Scotty,
I have a 2008 GMC Sierra 1500 4x4 automatic with a 6.0 liter Active Fuel Management engine. It has a new remanufactured engine from O'reilly and the transmission was rebuilt by Mr. Transmission. Both engine and transmission now have about 5000 miles on them. In the mornings the truck idles rough for a few seconds and then runs smooth the rest of the day. There's not any smoke or anything like that and it's not using any oil. I did install the upgraded valve cover when installing the engine. If it sits more than a couple days it seems to idle more rough and sometimes will give the P1400 Cold Start Emission Reduction Control System code. If I reset the code it might not come back for weeks/months. Any idea what might be going on?
Well it's got a problem in the emission system when it's starting up cold. Have a guy go through that with a fine-tooth cone with a GM enhanced scan tool. Could be something as simple as vacuum leaks on the intake system or my complex like sensor or computer failure
