't show it. Thanks.

Answer
Hello and thanks for the question...I'll try to help.
 The computer system in the Ford vehicle's works on a 5 volt signal from the computer. Also, your vehicle doesn't have a PFE (Pressure Feedback EGR valve). What I would suggest is that you check the connectors of the computer system, as well as the grounds of the vehicle.
Hope this helps,
Erik