[v12-engine] Spark Plug Shenanigans

V12-Engine
1

Hi all,

Let me open by saying that I KNOW I AM IGNORANT.

There.

Okay, here goes. My 1976 XJ-S had this symptom of blowing pale smoke out of
one exhaust only, on heavy throttle + high revs, the right side, this was
worse after the car had been driven gently for a while; also the exhaust was
stuttery. It also smelt somewhat rich across all rev ranges.

Well, I reasoned that it might be because a spark plug was fouled, so that
unburnt fuel was going through the engine. To my mind this explained the
stutter and the richness and the “smoke.”

At any rate, I thought, it couldn’t hurt to pull out all the spark plugs and
replace them.

This I did. (I’ll leave out the panicked moment when one of them cross
threaded. I believe that was the previous mechanic’s fault, and I think it’s
okay now - but I am going to have to have helicoils and tapping tools and
glue and a priest on hand when I change the plugs again, I think. Boy oh
boy.)

Was my conjecture right or wrong? I don’t know! The smoke symptom is 100%
gone. It’s no longer embarassing to bury everyone else at the lights. On the
other hand, the plugs that came out all looked exactly as used spark plugs
should. Good clean engine, you would think.

Bad news though - the engine is not smooth, not nearly as smooth as it used
to be. Under light throttle, maintaining a given speed, there’s a noticeable
judder, a sort of pulsing in the power delivery.

Oddly though, giving a bit of pedal eliminates the symptom and the car
seems to have no less power than before.

The only other thing that springs to mind is that the new plugs, which I
gapped according to the Haynes manual at 0.025", or 0.64 mm, looked more
closely gapped than the plugs I pulled out. I don’t know if that’s relevant.

So, what cured the smoke? And what is causing the judder? Could I have
gapped the plugs too closely? Could that cause a sporadic failure to spark
at low revs?

As I said, I realise I have a lot to learn here so don’t be afraid to tell
me I am an IDIOT.

Don’t get me wrong, it’s not running like a dog or anything. It just feels
juddery.

Any suggestions appreciated.

Jason

2

I will take this opportunity to confess my own sins.
In the past I have gotten confused and not gotten all the plug wires
installed in the correct order. I have ten fingers and twelve wires so
it was easy to misplace a couple of wires.
In my mind I just knew I had it right but when all else failed
I reread the firing order, double checked my work and found a big oops.

John and Sue in West Texas-----Original Message-----
From: Jason D. Korke [mailto:chimpboy@ozemail.com.au]
Sent: Tuesday, May 23, 2000 9:07 AM
To: v12-engine@jag-lovers.org
Subject: [v12-engine] Spark Plug Shenanigans

Hi all,

Let me open by saying that I KNOW I AM IGNORANT.

There.

Okay, here goes. My 1976 XJ-S had this symptom of blowing pale smoke out of
one exhaust only, on heavy throttle + high revs, the right side, this was
worse after the car had been driven gently for a while; also the exhaust was
stuttery. It also smelt somewhat rich across all rev ranges.

Well, I reasoned that it might be because a spark plug was fouled, so that
unburnt fuel was going through the engine. To my mind this explained the
stutter and the richness and the “smoke.”

At any rate, I thought, it couldn’t hurt to pull out all the spark plugs and
replace them.

This I did. (I’ll leave out the panicked moment when one of them cross
threaded. I believe that was the previous mechanic’s fault, and I think it’s
okay now - but I am going to have to have helicoils and tapping tools and
glue and a priest on hand when I change the plugs again, I think. Boy oh
boy.)

Was my conjecture right or wrong? I don’t know! The smoke symptom is 100%
gone. It’s no longer embarassing to bury everyone else at the lights. On the
other hand, the plugs that came out all looked exactly as used spark plugs
should. Good clean engine, you would think.

Bad news though - the engine is not smooth, not nearly as smooth as it used
to be. Under light throttle, maintaining a given speed, there’s a noticeable
judder, a sort of pulsing in the power delivery.

Oddly though, giving a bit of pedal eliminates the symptom and the car
seems to have no less power than before.

