I have a similar problem as Terradyne Kappa. After the engine gets warm, the RPM's run up to 1500 in idle. After I shut it off and start it up, it goes back to normal until i hit the gas. I am help less

Don't know the year ofyour truck, but here are some common problems with High idle

Throttle body is coated with carbon deposits
Maladjusted throttle stop
Vacuum leak(s)
Speed/Distance Sensor failure
MAP Sensor failure
Throttle Position Sensor failure

it is year 2000, 5.3L. Mileage 135 000. I took it to a shop today and I am waiting for their phone call.

Let us know what they say

They called me from the shop that everything is just fine. I told them it happens only after the engine warms up. He said they didn't run it for long, .... I guess they were busy to drive it for a while. But we have an agreement that tomorrow I will bring it straight to the shop when it's acting up. I'll keep you posted.

Engine Coolant Temp sensor can also cause this. Make sure they check what temp it is reporting to the PCM.

Colder engine needs more fuel.

//2000CAYukon

Very strange !! All of a sudden the truck runs fine. In the shop they said they didn't do anything to it. I didn't do anything to it. The only change is the weather. It has raining like crazy and the truck runs fine. Could the rain have anything to do with it ? I guess I will find out when it clears up and truck dries out.