I dont understand...

Are you saying the engine idles at 700 rpm and then settles back to its normal tick-over? Mine does that, I assumed it's normal.

Luca, you're not the only one who don't understand! we are going to have to work on developing your technical English!!!

First, what happens when you start the engine after, say, 12 hours? Does it behave properly? When it does misbehave what is the seven? Kph? 700 RPM? Or something else? You say 'something is strange' - what is something?

More details - Good! Brief description - not so good!!!

Whatever it is, it isn't likely to be leaking spark plug gaskets!! (IMHO!)

Sorry im not English, and im not good in technical English, maybe you can help me to improve it. Try to understand also that im one of the few italian that at least speak english.
The problem is that i dont know if its normal that when i dont use the car for one entire day so the engine is very cold, when i start it, it idles at 1100rpm and makes vibrate the car, if i push gas and i go it looks ok, and after a minute the engine comes back normal.
Im not surprised that at cold engine it idles at 1100 for a minute, its normal, always done, is the vibration that makes me worried, and remember to me 15 years ago when my saab 900 turbo was going 3 cyls
Sorry again for my technical english, i hope is not a big disturb

No need to apologise for your English - we just wanted to make sure we understood what you were saying.

Does the car vibrate when you rev the warm engine at 1100? If it does, then that the issue, not the cold starting. Do you get the vibration at any other engine speed? Do you have any other problems when it's running? (Lots of questions, no answers yet, sorry.)

No any other problem, is for this reason that im a bit worried ... i hope that it wont come some surprise ...

Mine's been sitting on my drive unused for a few days - I'll pay extra attention when I next start it and let you know how mine is.

Thanks so much friend !

You're right Luca, 1100 rpm is fine when cold, but the vibration sounds like a cylinder or two missing. Remind us what Model Year your XK8 is. Also the mileage on the clock. I wonder if the spark plugs are overdue for change? (As often as every 30,000 miles was the recommended distance for early XK8s) If not, then it could be a failing coilpack (The things stuck on top of each spark plug!) Keep the info flowing and we'll solve it somehow!!

My XK8 is a 1997, 158.500km.
I replaced the sparks 2000km ago, so i think is not about that, but it can be the coil pack, you're right. The strange thing is that after a minute everything becomes totally normal... very very strange... my saab 15 years ago when was going at 3 was always going at 3 ... mystery for me ...
Anyway tomorrow i need to talk to Dave (EuroJag) and i can buy a new coilpack set and replace all, bad is not for sure. What you think ?

Or do you think is better to buy new coil pack, (eurojag has good second hand) ?

Now i remember a thing that can be important... when we changed the sparks there was one of them, if i remember right its on the left side the first on the right, that was dirty of oil... maybe because of that i was thinking about some gasket... but im not a mechanical and i just supposed...

Was that sparkplug dirty/oily on the outside, that is around the white porcelain, or the inside, that is the electrodes that actually spark?

A leaking gasket on the cam cover around the plug can cause the external oiling that could result in 'tracking' of the high voltage making it misfire, but if the electrodes themselves are oiling up, (that will also cause misfiring) a more serious problem, such as worn valve guide seals or cylinder bore/piston rings wearing may be the cause. (But not usually just one cylinder)

Today it was perfect... no one vibration when switched on after 3 days... it was like airplane... acting very good... i hope she got miracled, pope was here in the weekend LOL

Blessings of the Forum be upon you!

i hope so.... i will tell you what happens next time i will start the car...