[xj-s] 6.0L V12 idle woes -- finally, some progress, sort of

In reply to a message from T Boy sent Fri 30 Aug 2013:

Hello Todd:

Thanks for the suggestion. If I haven’t mentioned it before,
it is entirely my fault. Too many things to list.

However, I DID clean both sensors last year, when I did a lot
of work in the fee. A few months ago, I actually replaced the
flywheel (first) and then the crankshaft position sensor.
Nothing in the running order changed.

I will say though, that on the 6.0L car, the ECU flags a code
if the engine speed signal is not optimal (FF77). Not sure if
the older cars with Marelli ignition do that.

Steve–
The original message included these comments:

connector, pull all plugs, check coils etc. I had a similar problem
a few yrs back and posted on it.To make a short story of it .I went
by a buddies shop who has a lift and put it in the air …sure
enough the front crank sensor had oil on it as I and most older
jags as yours probably has a small oil leak around the crankshaft
damper pulley oil seal.This leak drips directly onto the front TDC
or crank shaft sensor. This sensor takes the A bank info and
decodes it by sotware to determine the B bank position and the oil
keeps it from getting a good reading from the 3 toothed rink on the
crankshat.I checked the rear fly wheel sensor also but its
proteceted by a fly wheel dust shield and was clean. I sprayed the


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from sbobev sent Wed 28 Aug 2013:

Updates as promised.

  1. The Marelli jumper was removed today and the car driven
    w/o O2 sensors feedback – no apparent difference w/ or w/o
    the jumper.
    I also did not experience any difference before, when the O2
    sensors were connected.

  2. I communicated with R. Bywater from AJ6engineering. His
    response provides several very important clues

  • the 36CU does have a TIMER of some sort. No CTS or O2
    signal needed to switch from open to close loop. Feedback
    is always inhibited for one minute from start-up. This
    explains why the stumble always happens at the same time.

  • Mr. Bywater argues that there is an issue that causes the
    feedback to over-correct � possibly a slight misfire or
    perhaps injector fouling

  • with regard to running in limp mode, Mr. Bywater
    explained that ‘the system drops out of feedback and adds
    full throttle enrichment (not the same as acceleration
    enrichment) according the the signal from the throttle
    potentiometer and the throttle opening where this happens
    moves wider with increasing engine speed.’

All of the above makes sense, but there is always a but…

In the last 1000 miles I have done:
-all gaskets and vacuum hoses replaced
-new plugs
-new rotor
-new cap
-new coils
-new ignition amps
-all 12 injectors removed, ultrasonically cleaned and flow
tested
-pressure regulator checked, pressure gauge permanently
fitted on the rail
-new CTS, flywheel and CPS sensors
-swapped stock and S.E. ECUs
-new O2 sensors
-TPS voltages/sweep and idle switch operation confirmed
-dizzy alignment confirmed

I cannot find a sign of a bad misfire, exhaust flow is
steady and no po-pa-pop.

Where should I look for the culprit?

Steve

PS John, did you get the chance to put your beast in limp
mode?–
The original message included these comments:

The Marelli jumper should retard timing and allow for lower
octane fuel. I had removed it before when I was trying to
getting smooth idle in Park (one of the many previous
discussions we had not long ago regarding ‘rolling idle’),
but did not notice any difference.
Have not driven the car with the jumper out before or now,
with the O2 sensors disconnected. Will do when I get a
chance and will report.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from sbobev sent Mon 26 Aug 2013:

For what it’s worth… I had a similar problem to this on my 1991
5.3l XJS coupe. I had already cleaned the injectors, all new plugs,
wire, cap and rotor. It did not correct the problem (ran rough at
idle and reduced power when running). I started checking
connections, and when I touched the wiring harness for the
Crankshaft Position Sensor, the engine quit. I tried to restart -
no dice. I disconnected the CPS and reconnected it - started right
up. I replaced the CPS. The car ran good for a few days but then
started the same behavior. I decided to replace the injector wiring
harness. All day project. The old harness was cracked in spots but
no bare wires. After installing the new harness - no change. I went
and started checking the connections again and when I wiggled the
connector for the CPS it started running real rough and quit.

To bring this sad tale to a conclusion, I replaced the connector on
the harness side of the CPS sensor. The cable from the CPS to the
firewall connector is a coaxial cable. When I peeled back the
insulation I found a bad connection from one of the leads on the
connector to the shield of the coaxial cable.

For the center lead, I used a non-insulated butt splice, soldered
it and heat-shrinked it. for the shield connection I trimmed the
other connector lead about 1 to 1-1/2 inches longer than the center
lead, stripped it about 1-1/2 inches and wrapped it around the
outer shield, and soldered it.

That was about a year ago. Every time since, it has started right
up and run smoothly.

