348 idles high, rattles when ign turned off?

Idle air valve stuck open?

 

I tend to agree. There's some air getting onto the motor that shouldn't.

 

High idle may be a vacuum leak and the rattle is most likely from the viscus flywheel that needs to be serviced.

when they are low on grease in the flywheel or the grease is old, the weights inside rattle and also cause a jerky hot start condition.

Regards, Jim

 

To the OP, in the title of this thread you state the engine "rattles", in the text of your post you state the engine "sputters". Two very different conditions. If it rattles then Jim is exactly correct. The 348 flywheel is a clam shell arrangement around a spring dampened (and greased) mass. If the triple seals between the gearbox and the flywheel fail then gear oil enters the flywheel and contaiminates (thins out) the grease. Doubtful an Authorized Service Center will service this, you will need to DIY or find a good independent. Fill out your profile so folks here know what part of the country you are located.
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Easy to fix yourself. Unusual service level at american authorized service centers. Are we actually talking about authorized Ferrari service centers?

 

Yep, a few yeears back they considered this component non-serviceable, had to buy a new one.