[xj-s] 6.0L V12 idle woes -- it never ends

XJ-S
21

In reply to a message from Kirbert sent Mon 22 Sep 2014:

A very long shot:

Have you checked the valve clearances? I know it�s a hassle
and I don�t have a proper explanation as to why it might
cause this particular condition but from tinkering with many
a carburetor in vain and throwing a lot of money at
ignitions, timing and sonsors I now always start of with
valve clearances. Especialy as you say your 6.0 never idled
properly and smoothly.

A valve or two not closing fully on a hot engine can give
you the oddest effects.–
Tierfreund
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22

In reply to a message from Tierfreund sent Mon 22 Sep 2014:

Good point. No, I have not measured the clearances.
Should’ve done it when I did the covers/half-moons/banjo’s.

For the record, I know of only one 6.0L owner who has a legit
claim of silky smooth idle. Everybody else here with the 6.0L
is having issues of some kind.
Sounds creepy, but I’d allege that the engine wears, and the
management system ages, it gets harder and harder to keep all
the electronics in top shape.

Steve–
The original message included these comments:

Have you checked the valve clearances? I know it�s a hassle
and I don�t have a proper explanation as to why it might
cause this particular condition but from tinkering with many
a carburetor in vain and throwing a lot of money at
ignitions, timing and sonsors I now always start of with
valve clearances. Especialy as you say your 6.0 never idled
properly and smoothly.
A valve or two not closing fully on a hot engine can give
you the oddest effects.

–
'95 XJS convertible - V12 6.0L
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23

In reply to a message from sbobev sent Sun 21 Sep 2014:

That moment came sooner than I expected…

The transmission warning light came in this morning, on my
way to work, 2 miles before I parked. Now I am in ‘limp
mode’.

Apparently there is a faulty sensor somewhere – have not
had the time to pull the codes. Shift solenoids,
input/output speed sensors, pressure or temperature switches
– remains to be determined.

My understanding is that the line pressure will be increased
if there is a tranny fault, which in turn, could be the
reason for the idle ‘surge’.

Your thoughts on this one, please?

Steve–
The original message included these comments:

PS And yes, the transmission is the 4L80e. There is something
very weird with all that – transmission talking to the
Marerlli ECU talking to the Lucas 36CU, and I am sure I will
find out when I least expect it.

–
'95 XJS convertible - V12 6.0L
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24

In reply to a message from sbobev sent Tue 23 Sep 2014:

Another point, which a brief search in the archives produced
and I had overlooked:

‘common faults on the 4l80e are speed sensors and gearbox
wiring and also TPS sensors for engine or gearbox signals.’

Credit to the electronic guru Dave Reacher (540itouring) for
posting it.

Looks like I should have looked at the TPS earlier. The TPS
is where the TCM and engine ECUs get tangled into a web. It
has 6 wires, supplying signal to both engine and tranny
computers.

I did check it some time ago and all was good (Kirby, if you
change your mind and decide on a new version of the Book,
please make a note that the idle voltage for the 6.0L V12 is
0.6V, not 0.32V as for the 5.3L).

What’s on my mind now – was the transmission fault the
culprit to begin with or was it the TPS? I guess my winter
maintenance project will start early… :-((

Steve–
The original message included these comments:

Apparently there is a faulty sensor somewhere – have not
had the time to pull the codes. Shift solenoids,
input/output speed sensors, pressure or temperature switches
– remains to be determined.

–
'95 XJS convertible - V12 6.0L
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25

My understanding is that the line pressure will be increased
if there is a tranny fault, which in turn, could be the
reason for the idle ‘surge’.

I would think that a line pressure increase would cause the idle to
drop. It is, after all, an increased load.

– Kirbert

// please trim quoted text to context onlyOn 23 Sep 2014 at 6:16, sbobev wrote:

26

…I’d allege that the engine wears, and the
management system ages, it gets harder and harder to keep all
the electronics in top shape.

All true – but if not for the emission control requirements, it’d be
dead simple to make this engine idle like a purring kitten. Just set
up an open-loop EFI system with a manually-adjusted idle mixture and
dial it a hair on the rich side. The fact that a smooth idle is so
difficult is entirely because the EFI is trying to run it so lean and
trying to constantly correct to make sure it’s running lean. Trying
to keep the EPA happy with a lean-running engine and STILL have a
smooth idle, now that’s a challenge, especially as things get worn.

