Missing and backfiring wont run

It could be the throttle position sensor, MAP Sensor, or Mass Airflow Sensor (MAF) Symptoms of Bad or Failing Mass Airflow Sensor YourMechanic Advice
Symptoms of Bad or Failing Throttle Position Sensor
Symptoms of Bad or Failing Manifold Absolute Pressure Sensor MAP Sensor...

If you need further help, reach me via phone at https://www.6ya.com/expert/steven_a67ead41eb5fc62a

Check engine light on , what codes ? Map sensor , mass air flow sensor .

crank angle sensor or airflow meter/map sensor

run fault codes to find what faulty sensor you have missed

crank sensor would be my next bet. 40 mph is safety mode to not damage the engine. i.e get home mode.

p0300 random misfire can becaused by a few different thing's - low fuel pressure or flow ,weak fuel pump, clogged fuel filter , clogged injectors , vacuum leaks , incorrect sensor input to PCM - mass airflow sensor or MAP sensor , TPS - throttle postion sensor , Hooking up a professional or factory scanner an viewing engine sensor data should be the first step in the diagnostic process , before ever replacing any parts . What are the fuel trims at ? Adding or taking away fuel ,O2 sensors switching normally ?
016 Engine Misfire Misfire Scan Data

There are a number of things that can cause that, but, the most common cause is MAF, ( mass airflow ), sensor.
This is easy to check,
On the top, or side of the air intake duct, between the air filter and the engine, you should see a small plug that usually has 4 or 5 wires going to it. Carefully disconnect this plug and then drive the vehicle just enough to see if the engine has better power.

If this seems to fix the problem, you should try to replace the part as soon as you can. If you drive the car too much without the mass airflow sensor, it can lead to damage of other sensors, and could eventually cause ill effects to the catalytic converter.

If there isn't a change, my next best guess would be the oxygen sensor. A faulty oxygen sensor prevents the computer from being able to determine the proper fuel/air mix ratio. And that certainly can cause the engine to lose power as well.

I hope this helps you.

They are different types of sensors, but in your car the MAP (manifold absolute pressure) sensor, along with the BAROmetric sensor are used to estimate the required fuel flow through the throttle body injectors. These sensors measure pressures inside and outside the engine to estimate airflow. You do not have a MAF sensor. These are more common in later vehices with mulit-port fuel injection. A MAF sensor actually measures the airflow into the engine using a hot wire anemometer. You are correct in believing the sensors serve the same purpose, that is to calculate the fuel flow required for stoichiametric combustion. The oxygen sensor also feeds back to the computer to further refine the fuel charge by measuring oxygen in the combustion products. If excess oxygen is detected, the fuel charge is increased. This is known as closed-loop operation. Closed loop operation is not started until the engine is at operating temperature. If you have more questions, please let me know. I have pasted the location of the MAP sensor below. The sensor is mounted to the air cleaner, but it has a vacuum hose that leads to the throttle body, which is where the pressure must be measured.

Several things that cause poor gas milege! Bad thermostat,temperature sensor,MAP SENSER,OXYGEN SENSOR ,MASS FLOW AIR SENSOR,PLUGGED CONVERTER,FAULTY COMPUTER,CHECK IGNITION TIMING.
lOW TIRE PRESSURES. VACUM LEAKS,TO MAP SENSOR FUEL PRESSURE REGULATOR ,AND INTAKE MANIFOLD,PLUGS MISS FIREING BAD WIRES,DIRTY, WORN OUT SPARK PLUGS .

Things that cause this are usually carburetor problems....sometimes the fuel/air mix is off and the timing too.

What have you done so far to the engine that coould be related to either of these?