The only other thing that springs to mind is that the new plugs, which I
gapped according to the Haynes manual at 0.025", or 0.64 mm, looked more
closely gapped than the plugs I pulled out. I don’t know if that’s relevant.

So, what cured the smoke? And what is causing the judder? Could I have
gapped the plugs too closely? Could that cause a sporadic failure to spark
at low revs?

As I said, I realise I have a lot to learn here so don’t be afraid to tell
me I am an IDIOT.

Don’t get me wrong, it’s not running like a dog or anything. It just feels
juddery.

Any suggestions appreciated.

Jason

3

Hi all,

Thanks for the replies on this one! It’s sure puzzling me.

I don’t think I could have the spark plug wires in the wrong order because I
replaced them one at a time. Plug out, plug in, wire on.

What’s weird, to my mind, is that all I did was replace the spark plugs.
Nothing else. It didn’t misfire before, now it’s all juddery.

And what makes it weirder is that the whole reason I changed the plugs was
to eliminate a big plume of white smoke that emerged from the right hand
exhaust on heavy acceleration - and in that regard, I succeeded. Fixed one
thing, buggered up another!

Anyway, any suggestions still welcome; I’ll keep fooling around with the
thing meanwhile.

Cheers all,

Jason

I will take this opportunity to confess my own sins.
In the past I have gotten confused and not gotten all the plug wires
installed in the correct order. I have ten fingers and twelve wires so
it was easy to misplace a couple of wires.
In my mind I just knew I had it right but when all else failed
I reread the firing order, double checked my work and found a big oops.

John and Sue in West Texas

-----Original Message-----
From: Jason D. Korke [mailto:@Jason_D_Korke1]

Hi all,

Okay, here goes. My 1976 XJ-S had this symptom of blowing pale smoke out
of
one exhaust only, on heavy throttle + high revs, the right side, this was
worse after the car had been driven gently for a while; also the exhaust
was
stuttery. It also smelt somewhat rich across all rev ranges.

Well, I reasoned that it might be because a spark plug was fouled, so
that
unburnt fuel was going through the engine. To my mind this explained the
stutter and the richness and the “smoke.”

At any rate, I thought, it couldn’t hurt to pull out all the spark plugs
and
replace them.

This I did. (I’ll leave out the panicked moment when one of them cross
threaded. I believe that was the previous mechanic’s fault, and I think
it’s
okay now - but I am going to have to have helicoils and tapping tools and
glue and a priest on hand when I change the plugs again, I think. Boy oh
boy.)

Was my conjecture right or wrong? I don’t know! The smoke symptom is 100%
gone. It’s no longer embarassing to bury everyone else at the lights. On
the
other hand, the plugs that came out all looked exactly as used spark plugs
should. Good clean engine, you would think.

Bad news though - the engine is not smooth, not nearly as smooth as it
used
to be. Under light throttle, maintaining a given speed, there’s a
noticeable
judder, a sort of pulsing in the power delivery.

Oddly though, giving a bit of pedal eliminates the symptom and the car
seems to have no less power than before.

The only other thing that springs to mind is that the new plugs, which I
gapped according to the Haynes manual at 0.025", or 0.64 mm, looked more
closely gapped than the plugs I pulled out. I don’t know if that’s
relevant.----- Original Message -----
From: Richardson, John T. jtrichardson@alturaenergy.com

So, what cured the smoke? And what is causing the judder? Could I have
gapped the plugs too closely? Could that cause a sporadic failure to spark
at low revs?

As I said, I realise I have a lot to learn here so don’t be afraid to tell
me I am an IDIOT.

Don’t get me wrong, it’s not running like a dog or anything. It just feels
juddery.

Any suggestions appreciated.

Jason

4

Jason,
Are you sure you didn’t bump one of the injector connectors off.

John Ashcroft.----- Original Message -----
From: “Jason D. Korke” chimpboy@ozemail.com.au
To: v12-engine@jag-lovers.org
Sent: Wednesday, May 24, 2000 12:42 PM
Subject: Re: [v12-engine] Spark Plug Shenanigans

Hi all,

Thanks for the replies on this one! It’s sure puzzling me.

I don’t think I could have the spark plug wires in the wrong order because
I
replaced them one at a time. Plug out, plug in, wire on.