– Kirbert

// please trim quoted text to context onlyOn 23 Sep 2014 at 6:08, sbobev wrote:

27

In reply to a message from Kirbert sent Tue 23 Sep 2014:

Bummer. Is the tranny light a hard fault…in that
turning off and restarting the car does not get it to go
out? Mine goes to limp mode twice a year or so, but a
restart take care of it for a few more months. I refuse
to trouble shoot it until it becomes a hard fault that
won’t clear.–
John. '95 XJS 6.0L convertible. Southlake, TX
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28

In reply to a message from sbobev sent Tue 23 Sep 2014:

Same thing happened to my 1994 6.0 V12. After 3 months of
downtime, the third shop I had it towed to, found that the
throttle position sensor was bad. $150 and I was on my way.

Symptoms were high line pressure, chirping tires on the 1st
to 2nd shift and the 2nd to 3rd shift, eventually resulting
in limp mode.–
The original message included these comments:

My understanding is that the line pressure will be increased
if there is a tranny fault, which in turn, could be the
reason for the idle ‘surge’.

–
Donald D. Clary
Peoria, Illinois, United States
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29

In reply to a message from Don61604 sent Tue 2 Feb 2016:

Don:
This was ‘‘my thread’’ from some time ago. The transmission
limp mode is usually the speed sensor (generic AC Delco
items) or the harness.

The TPS could be the reason too, however, and this should be
in bold font, the TPS for the 6.0L V12 is the infamous
DBC2626 – NLA. They pop up here and there as NOS, but the
asking price is over $400. How could your shop find it and
charge you only $150 for the part and labor?

Thus, I regret to say it, but highly doubt what the shop
repair bill states.

Regardless, they fix the car for cheap, which is always a
good thing :-))

Best regards,
Steve

PS Supposedly DBC2626 interchanges with STANDARD MOTOR
PRODUCTS TH416, which also is very hard to be sourced, at
least in the US.–
The original message included these comments:

Same thing happened to my 1994 6.0 V12. After 3 months of
downtime, the third shop I had it towed to, found that the
throttle position sensor was bad. $150 and I was on my way.
Symptoms were high line pressure, chirping tires on the 1st
to 2nd shift and the 2nd to 3rd shift, eventually resulting
in limp mode.

–
'95 XJS convertible - V12 6.0L
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30

In reply to a message from sbobev sent Wed 3 Feb 2016:

The part was $150 - used. We couldn’t locate a new one.

Labor for putting the part on and diagnostics was $400. The
Jaguar dealership had it for two months and an independent
transmission shop had it for two weeks. Neither could
diagnose the problem. I actually don’t think they tried
very hard. Neither said they could diagnose the problem,
because they had no way to hook a computer to the OBDI port
(round). My Jaguar dealership’s Portable Diagnostic Unit
hasn’t worked in 8 years.

The third shop diagnosed the problem in a week and then it
took another week to get the part.

The car ran fine, except the high line pressure and
chirping of the tires, during transmission shifts from 1-2
and 2-3. They were surprised the engine ran so well, with a
faulty TPS. Eventually, after 10 miles or so of driving, it
would go into limp mode.

I have two 1994 XJS V12 6.0 - one coupe and one
convertible. So we weren’t guessing at parts, we swapped
the TPS from my convertible (that was running great) into
the coupe, to make sure that was the problem, before we
ordered parts.–
The original message included these comments:

In reply to a message from Don61604 sent Tue 2 Feb 2016:
Don:
This was ‘‘my thread’’ from some time ago. The transmission
limp mode is usually the speed sensor (generic AC Delco
items) or the harness.
The TPS could be the reason too, however, and this should be
in bold font, the TPS for the 6.0L V12 is the infamous
DBC2626 – NLA. They pop up here and there as NOS, but the
asking price is over $400. How could your shop find it and
charge you only $150 for the part and labor?
Thus, I regret to say it, but highly doubt what the shop

–
Donald D. Clary
Peoria, Illinois, United States
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31

In reply to a message from Don61615 sent Wed 3 Feb 2016:

Thanks much for the confirmation Don. I stand corrected :-
))

BTW, a bad TPS will display a FF17 code on the trip computer
(CEL may not be ON). Picture #4

http://www.jag-lovers.org/v.htm?id=1402280159

No PDU needed.

Best regards,
Steve–
The original message included these comments:

I have two 1994 XJS V12 6.0 - one coupe and one
convertible. So we weren’t guessing at parts, we swapped
the TPS from my convertible (that was running great) into
the coupe, to make sure that was the problem, before we
ordered parts.

