Had a 5.0 liter, V8, long block 302CI engine put in my 1990 Ford Club Wagon. Have had a tune up, new dist. cap & rotor, spark plugs and wires, fuel filter, IAC valve, set ignition timing, fuel and exhaust back pressures both ok, replaced the speed sensor. The car surges with no obvious pattern (ie, cold, warm, in gear, in park, in neutral) and dies occasionally, but starts back up fairly readily. Have had it in to several different mechanics and NO ONE can figure out what’s causing the surging. Computer? Catalytic converter? Any suggestions???
A lean idle mixture can cause a surge. Was your fuel system cleaned?
If your fuel injectors are dirty, they will spray less fuel than the engine needs. I encourage people to have their injectors cleaned by removing them from the vehicle and cleaning them on a bench (not just pouring “fuel injector cleaner” in the tank).
Dirty / worn injectors are much more common than people realize, because the symptoms are a bit ambiguous. But every time I install a fresh set of injectors, the difference is night and day.
You may also consider these:
Faulty IAC system (Other than valve)
Intake air (vacuum) leak
Faulty fuel pump or emissions problems.
Throttle position sensor possible…also too 