My 2005 Opel astra h (engine code-Z16XEP) has developed some oil loss issue. During autumn months while I've been only city driving, it has consumed very little oil in comparison of what it had consumed during summer (30min highway drive at 110kph - 3100rpm). In the summer it consumed like 0.6l or thereabout every 1k km. No smoke comes from the exhaust.
I've also tested the emissions, since the oil didn't leak anywhere, so it must have been burnt.
At idle the CO emissions were 0.14%(CO limit at idle is 0.3%), while at a higher idle (3000rpm) the reading were 0.5% (CO limit at high idle is 0.2%, which means i won't pass the next inspection). HC(ppm) were at 50.
I have also done a compression test on the warm engine, the readings were:
1st cylinder - 13.7bar
2nd cylinder - 14.1bar
3rd cylinder - 13.7bar
4th cylinder - 13.8bar
Also noted, the plugs on the 1st and 2nd cylinder were kinda darkish.
Furthermore, when I removed the injector wire from the 2nd cylinder the CO results were within acceptable limits(idle-0.09%, high idle-0.11%). And the 2nd injector isn't faulty/sticking/open/leaking, i have tested it and it works normally. No codes are stored in the computer as well.
So far from reading online about this engine, i have narrowed it down to two things, either the valve stem seals are bad or it is the oil rings on the pistons. Could it only be the bad valve seals, or piston rings, or both? The shop were I tested the emissions said it is most likely a bad cat(doubt it). Do you have any ideas?