The error codes I have had recently are: PO172, PO175, P1400.

I have a 2009 Sierra 4.8L 4x4 that idles very rough after sitting for awhile. It is usually worse when it is hot outside but the engine is "cold". It has been occurring ever since I bought the truck but has been getting much worse lately. Sometimes it even seems like it is going to die as the RPM's drop to near zero. They usually fluctuate between 0-500 when it is happening. It used to just last a few seconds but now it sometimes lasts 20-30 seconds. Giving it gas does not seem to even make a difference. Lately it is even happening while the truck is running like when I stop at a stop sign and then give it gas, it might take a couple seconds for it to react.

I have sent it back to the dealer 4 times and they cannot figure out what is wrong with it. They keep telling me that it is the gas I am using. They finally tested the gas and it passed the tests, but they still think that is what is causing it. Supposedly they added some GM additive to the gas and it solved the problem. They suggested that I only buy top tier gas. The problem is that I live in a small town and there is not any top tier gas sold here.

No one else I know has this problem. Could this really be due to the gas? Or is there something else that it could be?

Thanks for the help!

 

Maybe your local station has water in the gas. Next time it does it get the additive the dealer was using or an additive from the local auto parts shop and try it.