What’s weird, to my mind, is that all I did was replace the spark plugs.
Nothing else. It didn’t misfire before, now it’s all juddery.

And what makes it weirder is that the whole reason I changed the plugs was
to eliminate a big plume of white smoke that emerged from the right hand
exhaust on heavy acceleration - and in that regard, I succeeded. Fixed one
thing, buggered up another!

Anyway, any suggestions still welcome; I’ll keep fooling around with the
thing meanwhile.

Cheers all,

Jason

----- Original Message -----
From: Richardson, John T. jtrichardson@alturaenergy.com

I will take this opportunity to confess my own sins.
In the past I have gotten confused and not gotten all the plug wires
installed in the correct order. I have ten fingers and twelve wires so
it was easy to misplace a couple of wires.
In my mind I just knew I had it right but when all else failed
I reread the firing order, double checked my work and found a big oops.

John and Sue in West Texas

-----Original Message-----
From: Jason D. Korke [mailto:chimpboy@ozemail.com.au]

Hi all,

Okay, here goes. My 1976 XJ-S had this symptom of blowing pale smoke out
of
one exhaust only, on heavy throttle + high revs, the right side, this
was

worse after the car had been driven gently for a while; also the exhaust
was
stuttery. It also smelt somewhat rich across all rev ranges.

Well, I reasoned that it might be because a spark plug was fouled, so
that
unburnt fuel was going through the engine. To my mind this explained the
stutter and the richness and the “smoke.”

At any rate, I thought, it couldn’t hurt to pull out all the spark plugs
and
replace them.

This I did. (I’ll leave out the panicked moment when one of them cross
threaded. I believe that was the previous mechanic’s fault, and I think
it’s
okay now - but I am going to have to have helicoils and tapping tools
and

glue and a priest on hand when I change the plugs again, I think. Boy oh
boy.)

Was my conjecture right or wrong? I don’t know! The smoke symptom is
100%

gone. It’s no longer embarassing to bury everyone else at the lights. On
the
other hand, the plugs that came out all looked exactly as used spark
plugs

should. Good clean engine, you would think.

Bad news though - the engine is not smooth, not nearly as smooth as it
used
to be. Under light throttle, maintaining a given speed, there’s a
noticeable
judder, a sort of pulsing in the power delivery.

Oddly though, giving a bit of pedal eliminates the symptom and the car
seems to have no less power than before.

The only other thing that springs to mind is that the new plugs, which I
gapped according to the Haynes manual at 0.025", or 0.64 mm, looked more
closely gapped than the plugs I pulled out. I don’t know if that’s
relevant.

So, what cured the smoke? And what is causing the judder? Could I have
gapped the plugs too closely? Could that cause a sporadic failure to
spark

at low revs?

As I said, I realise I have a lot to learn here so don’t be afraid to
tell

me I am an IDIOT.

Don’t get me wrong, it’s not running like a dog or anything. It just
feels

juddery.

Any suggestions appreciated.

Jason

5

Jason,
Are you sure you didn’t bump one of the injector connectors
off.

John Ashcroft.

I checked that, but I will check it again.

I have a feeling that rather than knocking one off, I may have disturbed it
and cracked a wire. All of my injector wires are pretty brittle.

Grrrr.

Thanks for the reply!

Jason----- Original Message -----
From: John Ashcroft deerashy@bigpond.com

6

Jason,

Try measuring the resistance of the spark plug wires. They should all
be between 5K (shortest) and 20K (longest)ohms.

See what happens to the resistance when you wiggle the wire around,
there may be a bad connection at one end where the wire is crimped into
the cap.

If this doesn’t help try putting the old wires back on.

At least then you’ll know if it is the plug leads or not !

Regards,

Steve____________________________________________________________
Do You Yahoo!?
Get your free @yahoo.co.uk address at http://mail.yahoo.co.uk
or your free @yahoo.ie address at http://mail.yahoo.ie

7

Try measuring the resistance of the spark plug wires. They should all
be between 5K (shortest) and 20K (longest)ohms.

See what happens to the resistance when you wiggle the wire around,
there may be a bad connection at one end where the wire is crimped into
the cap.

If this doesn’t help try putting the old wires back on.

