[v12-engine] Idle and a series of unfortunate events

V12-Engine
1

Dear friends,

The last two weeks my1986 XJ V12 just started going downhill� I
have done something and can�t find what.
First the feedback monitor relay shorted (the diode inside) and for
an instant the relay that controls the supplementary air valve was
connected the wrong way� that was my doing�.
Replaced the relay but then the fuel pump was not working�.
Replaced the fuel pump, but old one turned out to be stuck and not
burned.
At the same time fixed some vacuum leaks that I had found.
But since then my idle went for a walk� It�s very low, extremely
rough, cyclic and nonexistent when cold, barely present when warm,
and absolutely nerve racking�
There is also some backfiring on the left side manifold.
It has a tendency to overheat on uphill cruise speeds, possibly
blocked radiator.
Also there is some pre-ignition happening although the car was
tuned four months ago.
Ever since I got the car the idle would fall very low after a long
drive and until now I thought that this was because of a stuck Aux
air valve, and even though it has a hot start sensor mounted on the
fuel rail hot starting was always difficult.
There is also a difference between how much gas I put in and how
much the trip computer says, about 5 to 8lt at a 45lt tank.
Otherwise the car is running just fine and three months ago drove
6,000 Km across Canada, Victoria BC to Montreal… a fabulous drive

The facts:

ECU replaced six months ago (6CU, DAC 6337)
All injectors are clicking and injector hoses are new,
Distributor was serviced a year ago; at least that�s what I�m told,
Auxiliary Air Valve is working, rebuild with a 74 degrees Celsius
thermostat bulb two months ago,
Throttle links adjusted according to the book.
Engine breather filter is clean.
Air filters are clean.

Opened the throttle potentiometer and has no excessive wear,
Vacuum advance system seems to work fine,
Do not know if the vacuum advance timer works but the supplementary
air valve works when 12v are applied.

When I disconnect the air temperature sensor idle comes back to
life,
When I push the roller microswitch on the throttle pot idle is good,
When I disconnect the vacuum-operated switch nearby idle is good,
When I cut off the vacuum from the left side fuel regulator, idle
is good, but there are signs of fuel leak in to the vacuum.
When I disconnect the jumper at the ECU, force the system in to
closed-loop, nothing happens,
Starter inhibit switch on the shift lever is not shorted and
switches on and off but when I manually push it, while being in
Park or Neutral and engine running, nothing happens,
When I disconnect the coolant temperature sensor things get worse,
as expected�

It’s been two weeks of strugle…
Thanks–
Aristides
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2

In reply to a message from Aristides sent Fri 18 Aug 2006:

Sorry to read of your troubles. I have an '86 as well and am
a wreck when it doesn’t run.

Two things occur to me – first the idle. Did you check the
throttle potentiometer under the bellcrank for proper
voltage at idle?

Second, you mentioned that you were told the distributor had
been overhauled, but that almost certainly means the
centrifugal advance is seized. That can cause overheating,
rough running, preignition, and maybe even backfiring.

Luck,
XJAson–
1986 XJ-S Coupe. 5.3L H.E. V-12
Northeast, United States
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3

In reply to a message from XJason sent Sat 19 Aug 2006:

XJason,
Thanks for the reply.
It’s funny how this car can effect your inner balance…
I will check the centrifugal advance but need to get a multimeter
to check the potentiometer.
I will keep yopu posted.
All the best,
Aristides–
Aristides, 1986 XJ V12, VDP
Montreal, QC, Canada
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4

In reply to a message from XJason sent Sat 19 Aug 2006:

What is the correct voltage at idle for the throttle potentiometer?
Is it .32V at idle and 5V and WOT. Does that vary at all? In other
words, are there any situatiuons where it should be more or less
than .32V?–
JagYouR, TonyJ, NL, Canada, 1988 XJ-S V12, 5.3L
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5

I’ve seen various “specs” bandied about, anywhere from .30 to .37.

Mine is at .35 at idle as I recall, and 4.8 at WOT…and I’ve “called it
good”

I’m not sure just how critical this is.

Doug Dwyer
Longview Washington USA
1995 XJR
1988 XJS V12 Coupe

What is the correct voltage at idle for the throttle potentiometer?
Is it .32V at idle and 5V and WOT. Does that vary at all? In other
words, are there any situatiuons where it should be more or less
than .32V?

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !From: “JagYouR” tony.joyce@freedom55financial.com

6

In reply to a message from Doug Dwyer sent Mon 21 Aug 2006:

Doug it is critical because I am not sure if my throttle switch
needs to be replaced. I thought I had the sluggish hesitation
pinpointed by fixing the throttle switch but when I performed a
repair the hesitation is still there. I followed the procedure
in ‘‘the book’’ and got .32V at idle but when I hit the gas from
stationary there is a hesitation and the RPMs drop by about 500
from 900 down to 400. If there is any strain on the engine such as
the A/C I am in danger of stalling. I thought the throttle switch
would solve that. The only question now remains is my throttle
switch up to par because even though I get .32V the resistance
coating is completely worn off one side. Any thoughts? Or, is this
worth taking about any further. Thanks.–
JagYouR, TonyJ, NL, Canada, 1988 XJ-S V12, 5.3L
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7

Equally important is checking for a smooth transition as you open the
throttle. With you meter hooked up is the voltage increase evenly as you
slowly open the throttle…or is it jumpy ?

Doug Dwyer
Longview Washington USA
1995 XJR
1988 XJS V12 Coupe

The only question now remains is my throttle
switch up to par because even though I get .32V the resistance
coating is completely worn off one side. Any thoughts? Or, is this
worth taking about any further. Thanks.

