My good friend in newyork has the twin too my xjs . He's got a high idle issue in neural and park? 1500 rpm I personally believe its a vacum issue but maybe someone else has delt with this?

 

AAV jammed open.

I did an answer to that a few bottles of JD ago.

Have a word doco on it if you want that, so PM me your email and and I will oblige.

ONE condition. If it sorts it, post back here so the system stays up to date.

 

I'd love a copy Grant! I'm running under 500 rpm in drive and around 1300 in Park & neutral. I'll post back with results.

1988 XJS - Sent from my LG-G2 using IB AutoGroup

 

Well as it turns out this indeed is his issue . It only does this when cold so this is the thermo bulb?

 

What is the "right" cold and warm Idle for a 5.3L V12?

 

That is as variable as "what is the best oil to use"..
In my years:

About 1200 rpm cold is what most are/have been. I usually knock that down to around 1000rpm when I do the AAV rebuild.

Around 700rpm hot in P or N is my settings and I find it quite satisfactory.

This hot idle will depend on waaaaaay too many items to nail 100%. Air con compressor (which one), smog junk, etc etc, all affect this number.

The tacho on the dash is NOT accurate, so estimate is alll that one can do. You get to know the engine speed after MANY years of listening to them. Its a feel thing for me.