At least then you’ll know if it is the plug leads or not !

Okely dokely. Just to recap: I changed my spark plugs and afterwards, the
engine is more juddery, far less smooth. It did fix the target problem,
though, which was unburnt fuel making its way through the system.

I have advanced my knowledge a little here.

Here’s what I’ve done in response to suggestions:

  1. checked that all leads were going to the right plugs, ie, confirmed
    firing order

  2. I liked Ashy’s suggestion of the injector connections, so, with the
    engine running, I just disconnected each injector in turn.

On most injectors, this resulted in an apparent decrease in rpm and greater
judderiness of the engine.

On a couple, the engine was definitely SMOOTHER with the injector
disconnected. I don’t know what to make of that.

I don’t think the disconnect-the-injector test is scientific enough though;
after a while I began to feel I was imagining a change in rpm, so, I am
going to try that 12V LED-in-the-injector-socket thing, as soon as I dig up
my box of LEDs.

I only have limited time to f*** around with this, though, as I am moving
interstate in three weeks and have lots of work to do in that time, so I may
have to take the car to a jag mechanic so he can cast a more experienced eye
over it.

I have decided to drive it to Brisbane, so I would like to fix this before I
go!

Thanks again for the suggestions,

Jason----- Original Message -----
From: Steve jagbuff@yahoo.co.uk

8

I have a feeling that rather than knocking one off, I may
have disturbed it
and cracked a wire. All of my injector wires are pretty brittle.

I quote Bywater’s Book:

“When a manufacturer specifies a certain make and type of spart plug,
fitment of an alternative, even though it may be catalogued as suitable, can
carry a slight risk of introducing niggling problems…”

“The writer (Bywater) once returned a set of plugs, of a well repected make
which had been used successfully in the same car before, to the manufacturer
for investigation after suffering persistent light throttle and idle
roughness which seemed to defy all attempts at diagnosis. Another set of
plugs cured the problem yet the manufacturer insisted that those returned
were perfect, and perhaps they would have been - in another engine”

It’s a long shot, Jason, but what this says is try a different set of plaugs
of a different make.

Craig

9

It’s a long shot, Jason, but what this says is try a different set of
plaugs
of a different make.

Okay, I’ll go buy some Champions.

Kidding.

I guess Bosch? The ones that are currently under suspicion are NGKs.

Thanks Dr Sawyers!

Jason----- Original Message -----
From: Craig Sawyers c.sawyers@tech-enterprise.co.uk

10

It’s a long shot, Jason, but what this says is try a different set of
plaugs
of a different make.

Okay, I’ll go buy some Champions.

Kidding.

Hate to say this but fit champions; not kidding. Yes I know familiarity
breads contempt, but… (Don Miles)> I guess Bosch? The ones that are currently under suspicion are NGKs.

Thanks Dr Sawyers!

Jason

11

Just to recap: I changed my spark plugs and afterwards,
the engine is more juddery, far less smooth. It did fix the target
problem, though, which was unburnt fuel making its way through the
system.

I hate to suggest this, but you might have fixed it. Before, you
probably had a bad wire, which was causing one cylinder to misfire.
This, in turn, dumped oxygen into the exhaust system. The oxygen
sensor picked it up, decided the engine was running lean, and
richened it up – providing a remarkably smooth idle despite misfire.
Now, all 12 are firing – so there’s no oxygen in the exhaust, and
the EFI system is leaning it out – which can make this engine idle
really poorly, sort of “stumbling”.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: “Jason D. Korke” chimpboy@ozemail.com.au

12

I guess Bosch? The ones that are currently under suspicion are NGKs.

Bang goes my theory! I’m using NGK’s with no bad effect, and they were
supplied by Jaguar in little bitty green Jag boxes.

Craig

13

Kirby, he said that he pulled the injector wires from two cylinders,
and the engine ran better.

Those two cylinders won’t fire, so the oxygen goes into the exhaust,
the sensor picks it up and richens the other 4 on that bank – which
causes them to idle smoother.

Is it possible that instead of having
spark plug wires misconnected, that he reversed two injector wires?
Would that cause the injectors to pulse at the wrong time, producing a
“juddery” engine?

