focussoft

Hello , This is my first post on this forum
Usually i find my answers by searching online no matter how long it takes
But this time i just cannot find anything specific and my knowledge about the detailed workings of the 4.6L fuel system are not enough to figure out the problem
First a little back story about the car, green '96 mustang GT manual transmission
It was a freebie given to me because it was in such a bad shape it wouldn't run, and they wanted it out of the driveway .
So far i have it running good , but i have a "closed loop feedback fault using 1 o2 sensor" ,without error code
I am using the app torque on my android phone with a bluetooth OBDII adapter to diagnose the system
The strange thing is it goes in to close loop for a while with no feedback fault , and both fuel trims are fluctuating around 32% ,and then suddenly after a few minutes it goes -20.3% fuel trim on bank 1 ,while bank 2 stays around 32% ,(indicating system things system running rich on bank 1 ? ), and the fuel status shows "closed loop using 1 O2 sensor (feedback fault)"
At the same time the engine idle turns real bad (probably because the fuel system compensates unnecessary due to wrong sensor info)
So it makes me believe something opens up and messes with the o2 sensor mixture sensing , but i can't find any information on what can do that on the bank 1 o2 sensor 1 .
It also has no power in high rpm and backfires like crazy on engine brake, but i guess thats to be expected with fuel mixture problems.
Any help would be appreciated !!

pure_kaos

So am I reading this correctly, you are only using 1 o2 sensor or is it that there is 1 o2 sensor that is sending a fault?

focussoft

No, it is that after the system goes in to closed loop, it runs as it should on 2 O2 sensors , but then after a few minuted suddenly comes with an feedback error and decides to use only 1 O2 sensor
the exact message i get is " closed loop using 1 O2 sensor (feedback fault)"
but i do not get an error code , but the engine also starts running real bad at the same time
From what i understand is that if the fuel trim is -25% it will throw an error code , but since its -20 , it doesn't
I am assuming that somewhere the emission system is opening something up that makes the fuel trim of O2 sensor 1 go haywire , but i don't know enough about this engine to figure out what can dump extra fuel on O2 sensor 1 ,to make it think its running rich on bank 1, after a certain amount of time , or at a certain temperature .
I need some help figuring out what (emission?) component can fail and make the mixture in bank 1 go rich from one to the other second.
It runs great when its cold and the first few minutes of closed loop.

imp

You do not seem to consider a possible faulty O2 sensor. Have you replaced them?

focussoft

well i would say i didn't consider that
I was actually the first think i considered, besides all the vacuum lines and tubes i checked and replaced
I bought 2 new O2 sensors, and then found out that the bank 1 sensor 1 was so close to the frame that the new sensor was to thick to fit on that side ,so i switched some sensors around .
But as i mentioned , the fuel trim on both sensors is around 30% in closed loop ,until after some time suddenly bank 1 goes to fuel trim -20%
So I believe the O2 sensors work fine now , else i would not get 30% on both sensors in closed loop ,imho
Its just that i cant figure out what can mess up the fuel trim on bank 1 when the engine gets on to temperature
it starts in open loop and runs fine.
it goes in to closed loop and runs fine , fuel trim on both banks 30%.
Then when the temp gauge climbs , suddenly bank 1 fuel trim goes to -20 and the engine runs like crap
Fuel system status shows a feedback error , but no fault codes show

pure_kaos

Sounds like what Imp said, a bad o2 sensor or even possibly the EGR.

http://www.autodiagnosticsandpublish...r-testing.html