Hey, guys, new here, but not new to Mustangs (86 GT and 94 GT engine in a Ranger). Seems I'm in a very similar situation. I'm going to keep an eye on this thread to see where it leads. I'm going to throw my similar situation out there, which may or may not be of help. It's going to be long, but thorough.
I have a 94 Mustang 5.0 in my Ranger. I have been having an issue that has continually gotten worse and I'm at my wits end to figure out what is going on. The engine is a stock engine with wiring harness, computer, etc from the Mustang. When cold the engine starts up and idles well. Going down the quarter mile driveway it seems fine and then upon acceleration it acts like I just pulled the choke on. It bogs down, has no power, occasionally backfires out of the intake. Then, for no reason it will just start running great with full power. I initially pulled an O2 sensor code and a 542 (fuel pump circuit). I'm not sure what the fuel pump circuit is supposed to mean. I have a low pressure and a high pressure pump and both are working well. I can check fuel pressure even during the low power issue while accelerating going down the driveway and it stays around 40 psi. So, I would like to think that I could rule out the fuel system. Now, the thing I don't understand is that when initially trying to pull codes I could not get the code scanner to work. When I plugged it in and turned the key on it would just beep in the hold position. When flipped to test it would do nothing but continue to beep. I ended up messing with the wires at the connection to the computer and found that if I wiggled a certain wire it would stop beeping, but if I touched another wire it would start back up. That certain wire was for the O2 sensor. So, I disconnected the O2 sensor harness from the injector harness and I could now use the tester. Figuring there was a problem with the O2 sensor harness, which I made myself 6 years ago (solder/heat shrink high quality job), I pulled it out and tested for continuity through all the wires (along with a visual inspection). I could not find anything causing a short. So, with the O2 sensors disconnected I reconnected the O2 harness to the injector harness and again could not pull codes. Figuring there was something wrong with the connection there I eliminated it and hard wired the O2 harness to the injector harness. Well, that just made things worse because I can not pull codes at all now.
Similar to the situation above it will occasionally go into self test mode without the scanner (idles between 1400 and 1800 on its own for a short period of time before going back to normal).
When this problem started I could let the truck warm up and then it would run fine. Now, it seems that it will barely allow me to drive it even with it warm.
I have already swapped the computer to verify that's not the problem.
Any ideas on this would be great. I hope we can solve both our problems.
Brian