IDLE CONTROL PROBLEM WITH 1991 GMC K2500 IDLES SOMETIMES AT 1100/1200, THEN WHEN YOU DO A RESTART IT MAY IDLE AT 650/700.

Remove the gas cap,and then remove the return fuel line off the back of the throttle body.Now take a air hose and blow in the line you removed to make sure it is not clogged up,you should be able to blow back threw the tank easy with compressed air.If you are able to,then replace the fuel pressure regulator on top of the throttle body.

Time for a IAC (idle air control ) motor

put some gas in a spray bottle. spray throttle body and vacuum line if engine raises found leak

Have you checked the idle air control valve (IAC)? This is a solenoid driven valve that controls the idle speed.

This sound like a fuel issue. Have you replaced the pump?
You may also check the fuel regulator valve

1600 is not idle speed my friend, 600 or 700 is idle speed. You need a tuneup on your 13 year old vehicle.

replace the coolant temp sensor,it`s values are off,it is defective.Also if this does not work,witch I think It will,change the throttle position sensor.

High HC?
****** ignition timing....this will lower HC but raise other values during test.
Do you have room to increase other values?
After test return timing to the most advanced position without spark knock under load for best MPG.

TRY THIS BEFORE U SPEND ANYMORE MONEY N PARTS.
Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

Check the fuel pump.