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !From: “JagYouR” tony.joyce@freedom55financial.com

8

JagYouR wrote:

What is the correct voltage at idle for the throttle potentiometer?
Is it .32V at idle and 5V and WOT. Does that vary at all?

You are supposed to adjust it to the .32V at idle. The other end is
whatever falls out, usually a bit less than 5V.

In other
words, are there any situatiuons where it should be more or less
than .32V?

No. If it’s not .32V, you need to fix it.

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9

Doug Dwyer wrote:

Mine is at .35 at idle as I recall, and 4.8 at WOT…and I’ve
“called it good”

I’m not sure just how critical this is.

It’s critical. It’s what tells the ECU that the engine is at idle so
it can proceed with idle mode instead of running mode. If the
setting is off in one direction, it will use running mode at idle –
which can result in an unstable idle and flunking emissions tests.
If it’s off in the other direction, you can get poor engine response
because you’re on light throttle and the ECU thinks you’re still at
idle.

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10

JagYouR wrote:

Doug it is critical because I am not sure if my throttle switch
needs to be replaced.

Easy enough to tell. With the VOM hooked up and the ign on, watch
the voltage as you move the throttle from idle to WOT and back
several times. The voltage should vary smoothly, no jerks, and
should always return to the same .32V at idle.

I followed the procedure
in ‘‘the book’’ and got .32V at idle but when I hit the gas from
stationary there is a hesitation and the RPMs drop by about 500
from 900 down to 400.

There are several possible causes of that problem. One is a throttle
pot that’s breaking up, so when you floor it you get a zillion fuel
injector pulses rather than a few, so the engine is severely
overfuelled. Another possibility is that the vacuum line to the ECU
in the boot has liquid or goo inside so the vacuum signal isn’t
getting through. There may be some issue in the fuelling that allows
the idle mode to work OK but the run mode does not – such as the
engine runs fine in open loop but not in closed loop. Maybe the
oxygen sensors are faulty. Finally, there’s some possibility of a
fuel flow problem, there’s enough fuel available for idle but once
you apply throttle it starts to starve.

If there is any strain on the engine such as
the A/C I am in danger of stalling. I thought the throttle switch
would solve that. The only question now remains is my throttle
switch up to par because even though I get .32V the resistance
coating is completely worn off one side. Any thoughts?

Sounds like a bad throttle pot. So, what does the voltage look like
as you move it slowly from idle to WOT?

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11

In reply to a message from Kirbert sent Sat 26 Aug 2006:

Thanks for being so thorough. If I hit the gas ever so lightly
(which is not likely when driving) it does start off at .32V and
consistently work its way up. On normal driving conditions I touch
the gas pedal, the volts jump to 1.04V and the RPMs drop by about
500 to 400 from 900. This is the only time it seems to do it. I
noticed when I had the TPS apart that one side of the resistance
coating is not only worn but it is completly gone.–
The original message included these comments:

Sounds like a bad throttle pot. So, what does the voltage look like
as you move it slowly from idle to WOT?

–
JagYouR, TonyJ, NL, Canada, 1988 XJ-S V12, 5.3L
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12

In reply to a message from Aristides sent Fri 18 Aug 2006:

Dear friends,

After some holidays, more search and study of Kirbert Palm’s book I
finally solved my problem !! and it was very simple…

First of all, because I was clearly having fuel starvation
problems, like intake manifold backfiring, I changed my fuel
filter, $12 from my local parts store, … $70 from Jaguar.

During all this time I was keep finding minor air leaks here and
there, which I was repairing, but never gave to much thought to
that fact. What actualy was happening though was that the ECU was
slowly getting higher vacuum, so the ECU’s fuel map was completely
out… so lean that the engine was almost not idling on P or N.
Then went to my ECU’s Feedback socket with my VOM and adjusted the
idle mixture according to the book.
The idle is smooth and perfect, no hesitation and no back firing.

Note that there was backfiring and slightly hesitating also when I
was driving around, as if that the fuel map was so out that the
oxygen sensors could not correct it 100%… something that made me
think that my ECU was faulty.

I also found an other way for double-checking the fuel map
adjustment.

I have installed in my car a Vacuum gauge, an instrument that I
find extremely useful.
During all my troubles the Vacuum was extremely low.
The adjustment on the baseline fuel map has a direct effect on the
engine’s vacuum.

When the engine is idling in P or N, I pulled out the red jumper in
the ECU, making it run on closed loop, meaning the oxygen sensors
correct the mixture to stochiometric.
That gives a certain Vacuum and RPM.
All I had to do was bring the same Vacuum and RPM in open loop by
adjusting the idle mixture, the red jumper connected.
It was at the same place as the VOM adjustment…
I also gave it one more click on the rich side to be sure.

I think that I still have to go to a gas analyser to see if what I
did is the right thing, but the car is now working fine.

Aristides–
The original message included these comments:

Replaced the fuel pump, but old one turned out to be stuck and not
burned.
At the same time fixed some vacuum leaks that I had found.
But since then my idle went for a walk� It�s very low, extremely
rough, cyclic and nonexistent when cold, barely present when warm,
and absolutely nerve racking�
There is also some backfiring on the left side manifold.
ECU replaced six months ago (6CU, DAC 6337)
All injectors are clicking and injector hoses are new,
Distributor was serviced a year ago; at least that�s what I�m told,
Auxiliary Air Valve is working, rebuild with a 74 degrees Celsius

–
Aristides, 1986 XJ V12, VDP
Montreal, QC, Canada
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13

In reply to a message from Aristides sent Fri 20 Oct 2006:

Congratulations on fixing your problem, and also on posting the
fix - the archives are very useful but ONLY if people post the
answers.

Happy motoring in your puuuuurfect car–
1985 XJS V12 HE TWR (Black Betty) - www.bryansplace.net
Gold Coast Qld, Australia
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