03-31-2016, 11:25 AM
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.
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.