I own a 2005 Chevy Express (Extended body) cargo van 4.8L V8 with 115K miles. I have an issue that my mechanic is unable to solve - he referred to it as a gremlin!
The van starts and runs without any great problems at first but if you drive it for a while and come to a stop sign, the van will often struggle to idle - the idle will be rough and choppy and will often result in a stall. But the van will always fire right back up again when it does stall. The second issue (i'm convinced they are all tied together) is that when idling with the AC running, the AC will often stop blowing cold (sometimes when it begins to struggle to idle) and sometimes when it seems to be running ok.
The last component, and the most troublesome is when driving down the road, holding a constant throttle position (on a flat smooth road - no changes in load) at any speed over 30 mph, the truck will either gear down, then up, then down then up - or, the power will begin to fade slightly, prompting me to press harder on the gas, which will gear it down (causing it to race) - but I want to go at the same speed so I need to lift off again - causing it change back up - and the problem persists. If I fully depress the gas pedal the truck will often gear down (as it should), allowing me to build up some speed. As I then let off the gas it will shift up into a higher point in the new gear range but will shortly start to do the same thing if I slowly try to accelerate. It is as though the engine and transmission are having a mis-communication and cant get on the same page. But my gut tells me the transmission is simply responding to an engine that is acting-out. Incidentally, when the truck does shift, the shifts are silky smooth every time. The transmission makes no odd noises and other than the random changes it seems to be fine.
The van doesn't seem to be misfiring, it sounds relatively sharp and crisp when revved in neutral. My mechanic has reached the end of his usefulness and has thrown his hands up. We have changed the fuel pump and fuel filter, one ignition coil (he said it was putting out a code one time when it was in his shop and it pointed to a particular coil bank), changed the plugs, the plug leads, and the throttle position sensor.
I think the last three parts were just thrown at the truck so he could justify his bill for chasing this "gremlin". I need a new avenue to explore as nothing so far has worked and I cannot afford to have someone keep making guesses. My old mechanic offered that the idling issue may be due to a faulty idle control sensor, but I am reluctant to pursue his advice at this point.
Thank you in advance for any assistance you can offer.