Hi guys, been browsing this forum for a year or so, now making first post. Trying to fix my grandfather's old 1986 302 EFI as it sat for a few years but struggling with code 31 (have an innova obd1 reader). Got a new egr and evp from napa a while ago before I fully understood the eec-iv and debug procedures so I know the egr at least is good.

I've done the tests described in shop manual but am still stumped...
Tested and have the following.
5v vref at sensor
0 ohm continuity from sig return to pin 46 at ecm i think, (don't have schematic in front of me)
0 ohm continuity evp signal from sensor to pin 27
Ecm grounds are good
Voltages at the sensor are same at ecm connector (did the paper clip trick)
checked resistance values of sensor and in line with manual
Measure. 0.06v evp signal with plunger fully extended and 3.9v with plunger pushed in all the way
I read the voltage threshold for this input is around 0.25v which I'm below and causing code

I even tried to "trick" the ecm by doing a scan on the reader with the plunger pushed in to give the overvoltage code but this didn't work either. Still gave code 31.

I tried the tests with 2 different sensors ( factory and new one from napa),and I have identical problems.

I'm totally stumped with this. Do you guys have any suggestions to try or somewhere else to look? Thanks in advance!

 

The guy in my book says to first determine are you getting code 31 during the KOEO or the KOER test. He says if you are getting the code during the KOEO test, that the computer doesn't like the signal it is seeing from the position sensor on top of the EGR. I assume you pull the battery cable for a minute to clear the codes (unless your code reader will do this) and then do the KOEO test and see if the computer sends the code 31 again without starting the engine.

He also says you will get a code 31 if the voltage signal to the computer when the EGR is closed is less than 0.2v. You said you are getting 0.06v with the EGR closed so that is going to set the code correct?

Another thing he said you can do is unplug the connector on the EGR sensor and take a paper clip and jump the vref signal wire to the signal wire that goes back to the computer. This should generate a code 35. If it does, this is a quick way to prove out the wiring to the computer, and verify the computer is working also. Code 35 is generated if the computer sees a signal greater than 4.81v on the signal wire.

So this signal wire has to be between .2v and 4.81v to not set a code.

 

thanks for your response. The codes I am getting are KOEO, shows as 31 O on the reader. According to the manual shown in attached pic, these are codes that are present currently and are not stored in memory.
The other pics are voltage at ecm backprobed for pin 27, 0.05v with evp sensors connected, and 5v with evp disconnected and jumper vref to evp at sensor. Still get code 31 O if I scan in both scenarios, which is why I am stumped.

 

The guy in my book says the voltage needs to be at least point 2 volts .2v. (I can hardly see that dot this evening). You are getting .05v, that is too low, so that sets the code. If you do not get a code 35 when 5v is sent to the computer, and you verified you are getting 5v at the computer terminal, then the computer is bad.