–
'95 XJS convertible - V12 6.0L
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32

Everydayxj has one for $145 NOS so act quick…

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33

In reply to a message from jagsandmgs@gmail.com sent Wed 3 Feb 2016:

John:
No they don’t. When you try to check it out, the following
appears:

‘‘Oops, I’m showing this is out of stock. You may want to
email to see if I have it. XJ6 XJ12 XJ40 XJS Throttle
Potentiometer - DBC2626’’

Steve–
The original message included these comments:

Everydayxj has one for $145 NOS so act quick…

–
'95 XJS convertible - V12 6.0L
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34

In reply to a message from sbobev sent Wed 3 Feb 2016:

Hi Steve…

My apologies for that…
I did have it, and did sell it for a great deal.
What I do is buy cars and inventory from either individuals
or shops that are closing. I don’t have what you need every
time, but when I do it’s usually a great deal. Just like
that part was to the person who nabbed it up…
I appreciate your having checked out my site. Hopefully I’ll
find some other things that may be of some help in the
future…

Cheers,
David
Shop.EverydayXJ.com–
The original message included these comments:

No they don’t. When you try to check it out, the following
appears:
‘‘Oops, I’m showing this is out of stock. You may want to
email to see if I have it. XJ6 XJ12 XJ40 XJS Throttle
Potentiometer - DBC2626’’

–
84 XJ6 (Driver) 89 XJS Convertable… Parts cars piling up.
Rockwell, NC, United States
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35

In reply to a message from DavidBoger sent Tue 16 Feb 2016:

Hi Dave:
No need for apologies. You’ve always been very helpful.
Actually, I had written to you and asked about the TPS and
knew you don’t have it.

In a post here from Febr. 3, this is what I had stated
(verbatim):

‘’… the TPS for the 6.0L V12 is the infamous DBC2626 –
NLA. They pop up here and there as NOS, but …‘’

All 6.0L v12 should be aware of this. The supply is very
tight and a new part may not be available when you need it.
I am hoping something who is more knowledgeable than me will
come up with an alternative of some kind.

Best regards,
Steve–
The original message included these comments:

My apologies for that…
I did have it, and did sell it for a great deal.
What I do is buy cars and inventory from either individuals
or shops that are closing. I don’t have what you need every
time, but when I do it’s usually a great deal. Just like
that part was to the person who nabbed it up…
I appreciate your having checked out my site. Hopefully I’ll
find some other things that may be of some help in the
future…

–
'95 XJS convertible - V12 6.0L
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36

In reply to a message from sbobev sent Wed 17 Feb 2016:

You’re in luck, theres a used on on eBay right now…

http://www.ebay.com/itm/Jaguar-XJ40-Throttle-position-
sensor-potentiometer-DBC2626-93-94-models-/272120919900?
hash=item3f5babb75c:g:4NQAAOSw5dNWrnh~

Price is right too…–
1996 XJS 2+2 Convertible with 4.0L AJ16
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37

In reply to a message from Veekay sent Wed 17 Feb 2016:

Don’t need used TPS (or alike electric gizmos), have one
already.
Thanks for the tip though.

Steve–
The original message included these comments:

You’re in luck, theres a used on on eBay right now…

–
'95 XJS convertible - V12 6.0L
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38

In reply to a message from Veekay sent Wed 17 Feb 2016:

That TPS is for 4 litre cars. I’m pretty sure the 6 litre is
different.
Some folks have cleaned the TPS by drilling a small hole, or two,
and squirting contact spray through it.

Rob–
The original message included these comments:

You’re in luck, theres a used on on eBay right now…

–
Robert Laughton www.leatheriquecanada.com 1999 Super 8
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39

In reply to a message from Robert Laughton sent Thu 18 Feb 2016:

Hello Rob:
I encourage you to visit the JDHT and check the part number
yourself. The link to the diagram is below (#4)

http://tinyurl.com/h73t7h5

DBC2626 is indeed used on the 6.0L V12.

Best regards,
Steve

PS The ‘fix’ you are describing is only temporary. If one
needs a permanent solution (or at least something that will
last another 20 years), NOS is the way to go–
The original message included these comments:

That TPS is for 4 litre cars. I’m pretty sure the 6 litre is
different.
Some folks have cleaned the TPS by drilling a small hole, or two,
and squirting contact spray through it.

–
'95 XJS convertible - V12 6.0L
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40

In reply to a message from sbobev sent Thu 18 Feb 2016:

Thanks for correcting me. That part fits both 4 and 6 litre. I
have a couple of 6 litre cars in the yard, and from foggy memory,
they appeared to be unique…oops.

Rob–
Robert Laughton www.leatheriquecanada.com 1999 Super 8
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