1990 GMC 350 engine stops while idling at light. Replaced distributor cap,rotor, spark plugs and cables, new battery. and air filter. Engine is running without missing and no hessitation. Starts strong and runs well. Alternator is generating 14.0 volts. Engine stops running at light and dash lights come on.

Have you checked fuel pressure ? Distributor cap and rotor ?
Is the check engine light on ?

your distributor make be a hair off if the bolt came loose, or your throttle cable may be a little loose

replace fuel filter, air filter, spark plugs, wires, distributor cap /rotor, if you have one. new oil with your filter ? just thought I'd ask, have a auto parts store pull code to see why your service engine soon light is on. this will tell you what sensor needs replaced, I'll bet its the oxygen sensor !

You can google the engine size and "firing order" to get a drawing or photo of the firing order online.
Example - - 1990 GMC 350 firing order diagram

Could be the EGR valve or the idle adjust valve. Check Engine light on? Any codes?

What year and is it the factory engine? If it is carborated the choke may not be opening enough and is starving it for air and running rich. What spark plugs are you running? Have you replaced the vacume hoses and gone over the engine tightening(torquing) intake, carb and other parts?

Check this procedure...

1. Disconnect the negative battery cable.
   
2. If necessary, remove the air cleaner.
   
3. Tag and remove the spark plug wires and the coil leads from the distributor.
   
4. Unplug the electrical connector at the base of the distributor.
   
5. Loosen the distributor cap fasteners and remove the cap.
   
6. Using a marker, matchmark the rotor-to-housing and housing-to-engine block positions so that they can be matched during installation.
   
7. Loosen and remove the distributor hold-down bolt.
   
8. Remove the distributor from the engine.

INSTALLATION
Engine Not Disturbed

1. Install the distributor in the engine making sure that the matchmarks are properly aligned.
   
2. Install the hold-down bolt and tighten the bolt to 20 ft. lbs. (27 Nm).
   
3. Install the distributor cap and attach the electrical connector at the base of the distributor.
   
4. Install the spark plug wires and coil leads.
   
5. If removed, install the air cleaner assembly.
   
6. Connect the negative battery cable.

Engine Disturbed
(see Figure)

1. Remove the No. 1 cylinder spark plug. Turn the engine using a socket wrench on the large bolt on the front of the crankshaft pulley. Place a finger near the No. 1 spark plug hole and turn the crankshaft until the piston reaches Top Dead Center (TDC). As the engine approaches TDC, you will feel air being expelled by the No. 1 cylinder. If the position is not being met, turn the engine another full turn (360 degree). Once the engine's position is correct, install the spark plug.
   
2. Using a long screwdriver, align the oil pump drive shaft in the engine in the mating drive tab in the distributor.
   
3. Install the distributor in the engine.
   
4. Install the distributor in the engine making sure that the matchmarks are properly aligned.
   
5. Install the hold-down bolt and tighten the bolt to 20 ft. lbs. (27 Nm).
   
6. Install the distributor cap and attach the electrical connector at the base of the distributor.
   
7. Install the spark plug wires and coil leads.
   
8. If removed, install the air cleaner assembly.
   
9. Connect the negative battery cable.

Fig. : The rotor segment should be aligned with the pointer cast in the distributor base, when the distributor is fully seated

Hope this helps (remember to rating this free asnwer).

You've already eliminated the spark plug & wireset, distributor cap & rotor.
Sounds like your car needs a tune-up, specifically, the PCV (positive crankcase ventilation) valve, and/of the EGR (Exhaust Gas Recirculation) valve.

Components checked/replaced during a tune-up are:
air filter
motor oil
motor oil filter
breather filter
fuel filter(s)
fuel strainer
PCV valve
EGR valve
spark plugs
spark plug wires
ECM or distributor cap & rotor
cabin air filter
transmission oil filter
transmission oil
engine coolant
check all vacuum hoses and lines

Troubleshooting the no-start condition - from the 1990 Mitsu Mighty Max Repair Manual:

Priority Action Part Type -- Cause
1 Inspect Battery -- Battery Discharged or Faulty.
2 Inspect Battery Cable -- Corroded, Broken, Shorted or Poorly Connected Battery Cable.
3 Inspect Distributor Cap -- Distributor Cap Cracked or Burned.
3 Inspect Fuel Filter -- Clogged or Dirty Fuel Filter.
5 Inspect Ignition Coil -- Faulty Ignition Coil.
6 Inspect Wireset -- Worn, Damaged or Faulty Spark Plug Wire(s).
7 Inspect Spark Plug -- Incorrectly Gapped or Fouled Spark Plug(s).
8 Inspect Starter -- Starter Motor Faulty.
9 Inspect Fuel Pump -- Low Fuel Pressure.
10 Inspect Solenoid Switch -- Faulty Starter Solenoid Switch.
11 Inspect Fuel Pump Relay -- Faulty Fuel Pump Relay.
12 Inspect Fuel Injector Pressure Regulator -- Faulty Fuel Injector Pressure Regulator.
13 Inspect Ignition Switch -- Improperly Connected or Faulty Ignition Switch.
14 Inspect Starter Drive -- Faulty Starter Drive.
15 Inspect Fuel Injector -- Dirty or Worn Fuel Injectors.
16 Inspect Fuel Pump Strainer -- Clogged or Dirty Fuel Pump Strainerat.
17 Inspect Engine Control Computer -- Incorrect Operating Information Being Delivered and Sent From The EEC.
18 Inspect Fuel Tank -- Fuel Tank Empty.
19 Inspect Carburetor -- Carburetor Flooded or Faulty Choke.
20 Inspect Distributor -- Worn, Loose, or Incorrectly Installed Distributor.

I Have a 1999 ford f150 4.6 that back fires and is just running really ruff when idling alot of nack fire a sord of like a pulse going on what is my problem?

let up hood let it idle to dry watch the check engine light . it should be okay.let it idle it will smooth out.