[x300] XJR6 High idle problem

Have had car in shop to diagnose high idle issue for 2 weeks with
no result.replaced tb with good used one/new stepper motor/new
temp sensor/cleaned MAF/check for vacuum leaks.(shows no codes)
Possibly using more fuel,not tested as yet as just
noticing.Mechanic seems to think a possible problem with
ecu(expensive hunch). My info is that this is unlikely.Can I have
the ecu tested.This morning when I stopped the car, idle at 1300
turned steering to full left lock idle dropped to 800 rpm. Full
right lock not as much of a response.How would this affect idle.
Should I go to another mechanic or auto electrician.Looking for
some direction as am stumped.–
matt60
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In reply to a message from matt60 sent Wed 9 Sep 2015:

Well, before I embarked on a new ecu adventure, I would find
a shop that has the software to do a TPS calibration
procedure. Also, check that the throttle plate is returning
to the same position relative to the closed screw. My
advice, not universally shared, BTW, is don’t mess with that
screw!

The idle speed control is not functional unless the throttle
plate is sensed closed. It is a very small window and the
only Jaguar prescribed method for adjusting the sensed
closed position is using the dealer level software such as
IDS or WDS.

It took me a year to figure out my idle problem. In my case
it was the TPS- followed by a calibration.–
Ross - 89 XJS, 96 XJ6 (I hope), 99 XJR, 02 XJ8, 12 XF-R
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In reply to a message from sparkenzap sent Wed 9 Sep 2015:

Was at jag related mechanic with the equipment for 2 weeks +,would
assume those items were already addressed as he installed TB and
would have examined and tested TPS when changed over.Lower idle
when full steering lock could be clue but have problem knowing how
to rule out other than somehow catching on throttle cable?–
The original message included these comments:

In reply to a message from matt60 sent Wed 9 Sep 2015:
Well, before I embarked on a new ecu adventure, I would find
a shop that has the software to do a TPS calibration
procedure. Also, check that the throttle plate is returning
to the same position relative to the closed screw. My


matt60
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In reply to a message from matt60 sent Wed 9 Sep 2015:

assume, huh?

The’‘lower idle’’ you refer to is a result of bottoming out
the servo valve on the power steering pump at the end of
travel and increasing engine load. Try turning the AC on
and off and see if the rpm changes (it should not).

Your car is not controlling idle. To control idle, the ecu
must think the throttle plate is at idle. It knows that
from the TPS. The ECU controls the stepper motor. So,
either the throttle plate is not at idle, the TPS is not
giving a idle position sense to the ECU or the new throttle
stepper is bad. Or something is mounted wrong or not
connected right.

BTW, if you trust the mechanic, then ask him why the idle
control that he worked on does not work!–
Ross - 89 XJS, 96 XJ6 (I hope), 99 XJR, 02 XJ8, 12 XF-R
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In reply to a message from matt60 sent Wed 9 Sep 2015:

Oh, yeah, I forgot you said that he identified a bad ecu.
So, the test procedure should include reading the TPS signal
on the WDS to assure the ecu thinks you are at closed
throttle, then look for stepper pulses as the engine load is
changed.

It is always possible that you do have a bad ecu, but most
reports on an X300 ecu problems are centered around water
ingress to the connectors. It probably would not be a bad
idea to look at the connector pins to assure you do not have
corrosion.

Good luck.–
Ross - 89 XJS, 96 XJ6 (I hope), 99 XJR, 02 XJ8, 12 XF-R
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In reply to a message from matt60 sent Wed 9 Sep 2015:

Matt,

Although you say you have checked, is there any sign of air leaks?
Humming, whistling etc. at idle?

Just a different angle to Ross’s. I do agree that the reduction in
idle speed is due to increased load from the PAS and that the idle
speed is not being managed. That means the reduction in idle speed
is telling, but also a Red Herring.

Regards

Steve–
Jags on gas
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In reply to a message from Jags on gas sent Thu 10 Sep 2015:

Had high idle 1000 - 1300…was told it was due to butterfly
valve not closing because throttle return spring not strong
enough to pull it shut against oily gunge coming from engine
breather. Before this replaced TPS and IACV, and calibrated.
When throttle body removed and cleaned…good idle. When
return spring tightened by snipping off end…good idle.
Wished I’d put a new stonger return spring on throttle as
per some TSB I’d heard of. My idle, when too high, can be
returned to 800 by blipping throttle.–
Ant XJR6
Kent, United Kingdom
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Recently bought a 96 XJR6 here in the US, was having this exact problem. Luckily for me, my mechanic is very well experienced and reasonable on cost. He just explained this cure, plus custom making a bracket that will better secure the stronger spring that he installed. Apparently this is quite a design flaw and hard to cure. Very annoying when you are going 60mph or 100 kmh, with no foot on the gas, much less horrible gas miles. Hopefully this post will inform others before breaking down so many other things to no avail! Thanks for sharing.

I’ve got to agree with these last several suggestions about tb/return spring issues. I’ve had similar problems on two past X300’s. I could apply a little more return pressure manually and it would get better. Thought about fixing a stronger spring, but I cleaned the TB in situ first. Made a whole world of difference, but still has occasional issues. The infamous “blip” of the throttle would clear that up usually. FINALLY, did a proper cleaning, completely off the car, in a bucket on the bench, with plenty of cleaner, a tooth brush, and a tiny little bore brush for some of those ports. Voila! Perfection! Never had another problem.

I thought I had gotten them pretty clean while still in place. The amount of grunge in the bottom of the bucket told a different story. Good luck!