Good luck finding the problem. I hope it doesn’t take you as long
as it took me. It was a real PITA but a good learning experience!–
1991XJS Classic Coupe
Tampa, United States
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In reply to a message from sbobev sent Sat 31 Aug 2013:

Steve,

Finally back and got my car back from the Airport after
the coil failed. I swapped the B coil, but have not
changed the amplifier. Here’s some info…

First, the engine idles much better with the fresh coil.
There is no longer any ‘‘roll’’ at all, and the stumble is
barely noticeable now. This makes me wonder if the B coil
was weak and skipping for some time. The idle speed even
increased by 50 rpm. Now it may be crazy, but it seems
like the tranny is even shifting more smoothly. That’s
impossible to quantify. Just seems that way.

I pulled off the O2 sensors. The idle dropped about 75
rpm and the check engine light came on. Other than that,
I can’t really say I notice any difference driving. It
stumbles briefly on rapid revving with no load. I didn’t
leave them off long enough to check the mileage.

Have you had time to work on yours?–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

‘‘but it seems like the tranny is even shifting more smoothly.’’

Should - better vacuum signal from the manifold.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

Hi John:
Thanks for doing this. I am confused more than ever.
My idle speed when the O2 sensors are pulled is actually
higher. You report lower idle, how could this be? AFR
ratio is 12:1 w/o feedback and close to 14.7:1 with
feedback?
BTW, the most noticeable difference between normal and
forced closed operation is 1-3 minutes after a start-up from
cold.

Mr. Bywater thinks I’ve got an injector that is not working
properly, therefore the O2 sensor trim off the fueling
pulses to cause the but I had them cleaned/tested by SD
Faircloth just a few months ago.
I guess my next project will be to fab a new injector
harness…

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

Steve,
I pulled off the O2 sensors. The idle dropped about 75
rpm and the check engine light came on. Other than that,
I can’t really say I notice any difference driving. It
stumbles briefly on rapid revving with no load. I didn’t
leave them off long enough to check the mileage.
Have you had time to work on yours?


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from lockheed sent Mon 16 Sep 2013:

Hi Alan:
I am afraid I am not following – the tranny is controlled
electronically, why would MAP matter?
Steve–
The original message included these comments:

‘‘but it seems like the tranny is even shifting more smoothly.’’
Should - better vacuum signal from the manifold.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

John:
If you replaced the original coils, yes, you might have had
an intermittent spark causing all sort of problems. The car
will still be running ‘‘fine’’ even if loosing 1 (or 2) out of
the 12 cylinders every now and then…your cats would have
suffered big time but you don’t have them, so all is good.
Keep a spare coil in the boot, just in case. I also got $40
Prenco coils for my engine, but doubt they will last as long
as the original Marelli colis (now my spares).
Steve–
The original message included these comments:

Finally back and got my car back from the Airport after
the coil failed. I swapped the B coil, but have not
changed the amplifier. Here’s some info…
First, the engine idles much better with the fresh coil.
There is no longer any ‘‘roll’’ at all, and the stumble is
barely noticeable now. This makes me wonder if the B coil
was weak and skipping for some time. The idle speed even
increased by 50 rpm. Now it may be crazy, but it seems


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from sbobev sent Tue 17 Sep 2013:

Steve, you are right, I forgot that yours is a later model with the
4L80E rather than the TH400. Probably just a smoother running
engine makes for smoother shifts.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from lockheed sent Tue 17 Sep 2013:

Sorry Steve, my test with the O2 sensors was all done with a warm
engine. When I get a chance I’ll try from cold.

The lower idle seemed to make sense to me. I know in the ancient
days of carbruetors, the speed of the engine grew faster as you
leaned the idle mixture…up to the point where the car would get a
lean roll to it, and finally die if you went leaner. I figured
defaulting the map to the richer setting would naturally make a
slightly lower, but stronger idle, like I experienced.

Now if moving your car to the richer map sped up your idle, you may
be on the rich side with the sensors hooked up? I’m not sure if
that would be possible, though. Just thinking out loud. Maybe
back to your temp sensor reading too cold? Or, your car could be
way on the lean side with the sensors connected, so the rich map
brings it back to a solid idle. Lean could be the injectors,
injector wiring, injector resistors, or the temp sensor again.–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Tue 17 Sep 2013:

John:
Thanks for the feedback. I’ve driven carburetored cars, but
was in my late teens/early 20s and do not remember any of
the carb-tuning basics.

What you are saying makes sense, but I wonder about the
effect of the EVAP system. IIRC, you capped off yours, am I
right?

If this is the case, your idle is never affected by the
carbon-canister, while my engine can breath through it.
I re-read the JDHT last night (again) and I noticed the
section on ‘Adaptive idle fueling trim’. See my other post
above.

