@Kramretsimis @mtl @Sylvester
Hi all,
I actually sold the car a week or so ago, but I just wanted to chip in here in case I can share anything that might the helpful.
(1) A compression test is certainly a good way to diagnose a stuck valve (i.e. stuck in the 'out' position / not sealing back into the head) or worn piston rings. I never did one because the problem was intermittent, and because I didn't have a compression tester with a long enough reach to get to the plug holes. But for those who haven't done a compression test before, you need to take out all the spark plugs, then one at a time push the tester against each plug hole while an assistant cranks the engine on the starter while holding the throttle wide open. That last point is quite important - before I knew any better, I confidently told a friend that his engine was scrap after doing a test without opening the throttle! Anyway, crank the engine until the needle stops climbing. Some compression testers come with nifty screw-in attachments so you can fix them into the plug holes, but I can honestly say from experience with older engines that one with a squashy rubber seal that you jam against the opening will work just as fine. I don't know if it's OK to do on a modern engine, but certainly on older stuff if I had poor compression on any cylinder I would then test again, but this time after pouring a tiny amount of oil into each plug hole. If the compression remains very low on one or more cylinders then you know you have a leaking valve.
(2) Broken valves are not a particularly common fault on any mass production engine, as far as I know, whereas a valve that isn't sealing for other reasons is quite easy to achieve with poor maintenance. My belief is that VW approved so-called 'long life' servicing on these engines as a sales tactic, safe in the knowledge that they'd have passed to third or fourth owners long before the sludge became an issue. Basically, sludge builds up in the engine because the oil isn't being changed often enough, and it blocks the strainer in the sump, and the oil ways in the head. The result is sticking valves, and worn lifters and camshafts. And that's why my engine sounded rough at tickover but fine (with plenty of power) at high revs, as the oil pressure rose.
(3) In my experience flushing and Wynn's Hydraulic Lifter Treatment really helped. In the months after doing all the work detailed on this thread I think the light came on three times, and the engine dropped into three cylinders just once. In all cases it happened when the engine was left ticking over or struggling at low revs for too long (when giving my wife driving lessons, for instance), which I think bears out my explanation.
(4) All that said, I would still start with the coil, leads and plugs. As a brilliant mechanic (my Dad) once said, "99% of the time when a car won't run, it's something electrical. And the other 1% of the time it's the electric fuel pump"
(5) If you do diagnose a sticky valve, and flushing / lifter treatment doesn't help, then the most sensible next step would be to take off the cam box to replace the lifters and camshafts (or get them reground, if that's possible on one of these engines). But really, these sadly aren't valuable cars right now, so if you can get the fault down to manageable proportions (i.e. a couple of warning lights a year) and emissions and performance aren't effected, you might be better just living with it for now.
I hope some of that is helpful. This forum was absolutely amazing in helping me out when I was having issues with my A2, and I just wanted to get a few thoughts down here for others who come across this thread