[xj-s] XJ-S 3.6 erratic high idle

I have a 1989 XJ-S 3.6 manual which has covered 111000miles.

Lately it developed an erratic idle. Starting off from cold the
idle would be as expected, a steady 700-800rpm. Once warmed up it
idles initially at 1500rpm when in neutral, then down to 1000rpm
slowly and steadily. Also, when I gear change on the run, the idle
zaps up to 1800-2000rpm before the clutch closes. On the over-run
it also revs at 1500-2000rpm.

I don’t like this high idle as it makes me look like a boy racer at
the traffic lights, revving the balls off the engine!!

Can you jag-lovers advise me what’s happening, and what I can do to
fix it please?

Thanks
teddykan–
teddykan
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In reply to a message from teddykan sent Wed 20 Dec 2006:

111000 miles. Hardly run in yet!! Quite often the throttle
potentiometer plays up and you might want to get this cleaned up
and checked. At least worth a look!

Nick–
ng4xjs
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In reply to a message from ng4xjs sent Wed 20 Dec 2006:

Yes the engine’s barely run-in by its ancilliaries are already
playing up!!

I spoke with my Jaguar specialist garage (Swallows Jaguar,
Somerset, UK) who advises me that it could be to do with the
throttle potentiometer sticking thanks to gumming up with all sort
of crud. Also he pointed out there’s an idle stick that needs to be
adjusted/cleaned too.

Can I clean this myself? Apparently no, as the pot needs to be
reset properly etc. What do you jag-lovers think? I prefer the
satisfaction of understanding the problem better and sorting it out
myself, if possible and practicable.

teddykan–
teddykan
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In reply to a message from teddykan sent Wed 20 Dec 2006:

My experience has been on AJ6 abd AJ16 engines, that idle problems
are almost always TB related. On both engines, the TB gets gummed
up over time due to how the crankcase is vented to the front and
rear of the TB. Removing the TB for a good cleaning is not
difficult and requires not much more than a can of cleaner and a
set of 1/4’’ sockets. On the case of the AJ6, you must check and re-
set the clearance on the TB butterfly, as it can have an effect on
your idle, not the case on the AJ16 which closes completely at idle.
The TPS on both engines has little to do with idle. The ISCV
or ‘‘Stick’’ as your mecahanic calls it, does. Often the cleaning
will cure any problem with this device. Remember, when the
throtlle is is closed (at idle) the only thing that allows air in
to the engine, is the ISCV or the ‘‘Stick/stepper motor’’. It is the
change in the amount of air in to the engine that causes the idle
fluctuations. Not fuel alone.–
The original message included these comments:

I spoke with my Jaguar specialist garage (Swallows Jaguar,
Somerset, UK) who advises me that it could be to do with the
throttle potentiometer sticking thanks to gumming up with all sort
of crud. Also he pointed out there’s an idle stick that needs to be
adjusted/cleaned too.
Can I clean this myself? Apparently no, as the pot needs to be
reset properly etc. What do you jag-lovers think? I prefer the
satisfaction of understanding the problem better and sorting it out
myself, if possible and practicable.


uncle
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In a message dated 12/20/2006 2:42:32 AM Eastern Standard Time,
richard.kan@fsmail.net writes:

Lately it developed an erratic idle. Starting off from cold the
idle would be as expected, a steady 700-800rpm. Once warmed up it
idles initially at 1500rpm when in neutral, then down to 1000rpm
slowly and steadily.

Does your 3.6 XJ-S have an AAV (Auxiliary Air Valve) to adjust idle speed
(like the V-12 XJ-S), or does it use an ISCV (Idle Speed Control Valve) like the
3.6 AJ6 in the early XJ40? The Service Manual suggests you have an AAV and NOT
an ISCV.

If you do have an AAV, then perhaps it is stuck in the open position, giving
you proper idle speed when cold, but too high an idle when warm. There is a
lot of information in the archives and in Kirby’s Book about freeing up a stuck
pistion in the AAV.

George
George Balthrop
Clifton, Virginia USA
85 & 89 XJ-S Coupes; 89 XJ40 VDP

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In reply to a message from GBalthropXJS@aol.com sent Wed 20 Dec 2006:

I’e located the throttle potentiometer. I think I would like to
clean the internal contacts and potentiaometer tracks.

The pot is right under the throttle body and fixed to it upside
down with screws. Access to these 2 screws is difficult as the oil
filter is in the way. Also, I don’t know whether it is mounted in a
certain way that needs readjusting if I disturb it.

Questions:-
How do I extract the pot with the minimum of fuss? I hope I don’t
have to remove the inlet manifold!

How do I extract the pot without upsetting any pot settings it may
have currently?

Thanks
teddykan–
teddykan
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