I have been trying to troubleshoot my church van for the last month and have had no luck. The problem is this:
When the van is cold and until it reaches normal operating temperature at 210F degrees, it will switch between Open and Closed loop, and when its in Closed loop when cold, it will chug and misfire like crazy on cylinders 2, 4, 6, 8. If you attempt to drive while its doing that, it will "chug" and have no power. Sometimes the transaxle service light will come on during this situation.
Again, it ONLY does this when cold. When its warm, it will drive fine and be in closed loop. So drive this 1/2 a mile down the road after letting it idle for 5 minutes, and suddenly its driving normal. If you clear the check engine lights at this point it will stay off the duration of the trip.

The van is a 2011 Express 2500 with a 4.8L LS engine. It has 162,000 miles on it.

What I've done:

1. Smoked the exhaust and intake system to find leaks but found none
2. Sprayed carb cleaner around the top of the engine, no engine RPM changes
3. Replaced the plugs and wires, they were original and needed it anyways.
4. Found a few cracked vacuum lines and replaced them all
5. Checked the exhaust header bolts and replaced the rear cylinder 8 bolt just because that one normally breaks, but the original was not broken
6. Put some techron in the gas tank to clean the injectors
7. Replaced the MAF (done by another mechanic)
8. Repalced the O2 sensors (done by another mechanic)
9. Done a exhaust backpressure test -- no movement on the gauge at all, so no back pressure no plugged cat

So some good troubleshooting I've done with my Foxwell computer. When its chugging the rear bank 2 sensor reads 0 and the front sensor reads lean. These are Denso sensors that a shop put in. I took a video of this, its here:
If I drive it down the road until its warm enough, it will suddenly flip to be 600-800 for readings on the zero sensor and its fine. But until it warms up it won't work and reads 0.

I'm looking for ideas. I've been troubleshooting this for a month now and have no solution yet. I bought genuine GM sensors and have a suspicion to look at the fuse for the oxygen sensor heater. Again, once its warm it drives fine. No more codes, no more errors.

For codes this throws a lean code mostly and misfire codes for random cylinders.

If someone has experienced this or have ideas please let me know. I'm at wits end. Its already gone to a shop mechanic who replaced the O2 sensors and the MAF and it didn't make a difference. Now I've been troubleshooting it but have had no joy.

Thank you!
David

 

I figured it out myself. In the end, the "mechanic" had switched the upstream and downstream oxygen sensors. I replaced those with genuine GM, put them in the right proper position, and now the van idles and runs great. But this makes sense now thinking about how the oxygen sensors were reading completely the wrong things and why it was running lean for so long.
Hopefully this will help someone in the future.
David