Reversing two injector wires on the same bank makes no difference at
all – all six are wired together and fire at the same time. Even if
swapped across banks (or on the same bank on the pre-H.E., which has
three on each side wired together) probably still wouldn’t make a
noticeable difference.

You might try going through the adjustment procedure for the idle
mixture at the ECU. It might fix the problem, or it might provide
more data.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: Gary Oakley gdoakley@excite.com

14

Pre- HE, Aussie car, no oxygen sensors.

John Ashcroft----- Original Message -----
From: “Kirbert” palmk@nettally.com
To: v12-engine@jag-lovers.org
Sent: Thursday, May 25, 2000 12:04 PM
Subject: Re: [v12-engine] Spark Plug Shenanigans

From: Gary Oakley gdoakley@excite.com

Kirby, he said that he pulled the injector wires from two cylinders,
and the engine ran better.

Those two cylinders won’t fire, so the oxygen goes into the exhaust,
the sensor picks it up and richens the other 4 on that bank – which
causes them to idle smoother.

Is it possible that instead of having
spark plug wires misconnected, that he reversed two injector wires?
Would that cause the injectors to pulse at the wrong time, producing a
“juddery” engine?

Reversing two injector wires on the same bank makes no difference at
all – all six are wired together and fire at the same time. Even if
swapped across banks (or on the same bank on the pre-H.E., which has
three on each side wired together) probably still wouldn’t make a
noticeable difference.

You might try going through the adjustment procedure for the idle
mixture at the ECU. It might fix the problem, or it might provide
more data.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979

15

You might try going through the adjustment procedure for the idle
mixture at the ECU. It might fix the problem, or it might provide
more data.

The thing is just that I am reluctant to stuff around with anything too far
away from what I started with… all I did was swap spark plugs!

Quick question: if I didn’t gap them correctly, what would that do?

By the way - Craig - my spark plugs are NGK, but they didn’t come in Jag
boxes, they came in Nissan boxes! So I don’t think your theory is blown
right out yet.

This weekend, I will pull the plugs again, take a look at them, and possibly
replace them again. I may even put the old Bosch’s back in and see where
that takes me.

I’m certainly open to further suggestions, but… I swapped the plugs, the
car runs badly… someone on this list said once, “suspect your own last
repair FIRST”… so that’s where I am inclined to start.

Plugs, injectors - those are the things I boogered with!

My only real fear is that the thread on one spark plug socket is damaged; I
could tell on the last replacement. I’m very nervous about swapping that
plug - the potential for a major disaster is a bit acute, and if I stripped
it, I simply wouldn’t know how to fix it.

We shall see!

Thanks again for all the help here - I know that eventually I will complete
the restoration of this basket case of a car!

Cheers,

Jason----- Original Message -----
From: Kirbert palmk@nettally.com

16

I didn’t get your original message, but from what I gather, you changed
plugs and now the engine idles rough?

Since I missed some e-mails, you may have already covered all the
following…

One assumption, you have checked your plug wires, they’re in the correct
order, right?

O.K.

I’ve had similar problems with my cars, and it was due to poor electrical
connections on the injectors. Moving the plug wires around, bumping the
injector connections and harness caused all my grief.

I assume you know which bank is running rough? I could always figure it out
by placing my hand lightly over the exhaust pipe, it would “flutter” on the
offending bank’s pipe (of course if you have more than one injector missing,
both sides might flutter).

Step one: Inspect your injector harness on the missing side. The wires are
very brittle due to the wrong kind of insulation used (PVC) in such a hot
environment (replace them with Teflon, but that’s another thread), the
insulation cracks and falls right off! I’ve encountered bare wires that
shorted out to various brackets and such, won’t run worth a darn like that!

Step Two: On the bad side I could usually “wiggle” each connector and find
the offending injector (but oddly enough, not always). I carefully cleaned
the injector’s electrical connection with contact cleaner and coated with
dielectric grease and reassembled (the dielectric grease keeps the
connections dry and clean for good, looks like Vaseline but isn’t. Your auto
parts store will have it). Ultimately, doing this to all your injectors is
not a bad idea, as long as your harness is in good enough shape to take a
little movement, many aren’t.

