[xj] rough running and misfiring S3--sometimes?

XJ
1

I’ve just acquired an inexpensive 84 XJ6 VDP to use as a daily driver, and
have an odd situation with how the car runs. When I first start the car in
the mornings (it’s cold here), it fires up immediately, and runs great…no
misfire, very quick and responsive. However, the car runs cold, it never
gets above 80 degrees. The weird thing is, the first time I shut the car
off, whether after a long run or a short run, it still starts right up, but
then misfires/stutters on acceleration. When I get up to speed and hold it
steady, even at 70mph, it’s fine, but idling it misfires, and punching it,
it misfires. Not horribly, but enough stutter to annoy the hell out of
me. Once I get it up to speed it’s very smooth.

The next time I start the car, it could be perfectly fine…there’s not
really a pattern to it, it just seems to happen sometimes. One consistent
thing is that the car definitely runs too cool, between 70 and 80, which
probably means a t-stat is stuck open. Would that cause a car to misfire?

Thanks as always.

Nick===================================================
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2

In reply to a message from Nick Pam Mitchell sent Wed 11 Dec 2002:

Sounds like a stuck or missing thermostat, and the ECU is not
sending the right pulse to the injectors.Put in the correct 190
degree thermostat and that should fix the problem.Also you mifgt
check the water temp. sensor on the water rail.–
Bruce Kimball
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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3

Nick: the cool running is something to be sorted out by replacement of the
thermostat; if you do that you will lilely find that the performance will
smooth out. But you may have, in addition, a faulty coolant temperature
sensor which is telling the ECU that the car is at full operating
temperature when it is not. I once had exactly the same symptoms in my 1984
Sovereign; the probklem was immediately solved by a change of thermostat.
Gregory----- Original Message -----
From: “Nick & Pam Mitchell” musnick@erinet.com
To: xj@jag-lovers.org
Sent: Tuesday, December 10, 2002 8:40 PM
Subject: [xj] rough running and misfiring S3–sometimes?

: I’ve just acquired an inexpensive 84 XJ6 VDP to use as a daily driver, and
: have an odd situation with how the car runs. When I first start the car
in
: the mornings (it’s cold here), it fires up immediately, and runs
great…no
: misfire, very quick and responsive. However, the car runs cold, it never
: gets above 80 degrees. The weird thing is, the first time I shut the car
: off, whether after a long run or a short run, it still starts right up,
but
: then misfires/stutters on acceleration. When I get up to speed and hold
it
: steady, even at 70mph, it’s fine, but idling it misfires, and punching it,
: it misfires. Not horribly, but enough stutter to annoy the hell out of
: me. Once I get it up to speed it’s very smooth.

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4

Yep, T-stat, I’m convinced that’s what it is. The car is definitely
running too cool, the heat doesn’t get as hot as it should, and the gauge
shows no erratic behavior. I’ll let you guys know if this solves the
problem. I’m always afraid to try to get those nuts twisted off when
changing a T stat, I’ve had one of them break off on me in the past and as
you all know that is very BAD. Hopefully this one won’t cause any problems.

Does anyone know, can a proper replacement t-stat be found at any common
parts store? It’s been a while since I changed a S3 t-stat?

Nick

At 11:52 AM 12/11/2002, Dr Gregory Andrachuk wrote:>Nick: the cool running is something to be sorted out by replacement of the

thermostat; if you do that you will lilely find that the performance will
smooth out. But you may have, in addition, a faulty coolant temperature
sensor which is telling the ECU that the car is at full operating
temperature when it is not. I once had exactly the same symptoms in my 1984
Sovereign; the probklem was immediately solved by a change of thermostat.
Gregory

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5

In reply to a message from Nick Pam Mitchell sent Wed 11 Dec 2002:

You can find a thermostat at a parts store, but I wouldn’t. You
should use the Waxstat from everything I’ve read here over the
years. All the usual suppliers have them, and you need to replace
the gaskets anyway, and I doubt those are available from a parts
store.

Cheers–
The original message included these comments:

Does anyone know, can a proper replacement t-stat be found at any common
parts store? It’s been a while since I changed a S3 t-stat?
Nick

–
Larry Karpman '87 SIII VDP (US), '79 SIII XJ6 (US)
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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6

Update on my 1984 XJ6 VDP running cold and 'stuttering". I just put in a
new, correct 190 degree t-stat, and it instantly fixed the problem with the
car running cold–it used to almost never get over 80 degrees, now it runs
right smack at the center of the gauge now, between 90-92. The heat is now
as hot as it can be. Problem solved, right? Well, those 2 problems are
solved…but the car still idles and runs very rough once the car gets up
to operating temperature.

