CEL, Piston Slap, Pinging - HELP!
 

This is for my buddy's car; I posted in OD because it's not a MB but I know all the real pros are here in tech:

http://www.peachparts.com/shopforum/off-topic-discussion/161897-chevy-tahoe-problems-help.html

Bump - Ttt

OK, we have some major problems here. He cleaned with 2 cans top end cleaner, everything went great. He described the process he used and it appears he did it properly. He also changed the oil afterward for fear of dilution, used DELO 15W40. Here's where it gets tricky.

Today after work I helped him run a compression test. Results were about 145 to 150 or so across cylinders 1 through 7. On cylinder 8, we got 138 which seemed kind of low. We put about 1 or 2 teaspoons of oil in the cylinder via a tube, and after about a minute took another compression reading. This time it came back at 152.

<<EDIT: When examining the old plugs, they are caked in flaky black carbon. As I said earlier, this vehicle is used for severe duty delivery-type driving. Lots of stop-and-go and frequent idling for extended periods. I recommended regular Italian tuneups.>>

Since the plugs were out, we replaced with brand new OE plugs from the dealer, properly gapped at .040. Started it up, and everything appears great! No pinging, very smooth idle, smooth powerful acceleration. It wasn't like this the other night when we test drove it. It's like a new truck. There is an ever so slight knock sound still present; I think it may be more noticeable now that the dieseling sound is gone. <<EDIT: The knocking is slight at low RPM under load, but at higher RPMs (WOT at 60mph) it is very loud.>>

Here's the kicker - it is leaving a thick cloud of burning oil smoke behind it! The smell is suffocating and the smoke is really thick. It is definitely blue and smells of burning oil. We drove the car in a mix of local streets and highway for about 20 to 30 minutes and the smoke did not go away. This is definitely not remnants of the top end cleaner because he lives about 20 minutes for me, plenty of time to blow out the top end cleaner.

Any ideas?

Here's my/our theory: Cleaning carbon revealed flaws in cylinder #8 which is now drawing up oil due to worn rings which no longer seal.

By the way, no more check engine light, at least in the 20 to 30 minutes we drove it.

Any ideas?