Hello all. I'm new to chevy and love my truck. Its in great condition and runs almost perfect at 164k miles. So here is the problem. Upon startup it seems fine in park. When you put it in drive it will go from about 1k to 600 rpms. Sometimes it will surge back up and drop down again. Then when you let off the brake to press the gas it will drop really low and feel like its about to stall. Once the truck is going, it runs awesome. Not a single problem. Sometimes when you're slowing down, once you are close to being stopped it will drop really low again and has actually stalled out a couple times while going slow around a turn. So far what I have done to fix it with no results.

New throttle position sensor
New fuel pump
New spark plugs+ wires
New distributor cap.
Took the iac valve out and cleaned it.

The air filter is new and I havnt checked the fuel filter yet. And it has an automatic transmission. Any advice would be appreciated. Thanks

 

Check mass air flow sensor. These are also known for head gasket leaks. If its sucking in too much air it can cause this. Smoke test for leak near head gasket. Or try spraying starter fluid around head gasket area. If idle increases, you've found your leak area. Be careful with starter fluid ! Just a thought.