1992 f150 351 motor idles good but when you give it some gas it back fires through the intake. cranks good everytime. just replace the oxygen sensor and the electrical modules

try finding a mechanic with an old style oscilliscope tune machine so you can actually see what the mixtures and spark are like running i suspect cam or crank angle sensors

vacuum leak. get some brakekleen and spray the edges of the intake manifold where the manifold meets the heads and when you get a smooth idle thats your leak. also try it on every vacuum line

It should have an idle air control valve which lets air into the intake to control the idle speed. The valve could be faulty, or dirty, or the throttle body could be dirty. The computer opens and closes the valve to regulate idle speed.

if it is an older dodge with electronic ignition the gap is closing up and the advancement plate is hitting the reluctor the star shaped reluctor on distributor shaft and that will cause it to backfire etc.
replaced distributor.

Is this a high mileage engine? Have you checked to see if a worn timing chain may have jumped a tooth? Sounds to me like it's out of time.

Sounds more like a timing problem than an O2 Sensor. Distributor out of Sync would be my guess. It ran better at Higher idles because the timing belt/chain was tight from tension at low idle the timing was off which caused your problems. Now the belt is either broken or so loose the timing is out and the vehicle won't fire. If it has a Distributor cap remove it and watch the rotor. If it turns the belt is intact but Timing may still be off.

I know you mentioned the EGR valve has been replaced, but you may have a bad 'new' part. I know the car is electronic, but has anyone checked its ignition timing (timing belt been looked at, I know it only has 77k on it). Is the fuel pressure regulator OK and it vacumm supply?

Sounds like you might have an oxygen sensor problem. Your catalytic converter(s) maybe defective or dirty. That in turn can dirty up the oxygen sensor. A defective oxygen sensor will send faulty info to the computer, interpreting the fumes as cold (even when the engine warms up). That causes the computer to adjust the fuel mixture to very rich, giving you poor gas mileage and the fuel smell. Hope it helps.

cars need fuel, spark and oxygen to crank over. must be a problem with the air intake system. O2 sensor / vacuum leak / something similar

check compression on all cylinders or the timing. both could cause puttering but no starting.