Remove the valve cover, you a now have access to the top of the timing chain and the top guide. If there is any discernable play it is loose; from there you can use the borescope to image the guides. The "inspection cover" is the access hole to the balance chain sprocket to make it easier to replace the water pump.

You are running Open loop until the OBD testing completes, that means the computer is taking a break, not being tested. Not relearning anything. Disconnecting the battery wipes the volatile memory might do what you are expecting. Calibrations that you think you are clearing are stored in non-volatile memory, you need a tool like a Tech2 to erase them.

The problem with brake cleaner is that it can dissolve the coating of the TB as well as the plastic and rubber parts, not even getting to what it can do to the various sensors in the air flow.
The only test I can find for the coils is checking for high resistance or short to ground or short to voltage or open. There are codes for that.

A while back we went through this sort of thing to find out it was a bad clutch.

 

Yes, you are correct, I stuck the camera in where the balance sprocket is in the top left of the photo (where the sprocket with the 3 bolts is located) and then followed the chain down to the crank sprocket... Like I mentioned, it was pretty a Mickey Mouse investigation, but I had a fairly decent poke around, and there was no indication that the chain has any excess slack in it when I move the crankshaft... but it’s certainly an area I can investigate further. Currently I’m resisting the temptation to remove the cam-cover until I’ve absolutely ruled out ignition (Plugs/coils) as the cause. I’ve only done a quick overall check, nothing obvious jumps out so now I’m methodically ruling things out step by step.

 

x2 to timming chain. Had this symptoms and then engine rattle occur

 

Well, I found a tiny hairline crack on one of the new plugs I replaced last month. It was on cylinder #3 with some scorching (see pic), I’ve replaced the plugs (Again), not sure if this is the root cause as I had the misfire before I changed these plugs, but I will test and update.

 

If your spark plugs fail, car won't run, oh well, get a new set.

If any one of the timing components fail, the car won't run, oh well, get a new engine.

 

I just wanted to say a big thank you to everyone who responded to this thread, I genuinely appreciate the support from this community.

So here is what the problem was: I had the original light misfire which started 2 months ago, last month I replaced the original plugs even though they only had 35,000 miles on them AND despite the fact GM says these iridium plugs are good for 100,000 miles (so they were only 1/3 of the way into there service life!).

However the misfire didn’t go away, and slowly got worse... After all the countless hours investigating and testing every other part of the car, it turns out the new OEM plugs I’d purchased from the dealership had a faulty plug that had a hairline crack in it which was arching and causing the exact same misfire symptoms as the original plugs... but didn’t trigger any codes.

What a nightmare and significant amount of wasted time... But at least we now know that if you have a misfire with no codes being thrown, it’s likely a damaged spark plug.... Live and learn.

Thank you again to everyone, and happy new year.

 

There are thousands of Cobalts and HHRs and G5s etc. with over 300,000 miles with original plugs.

The plug was firing so no misfire, just weak spark. That does not explain the absence of P0171/P0172 though.