Alright, i have a general question that been on my mind since buying this car.

The proper idle quality.

I will try to detail it as much as possible.

The engine idles well, but slight vibration. very slight. To put it into perspective, bit more than a well put series 3 v12 5.3. On par with a 2001-2002 mercedes S600, but more than the M120 mercedes v12 found in the 1992-1999 S600. And about barely noticeable more than the same era e32 V12 750il.

Here is the kicker. From the experts, i keep getting variety of answers. Some say this is normal as designed, some say its because of ethanol and need to find non ethanol fuel, some say its the idle valve going out. Or spark plug or vacc leak, or such.

Alright, these are things i did and changed with no difference.

Brand new spark plugs, coils, and wires.

Checked vaccuum line all good. Disconnected some to see and oh yeah ya be able tell difference there for sure. So that rule that out.

So then figured and seeked advice on idle control valve. And same conflicting thing. Dome say leave it be, its fine. If it wasnt idle would ne a hot mess and probaby wouldnt idle at all. Some say chuck it and get new one.

Bit more detail.

The car idles steady on point within standard idle, no skip no miss, no hunting, straight up steady. Driving wise, typical v12 smooth delivery and push it, it takes off like a scalded Jaguar as it should.

Now this thing isnt very bothersome just one of those things if i can improve ut, i will. If i cant, no big deal, its fine. Hell, this thing been solid reliable so far. Folk said "oh no its a money pit oh its unreliable oh it will always be broken". And this is no showroom car. Lot of bushings replaced, little electrical stuff done and still to be sorted, like the trunk buttons, normsl old car stuff. And small power steering fluid leak from the ps container. And alt pully put up a racket when it gets wet. Then lastly, think the small fuel dampers on the rubber fuel lines are shot. can feel and hear the fuel pulsating every so often.

Very good car despite it's reputation. Light years more reliable than the last XJ8 i had that only had 45,000 miles on it. Now THAT car was a pita.


Sorry for the length. I just love these cars. These x305s seem to be largely forgotten about outside forums and its a shame they discontinued it. On paper XJ8 give similar performance. In reality....No. Off the line, yes, but past 55, night and day. When the xj8 start to reach peak and revving 4,000 plus, and downshifting like a mad man just to pass at 70 mph, the xj12 barely touch 3,000. Effortless. Dont think i ever revved this thing past 3300 yet. And it has one gear less than the xj8. go figure.