Anyhow, there it says that if are no DTCs present the ECU
cancels purge flow and adapts the idle fueling. OK, now, if
I disconnect the O2 sensors, DTC FF44 comes, the ECU does
NOT close purge flow and does NOT trim fueling, will this
make the idle speed higher or lower?

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

The lower idle seemed to make sense to me. I know in the ancient
days of carbruetors, the speed of the engine grew faster as you
leaned the idle mixture…up to the point where the car would get a
lean roll to it, and finally die if you went leaner. I figured
defaulting the map to the richer setting would naturally make a
slightly lower, but stronger idle, like I experienced.
Now if moving your car to the richer map sped up your idle, you may
be on the rich side with the sensors hooked up? I’m not sure if
that would be possible, though. Just thinking out loud. Maybe
back to your temp sensor reading too cold? Or, your car could be
way on the lean side with the sensors connected, so the rich map


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from sbobev sent Wed 18 Sep 2013:

Steve, you have a scary good memory! Yep, my canister
purge simply vents into the air cleaner. Unfortunately,
that didn’t seem to help the stumble. But I do have to
say that with the new coil my car idles better than most
normal cars idle. I am even wondering if it will possibly
pass the emissions test next spring. It did pass once,
right after I bought it 4 years ago…so who knows.

I think you are right…the idle would have to rise a bit
while purging. You are letting more air in.
Anyway…I’ll be following your new thread…–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Thu 19 Sep 2013:

John:
Glad at least one of us has made some good progress.
The solution is almost always simple, determining the problem
is not.

In my case, coils, amps, sensors, wires, cap, rotor and even a
re-programed ECU did not make a difference.
Probably also getting closer to solving it, but who knows what
I’ve overlooked.

Steve–
The original message included these comments:

that didn’t seem to help the stumble. But I do have to
say that with the new coil my car idles better than most
normal cars idle. I am even wondering if it will possibly
pass the emissions test next spring. It did pass once,
right after I bought it 4 years ago…so who knows.
I think you are right…the idle would have to rise a bit
while purging. You are letting more air in.


'95 XJS V12 6.0L, saphire/tan
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Hello All,

These idle issues seem all too familiar. My 94 XJS has a similar issue and has had for the last 16 years. I’m running an AJ6 Engineering super enhanced ECU with the purge valve circuit disabled, no cats, simply performance exhaust. I’ve removed the Air Injection system. I have a miss on the right bank at idle that is frustrating it could be a weak spark on B4 but why it would only be on B4 is beyond me. All the obvious has long since been replaced plugs, wires, caps, injectors, crank sensor, flywheel sensor and the setup is correct I’ve been back to basics and set up the throttles correctly, yet the B bank misfire persists to torment my OCD. Car runs fine just not as smooth as I want or I have had and do have in other V12’s… I would like a picture of the rear coil wiring harness at the coil if anyone has such a thing. Mine has one space vacant on the terminal and some sort of fuseable link there. Looks like original crappy Lucas wiring but no real mention of it in the manual wiring diagrams.

Welcome to Jag-Lovers and thank you for the interest in my (now 7 years old) thread on this subject.

Coil on bank A has one more wire for the tach.
Some more discussion on the subject here:

here’s a picture
as said by sbobev, nothing wrong with the “missing” wire

Thank you very much for the prompt reply, it’s a question that’s been niggling at me for at least 10 years. My pursuit of the misfire goes on.

Regards,

Giles.

Giles Ashe
CEO
Wings Aviation
4000 Execuair Street
Orlando
Florida 32827
USA
Phone : ( 407) 344 0202
Fax: (407) 344 0203
www.WingsAviation.aero

One thing no one has mentioned is valvetrain; tight clearances on one or more valves will upset idle. Valve clearance closes with valve/seat wear on overhead cam engines with bucket followers. Also with dropped valve seats.
Another issue, although rare, would be base circle runout on the cam lobe; this causes the valve to open slightly when it shouldn’t. All you need to check clearances are feeler gauges and a bunch of spare time.
If you elect to check these things, valve timing should also be verified. Tools needed would be a dial indicator, the factory camshaft tool, and a degree wheel. Must verify TDC for any timing issues.
Disabling the oxygen sensors will smooth the idle by running on the fixed mapping and somewhat masks the diluted cylinder charge.
Another possibility would be tight valve stems due to carbon; used to see random misfire faults on Jeep engines because of this. Used to clean valve stems and ream guides .001 oversize to correct.

It’s been awhile and I will have to re-read the whole thread. If not here, within the other threads that I created back then, someone mentioned timing chain stretch and wear, in general, as contributing factors.

That was then, when I was still new to Jaguar’s and lived in imaginary world. Now, I have come to the realization that balancing a coin on its edge over the intake manifold may never happen, yet, the car puts a smile on my face every time I take her out of the garage.

:-))

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Agreed- they are fine machines and their foibles give them character.