87 Mustang 5 speed running really rich hard start when warm....
Reply #34 –
Can you confirm the thing charges properly? What is the battery voltage once you shut it down hot? Coil is easy enough to change. Can he lend you the coil outta his cougar/know anyone else with a Fox 5.0? Did you just obtain the DA1 computer or attempt to install it already? If you try another coil, AND the computer, and there's still no change...I guess I'd rip the harness out and look for evidence of critters having chewed wires up...previous owner botching things etc...cause I'm pretty sure you've replaced EVERYTHING!
Is there *any* observable change, any stupid boring detail that seems to act differently once the car has run for 5-10 minutes or so? I am guessing not since as Demon stated, unplugging the O2 sensor harness made absolutely no difference. So it's either not even entering closed loop, or some other externality is so bad that the comparatively teeny weeny input from the O2's commands no authority. I don't remember...have you checked vacuum? Ought to be 19 inches, I think even 18 would be too low. That's a speed density car so it needs every bit of 19 inches. Personal experience has shown me that 16 inches makes the exhaust smell HORRIBLE and the engine refuses to idle properly for any extended period of time, without opening the throttle manually with the stop screw to help out.
If someone you knew had a scanner that was OBD-I compatible and could watch the computer's signals, you'd learn oh-so-much. The idea has already been tossed out about the Engine Coolant Temperature sensor which you've replaced. I believe that sensor is supposed to provide a variable resistance that the computer measures. Find out the spec on this and replicate it with some RadioShack resistors. You might need to do this for several sensors should you choose this diagnostic route. Somewhere I think you'd find a broken circuit, and the computer is receiving no input, and who knows what it'll do with that.
Does it run "super-duper excessively rich" from the MOMENT you start it cold? Because we can all vouch for starting our car cold and not smelling a horrible rich condition. Meaning that the computer doesn't add THAT much fuel even for a car started at 5 degrees F. It's just not programmed to do that. If this is happening then I say vacuum leak or some disconnect or error between the MAP and ECM causing the ECM to believe the manifold has less than 19 inches of vacuum, which it interprets as throttle is opening, air is rushing in, acceleration is taking place, and extra fuel is needed. Again, I have driven around (another lame-ass long story of stupidity for another day) a 5.0 for 2+ years, running on 16 inches of vac, with speed density, and boy what a pain in the ass. Poor idle, misfiring/stumbling, harsh and rough harmonics in the cold (okay it also had a random unknown mild aftermarket cam, re-read the remark about dumb-ass story) were the order of the day until I found a new daily driver. (But around 3,000 RPM or so she'd really turn on!!! 
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