Again the symptoms are:

Starting the car in the morning, even in very cold temps, the car starts
immediately, and idles smoothly. The car runs great for about 10 minutes,
until it gets up over 70-80 degrees, then the stuttering starts…it feels
like it’s either being starved of fuel, or the ignition is misfiring. It’s
not misfiring on a plug, that has a more steady, consistently uneven feel
to it when that happens to cars…this is random, erratic miss that feels
more electronic or fuel related. Sitting at a stop light, the tack will
try to stay at around 800 rpm, but it will just randomly drop to 400 or
less, very quickly and randomly. Acceleration is stuttered, but once you
finally get up to 70mph or so, it will hold that speed pretty evenly and
the misfire or stutter is harder to detect.

I’m going to read through the ‘rough idle’ archive, but does this sound
very specific to anyone, the ECU maybe? Or a sensor of some sort? Oxygen
sensor?

Thanks!

Nick Mitchell

66 Mark X RHD
72 XJ6 S1 27k original miles
74 XJ12L RHD
84 XJ6 VDP>Nick & Pam Mitchell wrote:

> One consistent > thing is that the car definitely runs too cool, between 70 and 80, which > probably means a t-stat is stuck open. Would that cause a car to misfire?

Not directly, Nick. But the ECU may be confused by the sensors in
certain phases during the heat-up process. Which is of course prolonged
and altered by a cool running engine. Reputedly the ECU is not
continuous but must select between 128 reference points, and, as I say
might be confused.

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7

Nick: this is not the “rough idle” on the 4.2 that we talk about. That is
more aptly described as an uneven idled, a sort of tremor which disappears
totally at anything above idle speed. Certainly nothing like a 400 rpm
drop. You have something else amiss there, perhaps a vacuum leak?
Gregory----- Original Message -----
From: “Nick & Pam Mitchell” musnick@erinet.com
To: xj@jag-lovers.org
Sent: Thursday, December 19, 2002 5:54 PM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: Update on my 1984 XJ6 VDP running cold and 'stuttering". I just put in a
: new, correct 190 degree t-stat, and it instantly fixed the problem with
the
: car running cold–it used to almost never get over 80 degrees, now it runs
: right smack at the center of the gauge now, between 90-92. The heat is
now
: as hot as it can be. Problem solved, right? Well, those 2 problems are
: solved…but the car still idles and runs very rough once the car gets up
: to operating temperature.

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8

Having just read the rough idle list, you’re right, this is not the same
thing. I can’t find any vacuum leaks, all of the hoses and connectors on
this engine look immaculate, all of the rubber is in good shape and after
spending a good 20 minutes trying to find a loose hose, cracked T
connector, etc, I don’t see one (that doesn’t mean there isn’t one
somewhere). If there was a vacuum leak, would it pretty much affect the
car all of the time, instead of just after it warms up, and even then not
every time the car is driven? It does seem to be happening more often in
the last few days, where as a month ago it would only start doing it after
a long run of an hour or more, or after a 15-20 minute run, shutting the
car off for a while, then restarting it.

There are so many things that seem possible here, I’m just hoping to get
opinions on the most likely?

Nick

At 09:55 PM 12/19/2002, Dr Gregory Andrachuk wrote:>Nick: this is not the “rough idle” on the 4.2 that we talk about. That is

more aptly described as an uneven idled, a sort of tremor which disappears
totally at anything above idle speed. Certainly nothing like a 400 rpm
drop. You have something else amiss there, perhaps a vacuum leak?
Gregory
----- Original Message -----
From: “Nick & Pam Mitchell” <@Nick_Pam_Mitchell>
To: xj@jag-lovers.org
Sent: Thursday, December 19, 2002 5:54 PM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: Update on my 1984 XJ6 VDP running cold and 'stuttering". I just put in a
: new, correct 190 degree t-stat, and it instantly fixed the problem with
the
: car running cold–it used to almost never get over 80 degrees, now it runs
: right smack at the center of the gauge now, between 90-92. The heat is
now
: as hot as it can be. Problem solved, right? Well, those 2 problems are
: solved…but the car still idles and runs very rough once the car gets up
: to operating temperature.

