The truck in question is a 1992 GMC Sierra with the 5.7l 350
Okay so I consistantly have a low idle once hot and at a complete stop while in gear. In Park or Neutral it will idle around 500, but if I come to a complete stop while in Drive or Reverse, it idles around 100 to 300rpm.

It does not stall or sputter when the idle is this low, but if I turn the power steering it will stall(belt driven). When the idle is around 100 the Check Gauges light will come on, other than that my oil pressure is fine when driving or idling on a cold start. I have noticed however that voltage reading on the dash will move between where the red ends and 14. I am not verse in automobile mechanics enough to know if this is normal or not, and I do not know this car personally as it was owned by a family member who passed away. It does not go into the red zone on the gauge, however. The Service Engine light comes on and off intermittently too.

I don't have any rpm surges, or throttle hesitation, zero problem starting in warm or cold temp, everything on the truck seems to work absolutely fine other then when I am at a complete stop, the engine is hot, and I am in gear.

I've been looking around the internet for weeks now I get a variety of possible causes: IAC Valve, Ignition Coil, Ignition Control Mondule, TPS, MAP, and O2 sensor, Vacuum Leak, Throttle Body, Fuel Pressure.
I don't know where to start and I am on a limited budget at the moment. I do however have access to a couple of pieces that could be the cause, I just don't know where to start and I was wondering if anyone has had a similar problem and could weigh in.

Any help would be greatly appreciated, thanks.

 

I am having similar issues. Car starts from cold runs great. When I let off the gas and the rpms drop below 1500 it will usually just die. Sometimes cranks right back up. Sometimes it wont crank till it sits 6-8 hrs. It runs fine on interstate but 50/50 chance when it drops to idle while decelerating engine will die. If my battery is good and I keep cranking ( for up to 30 mins) it will eventually crank up. I have drained fuel and replaced with fresh, Changed fuel filer, Replaced plugs wires distributor cap and rotor button. replaced fuel pump regulator and injectors air filter radiator thermostat. Engine had pin hole leak out head and intake manifold used bars leak for intake manifold and it cleared up both leaks real well. Engine runs smooth but is not reliable HELP ANYONE HAVE SAME PROBLEMS!