I have a 2002 Chevrolet Tahoe with the 5.3L Flex fuel motor. Which is a joke since you can't buy E85 in Oklahoma, but that's not the point. The gas mileage has been steadily declining, even for a Tahoe, down to around 12 mpg in town for the past 2-3 months. Last week the Service Engine Soon light came on, I had it scanned at Autozone and they said it was both front O2 sensors and that the engine was running lean. I ran some fuel injector cleaner through the motor and the service engine light turned off almost immediately which seemed a bit quick. Now in the mornings on cold start the motor will drop idle for a brief moment then it will pick back up. As the rpms drop it almost sounds like a miss then as it picks back up the whole truck shakes. This continues but the severity lessens until the truck warms up to operating temperature. It does not affect driving, it only does this when cold and in neutral or park. Once the truck has warmed up it idles as normal and has no problems. What could be causing this?
Short Version: Sounds like a miss during cold start, truck shakes, rpms drop, then goes away once it warms up. Why???