===================================================
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9

Nick: have you changed your coil?
Gregory----- Original Message -----
From: “Nick & Pam Mitchell” musnick@erinet.com
To: “Dr Gregory Andrachuk” <@Dr_Gregory_Andrachuk>
Cc: xj@jag-lovers.org
Sent: Thursday, December 19, 2002 7:02 PM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: Having just read the rough idle list, you’re right, this is not the same
: thing. I can’t find any vacuum leaks, all of the hoses and connectors on
: this engine look immaculate, all of the rubber is in good shape and after
: spending a good 20 minutes trying to find a loose hose, cracked T
: connector, etc, I don’t see one (that doesn’t mean there isn’t one
: somewhere). If there was a vacuum leak, would it pretty much affect the
: car all of the time, instead of just after it warms up, and even then not
: every time the car is driven? It does seem to be happening more often in
: the last few days, where as a month ago it would only start doing it after
: a long run of an hour or more, or after a 15-20 minute run, shutting the
: car off for a while, then restarting it.
:

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10

On the contrary: the coil can indeed go bad and it produces very erratic
problems such as you are experiencing; the problems happen more frequently
when hot rather than cold. If you have any oil leakage at the main
connection of the coil, it needs to be changed for sure. Do not get the oil
on your hands (PCBs). The correct coil for the car is DAC3001 (Ducellier).
Gregory----- Original Message -----
From: “Nick & Pam Mitchell” musnick@erinet.com
To: “Dr Gregory Andrachuk” <@Dr_Gregory_Andrachuk>
Sent: Thursday, December 19, 2002 7:20 PM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: Not yet, isn’t the coil one of the least likely? From what I’ve read and
: from experience it’s pretty rare for the coil to go bad? That would be
: fairly good news if that was it, it’s easy to get to!
:
:

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11

At 10:25 PM 12/19/2002, you wrote:

On the contrary: the coil can indeed go bad and it produces very erratic
problems such as you are experiencing; the problems happen more frequently
when hot rather than cold. If you have any oil leakage at the main
connection of the coil, it needs to be changed for sure. Do not get the oil
on your hands (PCBs). The correct coil for the car is DAC3001 (Ducellier).
Gregory

I’m sold. I’ll give it a shot-it does make sense since this happens once
the car gets hot. I wish I had a good coil from another car to test it
here, but the other cars I have may have very different coils since the
nearest car is a 74 XJ12 S2, would that one be the same? Heck for $30
U.S. I’ll just order one!

I’ll let you know if that solves it, thanks as always Gregory.

Nick>Subject: Re: [xj] rough running and misfiring S3–sometimes?

: Not yet, isn’t the coil one of the least likely? From what I’ve read and
: from experience it’s pretty rare for the coil to go bad? That would be
: fairly good news if that was it, it’s easy to get to!
:
:

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12

In reply to a message from Nick Pam Mitchell sent Fri 20 Dec 2002:

Nick while you are waiting for your new coil to arrive, you might
want to check the connection on the coolant temperature sensor, or
add a CTS to your coil order.

My 80 4.2 SIII has always started easily when cold and idles at 750-
800. Lately it had been very difficult to start when warm and once
started would backfire and run rough. Apparently, the CTS was
malfunctioning and was telling the system the car was cold when in
fact it was warm. The additional fuel being asked for by the
ECU was not needed with flooding the result.
Just another thought.

Bruce–
1980 XJ6 Brazila Brown
St. Helens, OR, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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13

And a good thought, too.
Gregory----- Original Message -----
From: brucehugo@hotmail.com
To: xj@jag-lovers.org
Sent: Thursday, December 19, 2002 7:54 PM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: In reply to a message from Nick Pam Mitchell sent Fri 20 Dec 2002:
:
: Nick while you are waiting for your new coil to arrive, you might
: want to check the connection on the coolant temperature sensor, or
: add a CTS to your coil order.
:
: My 80 4.2 SIII has always started easily when cold and idles at 750-
: 800. Lately it had been very difficult to start when warm and once
: started would backfire and run rough. Apparently, the CTS was
: malfunctioning and was telling the system the car was cold when in
: fact it was warm. The additional fuel being asked for by the
: ECU was not needed with flooding the result.
: Just another thought.
:
: Bruce

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14

Nick & Pam Mitchell wrote:

Update on my 1984 XJ6 VDP running cold and 'stuttering". I just put in a
new, correct 190 degree t-stat, and it instantly fixed the problem with the
car running cold–it used to almost never get over 80 degrees, now it runs
right smack at the center of the gauge now, between 90-92. The heat is now
as hot as it can be. Problem solved, right? Well, those 2 problems are
solved…but the car still idles and runs very rough once the car gets up
to operating temperature.

