You have done a lot already. Are you certain there are no vacuum leaks? There is a vacuum hose that runs off the back of the throttle body and will have a 90 bend in it. They commonly dry rot and leak.
If that isn't the case, what code are you getting for the sensor? Also, are you sure it is the engine and not a transmission issue? I ask only because there are no misfire codes.
Keep in mind that the O2 sensor could cause an issue, but I would think you would see a code and the exhaust leak, as you already know, can be the cause. So I would really like to know the code you have.
Next, have you checked fuel pressure to confirm there isn't an issue? Pressure that is too high or too low can also cause a misfire.
Here are the specs for pressure:
1998 Ford Truck Expedition 2WD V8-5.4L SOHC VIN L
Vehicle Powertrain Management Fuel Delivery and Air Induction Fuel Pump Fuel Pressure Specifications Pressure, Vacuum and Temperature Pressure
Engine Running .................... 193-310 kPa (28-45 psi)
Key ON Engine .................... 242-310 kPa (35-45 psi)
Also, a partially restricted exhaust system (converter) can cause a misfire often along with a loss of power.
If you have a live data scanner, could you let me know what the short term fuel trims are?
Let me know.
Thursday, November 12th, 2020 AT 8:55 AM