In case anyone with the same problem finds this thread, here is an update:
To summarize, the idle surging only happens in Open Loop mode (disregard the thread title related to the air pump). The problem has never come up when the truck starts cold or hot, but somewhere in between.
For example; I drive the truck to work and it heats up all the way, I park the truck and come back 1-3 hours later, start the truck up again and the idle stumbles/surges in open loop. Idle smooths out once the truck switches to closed loop (after 15 seconds or so). Otherwise, the idle is rock solid and this truck runs like a champ.
In the past 10k miles, I have done the following:
- Spark plugs (5) - ACDelco Iridium
- Coil packs (5) (ACD)
- Cleaned MAF
- Replaced PCV Hose to resonator
- Replaced TB
- Replaced MAP
- Replaced air filter (Wix non-oil)
- Replaced upstream oxygen sensor (ACD)
- Replaced downstream oxygen sensor (ACD)
- Replaced idler pulley (ACD)
- Replaced belt tensioner and new belt (ACD)
- Replaced Engine Coolant Temp sensor (ACD)
- Replaced camshaft actuator solenoid (ACD)
- Tested fuel system pressure, held constant with no signs of leaking injectors (@55-60psi)
- Oil change (5W-30 synth)
- New battery (Group 65)
- Seafoam intake and gas tank
- Identified a battery drain issue with my remote start, removed remote start and unplugged all electronics
- Monitored for pending codes, never had a CEL, no CEL present
If you have the same exact problem I have, the above will not fix it.
To be sure you have the same problem, refer to the videos below. Remember, this problem does not happen every time the truck starts, only after a hot soak. If you experience this idle problem more often than not, I doubt we have the same issue.
<--Hard to catch, but stumble is at the 5-11 second mark
WHAT HAS HELPED: Gas quality
Since making sure I have only top tier gas in the tank at all times, the problem has been somewhat reduced. Is it eliminated? Nope. Running higher octane fuel has almost eliminated the problem, but at a cost obviously.
Why? Maybe this article about the volatility of winter fuel? After PM'ing a few people across forums (even Hummer owners), every 08 I5 owner I talked to has had this problem once gas stations switched to winter fuel (around September).
So for now, that's my only lead. Wait it out until the spring and hope it goes away! In the mean time, my coolant looks gunky, so I am going to get the fluids flushed/filled this weekend, brake and rear diff included. And who knows... that might just fix the problem!