> Again the symptoms are: > > Starting the car in the morning, even in very cold temps, the car starts > immediately, and idles smoothly. The car runs great for about 10 minutes, > until it gets up over 70-80 degrees, then the stuttering starts...it feels > like it's either being starved of fuel, or the ignition is misfiring. It's > not misfiring on a plug, that has a more steady, consistently uneven feel > to it when that happens to cars...this is random, erratic miss that feels > more electronic or fuel related. Sitting at a stop light, the tack will > try to stay at around 800 rpm, but it will just randomly drop to 400 or > less, very quickly and randomly. Acceleration is stuttered, but once you > finally get up to 70mph or so, it will hold that speed pretty evenly and > the misfire or stutter is harder to detect.

I’m going to read through the ‘rough idle’ archive, but does this sound
very specific to anyone, the ECU maybe? Or a sensor of some sort? Oxygen
sensor?

The latter is intended to interfere with fuelling, Nick, and is
definitely a suspect. Disconnect it and observe difference. You may want
to readjust CO with the O2 sensor removed, and replace the sensor if
indications warrant it.

ECU is basically temp insensitive and is little likely to cause the
described behaviour. But an overly temp sensitive ign amplifier is not
out of the question…

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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15

At 10:54 PM 12/19/2002, you wrote:

In reply to a message from Nick Pam Mitchell sent Fri 20 Dec 2002:

Nick while you are waiting for your new coil to arrive, you might
want to check the connection on the coolant temperature sensor, or
add a CTS to your coil order.

Is this different than the sensor that tells the temperature gauge the temp
of the coolant? The reason I ask is, it appears to be working fine (the
gauge) because when I installed the new t-stat, the temperature now is
right at 90 as it should be, and heat is much warmer. Is the CTS a
different one that interacts with the other electronics?

Nick===================================================
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16

Nick: the coolant temperature sensor is an essential part of the running
system; it is entirely different from the temperature sender for the gauge.
The CTS gives the computer constant informations aout the temperature of
the engine so that the fuel “mixture” can be varied accordingly. A typical
malfunction is for the sensor to report that the engine is fully warmed up
when it is not. The result in this case is a “lean” mixture (actually
briefer spray pulses from the injectors) and that means hesitation and even
stalling until the engine really does reach that 70 degree magic mark, when
magically it all smooths out beause the mixture now matches the engine temp.
This is a most important component, in fact.
In your case, you get hesitation even when the engine is warm; if the
CTS is involved, it may be reporting a cold engine, and the result would be
overfueling, rather than underfueling. If you do not have smoking or a
typical overfueling “choking” symptom, then this is not likely to be the
cause, and again I would suggest that you investigate the coil.
Sometimes a faulty thermostat, which does not allow the engine to warm
fully, is combined with a CTS that is slightly “off”. The result of this
situation is the sort of warm hesitation and lack of power. I had this
precise situation in my 84 Sovereign with a sudden thermostat failure. One
day the engine just did not get above 70 degrees; it ran normally during the
warm-up period, but beyond that, was not satisfactory. The solution here was
easy; I replaced the thermostat and the symptom diappeared instantly. Your
car is running at 90 degrees so this is not the problem.
Gregory----- Original Message -----
From: “Nick & Pam Mitchell” musnick@erinet.com
To: xj@jag-lovers.org
Sent: Friday, December 20, 2002 8:01 AM
Subject: Re: [xj] rough running and misfiring S3–sometimes?

