2006 4.8is Rough Idle
 

My dad just got a 4.8is and it has an issue idling. We've unplugged the MAF, Valvetronic Servomotors, Valvetronic sensors, and VANOS solenoids, separately and in various combinations, with no change in idle characteristics.

Those characteristics being rough idle upon startup and it hovering around 1000 rpm while revving a few hundred RPM. It eventually smoothes out at 1500 RPM, and once the engine is at operating temp, idles normally. It also throws codes for misfires on every cylinder when cold. When revving it in any condition, it doesn't seem to misfire once above idle rpm.

We replaced the IVM and it had no affect on the idle issue. Today I hooked it up to ISTA+ and changed the minimum valve lift from 0.3mm to 0.8mm and that actually seemed to fix the high idle, but the engine now misfired on all cylinders while warm and cold, but did so consistently between those two conditions.

What do you guys think this could be? the previous owner said they did mist of the coils, all of the spark plugs, and filled it with gas. I don't believe it's the coils or gas, because it has two different brands of coils and all cylinders misfire the same, and I assume the knock sensors would take care of low octane gas causing this issue.

Have you cleared all the codes? Unplugging all those sensors will set codes. Delete codes and start it up. Post the codes. If it started after a fill up, empty the tank and use good fuel.

Yes, the codes have been cleared every time and I don't know, as I said, the previous owner filled it. We literally just got it a few days ago. The only codes that consistently appear are the misfire codes for every cylinder.

Pull a plug and note the brand. Are they OEM four prong or updated single tip iridium? Check the maintenance the previous owner did. I've corrected a lot of issues with bad running cars due to owner recent maintenance. Either cheap or bad parts. Or incorrectly done.
Check fuel trims. What are short term fuel trims doing?

Sounds like a vacuum leak. I've had similar situations on other models and most times that has been the issue with your symptoms. I built my own smoke tester and found leaks easily.

The vacuum system doesn't interact with the intake on the N62 since it's a valvetronic engine. Otherwise it would fit, aside from it also idling fine after it's warm.

I was probably gonna check those next, since the previous owner replaced them all.

There is nothing worse than chasing an issue after someone "worked on it". When in doubt, replace replace replace it all. Plugs, filters (air, fuel, cabin)
Confirm the following:
There is no oil down in the plug wells.
The air intake does not have a bird body down in there somewhere.... :)
The throttle body plates are clean (Berkebile Gum cutter works great)
Charging voltage is above 13.5V at idle and is above 12.3V with key off.
The oil vapor diaphragms are working.

There is more but these will get you started