After an attempted cleanup of this sludge buildup of the top end, when car was started it developed a misfire on 2/4/6 even at idle, mainly on 2 and 4 which the ECU then eventually turns off the associated fuel injector (I was able to confirm this with the SOLUS PRO functions)
So essentially the car is running on 4 out of 6 cylinders and wouldn't idle but steady at 2000 rpm till #6-cylinder cuts out and then engine stops. Every now and then all cylinders would star firing for about 30 secs then die again.
My mate swaps or replaces everything he can think of, plugs, PCVS, ignition coils, cam phaser control and sensor. Nothing helps.
I decide to do a compression check, 1/3/5 all good at 180 psi and pressure builds up within 2 or 3 engine revolutions.
Bank 2/4/6 also made 180 psi minimum but took 10 or more engine revolutions to reach 180 and in gradual increments, also was a strange popping sound separate from the compression gage. Sounded like from the inlet's passages on block.
I figured it had to be a valve timing issue and we removed front timing cover.
Bank2/4/6 cam chain tensioner was at full extension and still had slack in chain.
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The sludge made it too hard to verify timing marks, but the cam chain became so tight and then very loose as engine was rotated by hand.
Forgot to mention earlier, the starter was unable to turn engine over every now and again, this now made sense as the 2/4/6 bank timing chain made it very hard to turn engine over with an extension bar in certain places. Seemed it was fighting the other cam timing chain.
At one point moving by hand, the timing chain slipped a few more teeth at the idler drive gear. So, i can't tell you how many teeth had slipped initially, but i would say at least 1 or 2 teeth on the cam idler gear minimum.
The other chains were not quite as worn, maybe tensioners were at 60-70% extension.
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So, at this stage thinking this engine is a write-off, but found this FORUM and seeing some of the sludge buildups that had been successfully removed and that the compressions were good have decided to repair.
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I also have a differential cylinder leak down tester, which is how we do aircraft engines, they are checked every 100 hours.
This would have revealed which valve was decreasing compression buildup, but I figured it was the inlet valves opening early.
Like maybe 20 degrees Ish.
So, in summary, a high-end diagnostic tool with waveform presentation and maintenance functions is invaluable to diagnose an unusual fault. My SOLUS PRO is pretty dated, and I only have software in it up to 2014 cars, bought it off a mechanic for $600 and added a few keys for BMW and other European cars.
Also, from what I have read on this and other forums, quite often the dealers are unable to completely diagnose faults correctly, before spending big $$$ on replacing parts unnecessarily. I think going to a good known independent diagnostic mechanic may be worth it in the long run.
