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Hello ladies & gentlemen!
I've got an issue with my car and I can't say whether it's serious or not. If you have any input to help me in this mission to sort it out so I can feel comfortable selling it once I'm done in Australia.

So this has been going on since I bought it.
It shakes at start, normally when it's been dead for half a day or more. During idle after start or after a long drive it goes into rough idle, if I press the gas just a nanometer it's smooth again.

I performed an engine flush with lots of oil to get all junk out of it, changed to a Nulon oil and added archoil(friction modifier). That made it a lot better, huge difference. Now after last service without archoil it's back to the same symptons.

I also lent it to a friend(donkey) who managed to mess it up a little bit, dents, scratches.. and more lovely things you'd like to see on a former 180k beauty without any earlier forms of love. That wasn't the problem, after he had it I made sure that a family member of mine could take it from Brisbane to where she is now so that she'd get a good job etc not having to drive uber for 4 hours haha. She's a great lady but forgot to do a little procedure of just starting it a little now and then for a month.

So to the point(sorry). I started it, and wtf... it shakes, the sound of metal having an all in or nothing party. I quickly killed it, turned on the ignition again and wondered what the hell it could be.. if the downtime of the whole engine caused it to just slowly prepare for suicide. Started it, same shake same sound... I pressed the gas and after a couple of secods it turned back to normal state(no shake, but rough idle) had it idling for 15minutes and no symptons of a major damage. Drove it, nothing, drove it, had to top up with 1L oil(normal interval hence the "manual").

I'm wondering what It could be.
When I drive the vehicle, there's no symptons. My carly adapter gives thumbs up in diagnostics, all parameters are 100%, engine rev stays the same on the digital measure but the analog revstick is fluctuating as I drive and as it idles.
What is the actual value on engine speed/rpm & gearbox speed that I need to have? Is there any way to have a specialist manipulate it and higher the rpm with like 2%?

Could it be vanos? Sensors? Iac? Disa? Chain? Cam? Mounts? Former owners who didn't service it properly? (I service it every 9k)

I'd never sell a car that is messed up to someone, too many selfish people like that(win/lose in all areas, donkeys).

I want a win/win and rather sell it with an aknowledgement of what it is to next owner.

Thank you very much & have a great day
Jonatan