The throttle rods must be set with a bit of play in them, say 1.5 to 2mm, so that the capstan is slightly turning and the TPS slightly increasing the signal, just before the butterflies start opening. Grant recommends using a spacer in the capstan stop to set them both the same.

 

Doug,

1AM here, so COFFEE please.

When setting the cross rods a 2mm spacer is placed between thr idle stop vertical and the capstan peg. This holds the throttle mechanically OFF idle by 2mm.

Set thes 2 rods, many times, until you are 100% sure they are the same.

Remove the spacer, the electrics returns to 0.032-0.036v.

Then, when you press the go pedal, the electrics are just a bees d**k ahead of the mechanicals. The ECU applies just a tad more fuel as the discs open, and away you go.

That is a simplistic wording of what happens, but is more than sufficient for all of us.

SETTING THE THROTTLE CAPSTAN AND RODS GRANT.doc

 

I'll check this stuff too but I'm having a hard time imagining why improper gap would only cause an issue for 5 minutes?

 

Were this to be the cause of the trouble, one explanation would be: Because after the system warms up fuel vaporisation is better and so the engine is able to swallow a leaner mixture for a second or two without stumbling. What we are suggesting is your engine has symptoms that are akin to a carburetted car that has the choke pushed in too early and bogs on a too-weak mixture.
On your engine, if the throttle butterflies open in advance of the TPS asking the ECU for more fuel, when cold this could be a cause of the cold-engine bogging.

 

AND

The most important of all, and I have stated this for 40+ years.

ALL Jaguars, and in particular, V12's, are Female, so constant attention, fine tuning, pampering, GIFTS (as in spark plugs, clean fuel etc) are all required from time to time.

MAYBE, just maybe, the adjustments are NOT your issue, but they are part of a very long list, and must be checked off systematically until SHE settles down.

There is no one answer, IF ONLY, we V12 owners all dream of that.

Time to offer "gifts" to the 2 V6's in the shed, same needs as a V12, but in smaller doses.

 

I'll be damned. Never heard of it before.

Proof, once again, that we can learn something new each day ! When it comes to Jag V12s there seems to be an endless fountain of tidbits to learn and know.

Cheers and thanks
Doug

 

While I haven't checked in a little bit, the throttle should be gapped properly, I've even had the throttle tower out and back in since this has occurred with no change, but its an easy check to throw on the list, thanks for the idea. The linkages themselves should be really good, as I've replaced the bronze bushings in them, but its all working checking on.

We'll have to agree to disagree on needing constant attention. I drive the XJS daily and its very no fuss. I put gas in and it takes me where I'd like to go, and that's been our relationship for close to 8 years now. Its a great f***ing car.

 

SS
See post 18, it is not just the rods' adjustment. If the TPS voltage is way too low at the closed setting, this will have the same effect - ie the throttles open before the ECU get the signal.
Greg

 

Mine was the DD for 16 1/2 years, and once sorted from the slack precious caretakers, as good as any Toyota etc.

The "fiddling" is just a Jaguar thing, and comes with teh territory.

Some fine tuning is accepted, BUT, when she gets testy, then that list must be worked through, A to Z, forgetting what "should/might/will be OK. They may well be, but until crossed off, who knows.

Always mindful that the V12 is a Mid 1950's design, and the P Digital EFI is late 70's, and that is back in the era that daily "fiddling" with machinery was a given.

I had 4 V12's at the time, so I fiddled a lot.