96 Formula Running Very Rich

I am having a problem with the car running very rich. I am getting P0152 and P0132 o2 Circuit voltage high codes. When I look at the live data while it is running, both o2 sensors are switching between .800-.990 volts. fuel trims for both sensors are also showing 0. I went through the factory diagnostic chart for the codes, and One of the suggestions in the factory manual was to unplug the MAF and then check the readings again.

When I unplugged the MAF while the car is running the passenger side o2 will start switching normally between .1-.8 volts. The drivers side o2, just stays in the .800-.900 range. I also start to get negative fuel trims on the passenger side sensor, and still 0 on the drivers side. I think I have a fouled o2 sensor on the driver's side. I also checked to make sure the MAF was installed the correct direction and it is.

The car came with a jet pwr-flo maf sitting in the back. I swapped that one in to see if I needed to get a new factory one. When I put that one in, it starting doing the same thing, unless the maf was unplugged. Could the MAF wiring or connector is an issue? The one fouled out o2 screwing the whole works up? or something else? I got a new o2 sensor coming for now.

Swap the two pre-cat O2 sensors side-to-side to see if the problem with the driver side sensor follows the sensor (defective sensor) or stays on the driver side (defective wiring).

Are you reading long term or short term fuel trims? When one of those codes sets, that bank goes to open loop, so short terms/learning will be disabled on that bank.

As far as the MAF sensor, if you have a multi-meter with a frequency scale, check the frequency (Hz) on the yellow signal wire (vs. a ground). At idle, should be in the mid 2,200 -2,800 Hz range. If you have a high voltage wire running too close and in parallel with the MAF sensor harness wire, could be high voltage interference with the frequency signal as well.

After thinking on it, my first thought was a miss fire on the driver's side bank causing it to run rich. I pulled the plugs on the driver's side and inspected them. The plugs were light brown and pretty clean. I then checked the wires on that side, since the drivers side header seems to like eating them. They were also in good shape. I pulled out the driver's side o2, and it was jet black. I wiped all the soot off the sensor off and put it back in. I then did the key on engine off test of the o2 sensors, both pulled down to .080 volts with in a couple of minutes and they were warm/hot to the touch. I believe the o2 signal and heaters are working on both banks. I then checked the three wires in the MAF connector. The ground and 5 volt reference wires tested good. The yellow wire had 11.XX volts on it. I don't have a DVM with a Hz reading.

The codes were set when I tried swapping MAF's, and I was looking at short fuel trims after swapping MAF's. I did not know it ran in open loop when they were set. I cleared out the codes and tried again with the swapped in MAF. When the car went into closed loop, the o2's were switching up and down quickly and I was seeing negative short fuel trims from both banks. I then put the factory MAF back in. Car ran super rich again. Both o2 sensors were switching at .880-.990 and short fuel trims were 0 again. I believe the factory MAF in it is bad, and needs to be replaced. I also changed out the oil, since it smelled like gas pretty bad.

You indicate you got 11.XX volts on the MAF signal wire. That’s not right. With key on, connector off the MAF you should see 5 volts between the yellow wire and the black/white ground wire. When you reattach the connector, the voltage on the yellow wire will vary, generally less than 5 volts because the frequency signal is riding on the 5 volt signal.

Key on, harness off you should have ~12 volts on the pink wire. If you only saw 11.XX volts on the yellow wire when the connector was on the STOCK MAF (and not when it was on the aftermarket MAF), could indicate an internal short in the stock MAF between the 12 volt components and the 5 volt components.

Consider replacing the O2 sensor that was badly carboned up. If you just wiped the carbon off the part of the sensor you can see, that is just a protective shell, with holes or slits in it, to allow the exhaust to enter and contact the sensor “thimble” that is inside the protective shell. You also have to inspect the outer part of the sensor where the wires are. Any contamination - oil, penetrating oil used to loosen the sensor, an exhaust leak impinging on the sensor - can prevent ambient air from entering the inside of thimble. The thimble develops a voltage that adds to or subtracts from ~0.450 volt bias that the PCM supplies, based on the difference between the oxygen content of ambient air inside the thimble and the oxygen contained in the exhaust on the outside surface of the thimble. My wife's Jeep was running pig rich, and I found the power steering reservoir was leaking and PS fluid was running on to the exhaust pipe and had soaked the O2 sensor.

I will recheck the wires on the MAF connector. I may have gotten my wire colors mixed up. I will just replace the o2 sensor along with the MAF.