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I built this pickup a few years ago and really haven't had any problems with it until now. it's a 1980 stepside with the engine, wiring harness, fuel system and basically everything else from a 1986 efi. One day it just started running really rich. I ran the codes on it and it's telling me the MAP, EVAP, and O2 sensor are all bad. It's also giving me ton's of codes that say testing is out of range.
Is there any one thing that can confuse the computer this bad or did something cause everything to screw up all at once? If i can't find the culprit, i'm going to start going through all the wiring, (it needs some work in that department anyways)
I would look at the wiring. Groups of sensors on the engine are supplied with 5 volt power. If something happens to this power supply, it can cause multiple sensor errors, since they are not being fed the correct voltage. Also check the vacuum line going to the MAP sensor, and make sure it doesn't have holes or cracks.