90 gmc rough idle

My 1990 Sierra 5.7lit. at an idle hunts up and down and will stall, plugs cap rotor wires are recent,its burning more gas than usual and when driving service engine light sometimes comes on and while its on engine runs smoother.:shakehead

Sounds like your tps has went out, but you need to pull the codes to be sure.
You dont need no scanner to do this, you can do it yourself by turning your ing on, not starting it, and shorting out your A & B on your plug under your dash. These will be the two on the right top of the plug. Do a search for obd 1 codes for chevy. Good luck

Look at www.troublecodes.net/GM for instructions on reading codes and code definitions.

I own a 90 K1500 that I bought new. I had a hunting idle for a while and eventually found 2 vacuum leaks. One at the MAP vacuum hose at the back of the TBI. The rubber elbow no longer sealed around the plastic vacuum hose. 2nd leak was at the PCV gromet. It had hardened and no longer sealed very well.

Since it runs better when the check engine light is on, it means that idle problem happens in closed loop operation. Closed loop is when the ECM uses sensor inputs to determine fuel trims. Has the 02 sensor ever been changed? Look for vacuum leaks. Finding out what codes are set will really help.

//2000CAYukon

remove the air cleaner, start the engine and watch the fuel being dispenced by the fuel injectors. This should be a fine mist type spray, equal.
Watch for drips, drops of fuel coming off the injectors tip. If there is, rebuild the injectors, there is probley a bad O-ring. then vacume leaks would be my next choice. TPS is another.