Step Three: If cleaning doesn’t solve the miss (with the assumption that you
identified a specific injector) make sure the metal pins inside the wiring
harness end of the connector are not bent far enough apart that they make
poor contact with the injector. With a very small tool (screwdriver, etc)
you can “nudge” the pins back together, where they almost touch (they are
fragile so be careful).

I’ve don’t all of these to my XJ-S’s with good success, but if all else
fails, cuss a lot and throw things!

Mike Waldron

17

Those two cylinders won’t fire, so the oxygen goes into the exhaust,
the sensor picks it up and richens the other 4 on that bank – which
causes them to idle smoother.

Do the O2 sensors only control their own bank? IOW, left O2 sensor only
controls the left bank of injectors? No interrelationship at all?

Is there a failure mode (or wiring screwup) that would have the left O2
sensor trying to control the right bank, and vice versa?

Just wondering.

JohnOn Wed, 24 May 2000, Kirbert wrote:

18

Those two cylinders won’t fire, so the oxygen goes into the exhaust,
the sensor picks it up and richens the other 4 on that bank – which
causes them to idle smoother.

Do the O2 sensors only control their own bank? IOW, left O2 sensor
only controls the left bank of injectors? No interrelationship at
all?

That’s what Roger Bywater says. It’s not like I would actually know.
Of course, when oxygen sensors were introduced, the wiring was
changed to group all injectors on a bank rather than 3 per bank as
the pre-H.E.'s were wired. Not much reason to do that UNLESS you
were controlling the mixture by bank.

Is there a failure mode (or wiring screwup) that would have the left
O2 sensor trying to control the right bank, and vice versa?

I suppose a wiring screwup could do it, but you’d almost have to do
it deliberately.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: JGN jgn@li.net

19

On a couple, the engine was definitely SMOOTHER with the injector

disconnected. I don’t know what to make of that.

I’d suggest investigating the cylinder which is firing 360 degrees opposite
of the one which smoothed out the engine when you disconnected the
injector. Examine the firing order, and add (or subtract) 6 from the
cylinder which you noticed smoothed out the engine when you
disconnected it.

Why do I suggest this? Many years ago, I was on a ski trip with my folks.
After running the car through the carwash, the engine started vibrating.
I diagnosed a faulty distributor cap (carbon tracked); but removing an
ignition wire opposite the faulty cylinder resulted in a much more smoothly
running engine. Replacing the cap and the opposite ignition wire cured it.

Hope this helps…
Steve Wilke
'88 XJ-S H&E Conv.,57kmilesFrom: Jason D. Korke chimpboy@ozemail.com.au

20

Hi all,

Just an update. First, thanks to all who advised. Problem is still not
solved, but I have a bit more data.

To recap: I replaced my spark plugs, aiming to correct a tendency of my 1976
Aust spec XJ-S to send a plume of unburnt fuel out one exhaust pipe on heavy
accel.

The plume is gone, so that’s good, but now that it is evidently sparking on
all 12, I have a slight judderiness of the engine, kinda like low-level
bunnyhopping, at idle and on light throttle, even at high speed, 5th gear.
It’s a bit annoying when cruising.

Latest news is, I definitely have an injector pulse at all 12 injectors, and
I definitely have spark at all 12. Furthermore, temporary disconnection of
the injector plug at each cylinder results in a definite decrease in output,
so it’s pretty certain that I have power being produced at each cylinder.

I am leaning at this point towards the non-genuine spark plug explanation
offered by Craig Sawyers.

However, unless someone tells me I could be doing terrible damage by driving
the car in this condition, I am not going to attempt to replace these until
my interstate move is completed in a fortnight or so. The reason is that if
I have an intractable spark plug thread problem, which is possible, I simply
can’t have the car off the road in the interim. My arrival in Brisbane is
strictly non-negotiable.

When I get to Qld, I’ll replace the spark plugs, If that doesn’t fix things,
I guess I will be thinking that the car was tuned etc while sparking on only
11 cylinders, and that hence some retuning is required.

Anyway, thanks for the help so far. Any other suggestions are welcome also!

Best regards,

Jason Korke
1976 XJ-S, with Supra 5 speed gbox