I have a 90 C1500 454SS with fuel injection

Is this tbi fuel injection or mfi, I haven't looked at your wiring? Fuel pressure is correct? How long since a tune-up? Any applicable trouble codes? At idle block off the mouth of throttle body, it should kill the engine. If very little or no effect on the idle, you got false air getting in there from somewhere. Make sure the throttle plate closes. Check and clean anything that has to do with idle air system.

Just wanted to add, sometimes with high mileage engines, you adjust the idle in drive so it won't stall every time you stop at a red lite or stop sign. Yep, if you do that, when you put it in park or neutral, the idle will go high. Not much load on the engine in park or neutral. There is a load when you put it in gear.

Fuel pressure on a 90 Chevy 454 SS tbi