EFI Problem - Solved
OK - Sorry this was not on a Mustang but the diagnostic technique and solution will apply across the board.
Problem: Very rough running with no meaningful fault codes available from the ECU. Engine running lean then rich; lean then rich and some times over fueling to the point of stalling.
Diagnostic Procedures: 1) Check fuel delivery - Pump working and filter not blocked. 2) Check fuel rail pressure / pressure relief valve with vacuum pipe on and off. Within limits.
If both of those are good you are about half way there. I admit I was lucky to be able to check what the air and coolant temperature sensors were reading (the same to within 1*C) on the cold engine.
The next 'Best Guess' (OK my mentor knows his stuff) was the ECU unable to 'find' the correct injector width pulse for even running. It was running lean for some seconds and then dump in fuel which sent the mix fully rich and then back to lean - all the time the engine missing and shaking. Restricting the return line (raising the rail pressure) made the engine run better.
He surmised the ECU could not receive a signal it could use to stablise the mixture, so it was decided to disconnect the the O2 senders. This makes the ECU revert to its inbuilt, factory set 'best guess'. The engine ran very much better (would have got you home easily).
As the engine ran pretty well with no O2 sensor input it was decided to replace them with new. Result? Total transformation - back to its old self.
I actually used BOSCH Premium units (Pelican Parts have them). I have been told (now have proof) that O2 senders have a life of about 50-60k miles and can then become 'unreliable'.
Hope this helps someone out there.
Yep the o2sensor can wreak havick on an engine. I had an 87 Ford F150, and had an o2 sensor stick in the rich mode, it would run rough and almost stall at idle. The gas mileage also went from 15-16 to less then 10. took it to a friend at a local shop cost me 20 bucks over the cost of the sensor, and I felt LUCKY! Now I know what is going on, back then I didn't have a clue. Semi Trucks didn't have that stuff. They were just starting to use electronic controls. In the early 90's. Good diagnosis. JTS 71 Mach1
Good information to know. I have an old 97 Ranger that is starting to show some engine symptoms now so I will keep your diagnostic in the back of my mind.

Thank you!

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