91 xjs 5.3 litre v-12 on startup engine runs fine for 2 minutes then revs between 700 and 1700 rpms for 3 to 5 minutes depending on how cold it is any help would be greatly appreciated

 

If the engine cycles up to 1700rpm, then cuts out and slows down to a few hundred rpm, and then repeats this; it is close to a certainty that the Auxiliary Air Valve (AAV) is sticking.

What this does is to admit extra air on startup, and should gradually close as the water temp rises until the car idles normally when up to temp. If this valve sticks open, or does not close progressively as designed, when partially warm the revs rise too high, and the 'overrun cutoff' in the ECU cuts the fuel injectors until the revs drop. Hence the cycling.

The overrun cutoff is a fuel saving device that cuts the fuel when (eg) you lift off at 60mph, so fuel is not wasted as you drift along on a closed throttle. The ECU starts the injectors again once the revs drop to about 700, so the car does not stall. Trouble is, being an early system, the ECU dos not know if you stationary, or are blasting along.

The Great Palm's book has loads about the AAV, here is a pic of it, B bank at the cabin end, the big bolt sticking out of it is the tickover adjustment.



You can unbolt the thing once the aircleaner is off, and probably free it off with a bit of careful work, penetrating oil etc, if it is not too far gone.

Greg