[MJS]

Bone stock, check engine light intermittent, mainly on with foot on the gas, light accel or cruise, off when coasting with foot off the gas. Three codes came up, 31, 91, and 95. Replaced O2 sensors, code 91 went away. Just 31 and 95 left. The scanner book and different online sources have conflicting information about the 95, with either a Fuel pump circuit problem or thermactor air diverter circuit problem, while 31 says canister or EGR valve control system. Neither gives much insight as to what might be the bad part. The car does have a fuel pump cutoff switch for an anti-theft measure, but the fuel pump runs fine and the car operates normally.

So far, I've replaced the EGR valve, EGR position sensor, and EGR vacuum solenoid with known good, low mile takeoff parts from my stash - no changes in that 31 code.

Thanks for any ideas!

 

[indy2000]

when a CEL, has no relevance to how you are driving.

a code does NOT mean a part is bad

wiring is the cause of 99% of all electrical issues.

that is why when you get a code 91, you don't buy a part, you check to see if it is in spec.

if you changed parts and the code persists, it only the wiring that is left.

 

[MJS]

The 91 was the O2 sensors, which had around 80K on them. Changing them made the code go away, one down, two to go.

 

[MFE]

That intermittent part-throttle CEL is almost always the EVP, but you changed that. When I see multiple codes I start to distrust the 10-pin connection integrity.

 

[MJS]

Fixed. I'll give a complete rundown here, since this code 31 on a Fox Mustang seems to be a popular Google search, as well as in the F-150/Bronco community, so maybe this will help others in the future.

The Sunpro scanner book says Code 31 - EGR valve position (EVP) sensor or pressure feedback EGR (PFE) sensor - signal voltage is below minimum specification or EGR vacuum regulator (EVR) solenoid circuit problems.

I took MFE's suggestion and pulled the 10 pin connectors apart, (salt and pepper plugs on the back center of the intake) cleaned them up with electronic cleaner spray, and applied dielectric grease to all pins before plugging them back in. Test drive: No changes. Next, check the resistance range of the EPS. Removed it from the EGR valve and probed the connections. Around 0-4 ohms throughout the pin travel, linear action. Tested another one I have, identical readings, no problem there. Next, check EVP voltage, key on: 4.5 volts - in range. Next, check EGR vacuum from the vacuum regulator solenoid, engine running. You are not supposed to feel any vacuum at idle, but I did - a little. I learned of a foam filter in these units from a Bronco site, they get dirty and plugged. I pried the cap off the top and pulled the filter out. It was packed with dirt and just about solid, so I soaked it in water and dish soap, cleaned it up, rinsed and dried it. I also blew the dust out of the unit with compressed air, and hit it with a shop vac for good measure.

I also had to replace three plastic vacuum hoses on the tree, because they get brittle with age and all split when I unplugged the tree including two right at the rubber. Had to drill them out. Fortunately, I have a pile of these hoses from boneyard Tauruses and such, so I was able to get it all back together and looking decent - albeit with a few different color hoses.

Test drive with the cleaned vacuum solenoid filter - no CEL. None. I now have two days and four errand cycles on it, no CEL. Seems it's fixed. The plugged filter didn't allow the diaphragm to move enough to supply vacuum to the EGR valve, and the EVP reported insufficient movement to the EEC and lit the CEL. It would also explain the foot on/foot off gas CEL on/off. So while the code didn't tell exactly what to look for, it did provide an area to look in.

 

[MFE]

I didn't even know there WAS a filter on those solenoids. Also, the 10-pin connectors are never fully fixed until you use a dental pick to distort either the male pins or female receivers for a really tight connection.

 

[MJS]

Neither did I, thanks Bronco forum! Might want to check yours, I'll bet it's dirty and may not be letting the EGR open all the way. The TAB and TAD also reportedly have the filters, but I couldn't get the caps off without removing the vacuum solenoid, and since there wasn't a problem with them, I left them alone.