Let off of gas truck goes back too idle and then stalls?

Hi:

The most common cause of a rough idle or low RPMs is an engine vacuum leak. When a leak occurs, unmetered air enters the engine causing it to run lean which can lead to the symptoms you described.

Here is what I suggest. First, check for a leak. Visually inspect the air filter box and inlet tube to make sure nothing is disconnected or damaged. Next, confirm there are no rubber vacuum hoses that are disconnected, damaged, or dry rotted. Here is a link that explains how to locate and repair an engine vacuum leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If nothing is found, suspect the idle air control valve (IAC) is failing. The IAC is responsible for maintaining the engine's idle speed and is located on the throttle body.

If you look below, I attached the directions for replacing the IAC. When you have it out, also confirm there is no excessive build-up of carbon where it goes into the throttle body. Before you just replace it, I would suggest servicing it first. Here is a link that explains how it's done.

https://www.2carpros.com/articles/idle-air-control-valve-service

I hope this helps. Let me know if you have other questions.

Take care,

Joe

See pics below