You may need to do a fuel pressure check and make sure the fuel pressure is consistent with key on and when engine is running also driving.
A bad fuel filter or a faulty fuelpump relay could cause the issues youre experiencing so can dirty connector pins at the salt n pepper shaker connects that control the fuel injectors and send readings to the ECU from the sensors..........
Most times an issue like yours can be related to an airleak anywhere in the system............A DIY smoke machine or a cigar will do the trick and tell you where every leak is as those vacuum tree lines are highly prone to cracking.....
A bad coolant temp or air temp sensor could be giving the wrong temp & could be the culprit too.........
Even a faulty TPS can throw a lean code just like advancing the distributor too far too.........
Putting a bigger throttlebody or a bigger MAF or modifying the inlet tube or relocating the aircharge temp sensor into the intake can cause huge issues too..
What I do is start with the basics like verifying the MAF is working as the engine load is determined by what the MAF reads...
What most dont know is with a Mustang computer is you can use an adapter wire to connect the car up to an OBD2 scanner or you can get an OBD2 scanner that also can read OBD1 codes so you can read live data .............
Its great to see everything thats going on while the engine is operating..................
To rule out the O2 sensors I first make sure that the ground wire coming off pin #49 is hooked to the intake manifold and I verify its grounded good then remove them from the car and clean them in straight lemon juice for 20-25 mins then rinse with rubbing alcohol then blowdry with air...
Another thing to note is if you disconnected the EGR and dont have a shunt or resistors or a tuning chip to trick or turn off the EGR you wont get any spark advance too and will stay locked at 30* of timing as a default variable because the lil electrical doo-dad is a potentiometer mounted on top of the vacuum diaphragm and tells the computer whether to advance or retard the ignition depending on vacuum signal and Pin position plus BAP reading.................
Theres 2 methods to disabling the EGR Im partial to and one is the chip and the other is just by blocking off the EGR port with a simple blockoff gasket and letting the EGR do its thing without injecting dirty air into the intake stream but if youre set on using the resistors buying a wideband controller and using the EGR pin to control lambda would be a good idea................
Another downside to doing the EGR delete is engine detonation if premium 93 octane or higher isnt used,,,,,,,,,,,,,,,,,,
It literlly took 5 mins to disable the EGR on this 351w using a piece of self sticking foil tape over the hole and reinstalling the EGR............
As you can see from this picture this is how to block them off fast n easy then reinstall the diaphragm and sensor.....
I dont like doing the resistors because all they do is trick the CEL and spark advance is taken over by only the BAP throwing off calculations but can also incorporate the thermactor delete into the EGR resistor trick too............
Good Luck
