Running rich but reading lean - Bad ECM?

Discussion in 'Fox 5.0 Mustang Tech' started by bullitstang1313, Aug 21, 2006.

  1. Hey guys, I've been having some trouble with my car since I added new 24#'s, 80mm Pro-m and Trick Flow intake. My car is running extremely rich. The maf is calibrated for the 24#'s and I used my Tweecer to set the injector slopes and MAF curve. The computer keeps giving me fault codes 41 and 91 for lean. I replaced the O2 sensors because the heater element went out in one of them. I also ran new wires to the O2's from the computer to try and eliminate that. When I datalog with the tweecer the O2's read basically 0 volts. Anybody have any ideas?
  2. It's fairly common for an 02 sensor to read lean when you have an extremely rich condition.

    Here is a quote from the megamanual (megasquirt) "Beware that if you go so rich that you start to see misfires, the lambda sensor will read the unburned mixture as dead lean (since unburned oxygen is present)"
  3. I didn't realize that. So what can I look at to see why its running so rich. Even in open loop mode it is extremely rich.

    If this is the case, when it gets to closed loop it thinks it sees a lean condition and continues to increase fuel but never sees the lean condition go away. So what should I look at see why its running so rich to begin with.
  4. Hmmm…your on your own as to why. I'm not familiar with the "FAST" systems. One possible reason could be your fuel pressure being to high. Another could be a faulty MAF meter.

    If it's computer related I would imagine it's going to be tough to troubleshoot without a wideband.
  5. MAF meter is a new Pro-M/PMAS 80mm calibrated for the 24#'s. Fuel pressure is set at 39-40 psi with the vacuum line disconnected. I am looking into wideband O2 sensors and have thought about the FAST system but I don't really want to put that much into the car right now as I've dumped alot into it lately. I'm looking at just putting the old Stang in the Corral for the winter and getting me a new truck.
  6. All I can think of is the ground wire for the O2's.

    Sometimes the ground is left off, or poor contact is made due to fresh paint.
    The O2 ground should be an orange wire mounted to the firewall with the block ground strap on the drivers side of the engine bay.

    Hope you get it sorted out.

  7. I ran new power, ground and signal wires, maybe I'll double check all of them later. I guess its one of those things where you have to take a break from it to clear your head.
  8. Okay, wiring looks good. Still can't seem to think why it would be running rich even in open loop mode. Okay, so what in general can cause an engine to run rich, here are some things I thought of:

    Intake Air Temperature (IAT)
    Engine Coolant Temperature (ECT)
    Mass Air Flow Meter (MAF)
    Improperly Matched MAF and Injectors
    BAP Sensor

    Any other ideas?
  9. I had a 91 mustang GT that had the same problem that you are discribing. My car was running rich because my ECU was dumping more gas into the engine to correct a lean problem. I changed the MAF sensor and still had the problem, so I looked for leaks by spraying carb cleaner around the intake and heads to see if that would find the probelm, but this didn't work either. As i was reading my fuel injection manual (forgot the name of it) I noticed that there was a vapor canister that is on the passanger side that a hose was going into from the upper intake. There is a little ceramic sensor that was attached to a wire that I believed was gone bad (my book that I bought directed my to this canistor). I went to a salvage yard and replaced the canistor and sensor and for some strange reason this fixed my problem with the rich (really lean) condition. Hope you find the problem. Keep me informed.
  10. I'm having the same problem with my 347. Codes 41 and 91 but getting only about 7 MPG.

    Runs fine upon initial start but once it warms up the idle starts to hunt and the check engine light comes on.

    I have new o2's, TPS, wires, cap, and rotor. I cleaned the MAS, IAC (twice), and throttle body.

    Good luck and I'll update if I get this solved.
  11. this is a long shot but i swapped everything from one mustang someone totaled for me to a theft recovery "shell". it was just a rolling unibody. i completed the install and it ran really rich so i cleaned every snsor i could find to no avail. i eventually sold it to an acquaintance who fixed it. where i unscrewed and cleaned the mass air sensor, he simply unplugged it then plugged it back. i kick myself still.