: At 10:54 PM 12/19/2002, you wrote:
:
: >In reply to a message from Nick Pam Mitchell sent Fri 20 Dec 2002:
: >
: >Nick while you are waiting for your new coil to arrive, you might
: >want to check the connection on the coolant temperature sensor, or
: >add a CTS to your coil order.
:
:
: Is this different than the sensor that tells the temperature gauge the
temp
: of the coolant? The reason I ask is, it appears to be working fine (the
: gauge) because when I installed the new t-stat, the temperature now is
: right at 90 as it should be, and heat is much warmer. Is the CTS a
: different one that interacts with the other electronics?

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17

In reply to a message from Dr Gregory Andrachuk sent Fri 20 Dec 2002:

Nick the coolant temperature sensor is the third sensor from the
front on the water rail. The first sensor from the front is the
Thermotime sensor which triggers the cold start injector. The
middle sensor is the one that sends the signal to your dash
temperature guage. Dr. A is absolutely correct in his analysis.
If your car is behaving as if a ‘choke’ were on, your problem lies
with the cooloant sensor and/or the Oxygen sensor (located under
the exhaust manifold). These two sensors have a great deal to say
about how much fuel is shot through the injectors. Too much fuel
and you have the ‘‘choke’’ on; too little and you are starving the
engine.–
1980 XJ6 Brazila Brown
St. Helens, OR, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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18

I had a long running problem of this sort some years ago, and it turned out
to be the coil. Symptoms were:
Below an indicated 70degC very little power available (i.e. no
acceleration) above 3000rpm. Eventually worsened to very poor at starting
when cold. I went through all the usual things like AAV, Temp sensor,
Thermotime switch, clogged injectors, dodgy air filters, poor connection to
cold start injector etc.

I’m still unsure why the faulty coil should have caused a well-defined
effect at 70deg. This would imply some step function from the ECU making
life easier for the coil. Perhaps a weaker mixture is easier to ignite? -
and it would be weaker as the temp sender tells the ecu the engine has
warmed up and thus leans it out. But certainly lack of power at highish
revs is consistent with the coil: bigger bangs more frequently are tough
for a leaky coil or one with shorted turns.

Chris===================================================
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19

I just put in a new coil, same problem. I only took it for a short run,
and the problem didn’t seem quite as bad, but it’s still ‘stuttering’.

Today was very typical (before putting in the new coil). Started the car
after it had been sitting in the cold all night, it started right up, and
ran great for about 10 minutes, then the ‘skipping’ or ‘stuttering’ begins
on acceleration. Once I get up to 70 (or any speed), it will hold it
pretty well and not misfire, but any pressure on the gas and the stuttering
begins. If it wasn’t for the fact that this only happens once the car is
warm, I would bet on it being fuel related, but because it only does it
when the car is hot, I’m still thinking it’s electronics.

So, what’s next, coolant temp sensor? And, to re-ask a previous question,
is CTS the same one that the coolant gauge in the cockpit relies
on? Because if it is, I doubt it’s that sensor, becuase the gauge worked
perfectly, showing the newly achieved 90 degrees when I put in the new t-stat.

Thoughts?

Nick

At 10:25 PM 12/19/2002, Dr Gregory Andrachuk wrote:>On the contrary: the coil can indeed go bad and it produces very erratic

problems such as you are experiencing; the problems happen more frequently
when hot rather than cold. If you have any oil leakage at the main
connection of the coil, it needs to be changed for sure. Do not get the oil
on your hands (PCBs). The correct coil for the car is DAC3001 (Ducellier).
Gregory

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20

Sorry guys I just saw that both Bruce and Gregory answered my question
about whether the CTS was different than the temperature gauge sensor. I’m
betting that it is the CTS after all, that is what I’m going to try
next. After that, the dreaded O2 sensor.

Thanks as always

Nick

At 12:17 PM 12/20/2002, brucehugo@hotmail.com wrote:>In reply to a message from Dr Gregory Andrachuk sent Fri 20 Dec 2002:

Nick the coolant temperature sensor is the third sensor from the
front on the water rail. The first sensor from the front is the
Thermotime sensor which triggers the cold start injector. The
middle sensor is the one that sends the signal to your dash
temperature guage. Dr. A is absolutely correct in his analysis.
If your car is behaving as if a ‘choke’ were on, your problem lies
with the cooloant sensor and/or the Oxygen sensor (located under
the exhaust manifold). These two sensors have a great deal to say
about how much fuel is shot through the injectors. Too much fuel
and you have the ‘‘choke’’ on; too little and you are starving the
engine.

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: Jag-lovers Forums - Jag-lovers

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