my '83 6 III runs like a top-----but on days above 28 C
it misfires after about 2300 rpm . until about 2600 rpm.
Higher than that and it is smooth again.
You can see the Tach ‘‘kick’’ down a couple of marks each
time until say 2700 rpm.
In the fall (Oct–Nov) this is not happening.
Is that the ignition module acting up?–
wpw
ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from wrigs sent Wed 6 Jul 2016:
As guesses go, the ignition module…or ignition coil…are
good candidates. Both are known to give trouble after
absorbing lots of heat. Some report problems just absorbing
ambient heat…never mind engine bay heat
I’d say it could be a number of things, including plugs, wires, ignition
amplifier, coil, distributor cap and rotor.
I would start by doing the things that are free (except for the time to
do them) and first remove the plugs to make sure they are correct ones and
that they are properly gapped. Then do a compression check while the plugs
are out. No better time to do it. Then take a careful look at the spark
plug wires, distributor cap and rotor. Replace anything that looks
questionable. One way to check spark plug wires is to run the car at night
while it is very dark and look into the engine bay for any arcing and
sparking due to failed wire insulation.
The GM module inside the ignition amplifier is easily and inexpensively
replaced. I have had a few of them fail on me (it’s the same module in my
1990 V12 VdP). The coil may also be the culprit, but I would probably
change that last and only if the other items don’t fix the problem.
It’s best to do one thing at a time, that way you will know what fixed the
problem. Keep any good parts removed as known good spares just in case.
I have the luxury of having two XJ6s (and 2 V12 Jaguars) so sometimes to
troubleshoot problems like this I swap parts between cars to see if the
problem migrates to the other car. If it does, then I know that I have
found the culprit. You may not have that situation, but if you have
someone nearby with a Series III who is willing to help out, that is also
possible. I have swapped known good parts from one of my Jaguars to help
out other local Jaguar enthusiasts when they had problems with their
Jaguars. Perhaps someone nearby can help with a coil or ignition
amplifier.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA
P.M.Novak7@gmail.com-----Original Message-----
From: owner-xj@jag-lovers.org [mailto:owner-xj@jag-lovers.org] On Behalf Of
wrigs
Sent: Wednesday, July 06, 2016 8:05 AM
To: xj@jag-lovers.org
Subject: [xj] missfire in hot weather
my '83 6 III runs like a top-----but on days above 28 C
it misfires after about 2300 rpm . until about 2600 rpm.
Higher than that and it is smooth again.
You can see the Tach ‘‘kick’’ down a couple of marks each time until say
2700 rpm.
In the fall (Oct–Nov) this is not happening.
Is that the ignition module acting up?
my '83 6 III runs like a top-----but on days above 28 C
it misfires after about 2300 rpm . until about 2600 rpm.
Higher than that and it is smooth again.
You can see the Tach ‘‘kick’’ down a couple of marks each
time until say 2700 rpm.
In the fall (Oct–Nov) this is not happening.
Is that the ignition module acting up?
The tacho is driven by the ignition, sir - so your suspicion is not
unwarranted…
With tacho movement unrelated to change of engine rpms, ignition is the
prime suspect - but it is important to carefully notice that the tacho’s
reaction IS different from actual rpms. The ign amp function is to ‘make’
and ‘break’ the coil’s ground connection - and ditto for the tacho. If the
amp ‘stutters’ so will ignition, so will the tacho - and so will the
engine…
That this happens at a specific rev range is somewhat unusual for an ign amp
failure - such ‘flat spots’ may have variety of causes, and remedies, as
Dough and Paul implies…
Franmk
xj6 85 Sov Europe (UK/NZ)-----Original Message-----
From: wrigs
Sent: Thursday, July 07, 2016 3:04 AM
To: xj@jag-lovers.org
Subject: [xj] missfire in hot weather
In reply to a message from Doug Dwyer sent Thu 7 Jul 2016:
Agree on two levels:
The tach is a great clue
on ignition misfires.
Heat, a major enemy of all things
electronic. Some say it’s munt on the
water manifold is good. Were it mine,
I’d choose a spot in the airflow
on the body tin. And perhaps a
larger alloy heat sink.
Carl–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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Good day Sir. I finally got into New Forum. Regarding a stuttering 4.2 L engine. I tried most of the possibilities that were suggested. New AC delco 1906 bit in Amp. no change. fresh coil. still jumping tach between 2100 to 2500 rpm. now I am preparing to swap the Air flow meter with one from a previous 84 xj6. perhaps the injector pulses from the ECU are messed up. This is definitely a temperature sensitive issue. first thing in day you can drive for 45 min at any revs between 2000 to 2500 and it is rock solid. hums beautifully. then later in day not too long after getting going its terrible after 2000 rpm up to 2500 . the revs just kick and swing about like mad. .up and down like a Metronome. but hold throttle up over 2600 and peace again up as far as I dare say 160 kmph. I put the car way in the fall and this spring the issue is more pronounced. could the Oxygen (lambda) sensor be causing the injectors to go strange I find it hard to say im the problem is ignition of fuel supply. thank you
**
To make it clear, Wrigs; is it the engine rpms that oscillates, or just the tacho…?
To be subtle; there is a certain ‘slack’ in the gearbox converter between the engine is driving the wheels and the wheels driving the engine as the engine lose power. However, power loss is detectable, while an ign amp fault may just affect the tacho - though nominally may also influence injection, causing loss of engine power.
There is no particular reason for an ign amp to fail only in a specific (2000 - 2500) rpm band, working OK above and below - but electronics are quirky, and the tacho may be more sensitive to amp errors than the rest of the system. Nor that fuelling should be affected in this way for any reason whatever…
The AFM may of course have a flatspot, but is not heat sensitive - but a flat spot will be evident at a specific flap position, and will in itself not influence tacho readings…
As for the Lambda system; it doesn’t work this way - but is easily eliminated by simply disconnecting the Oxygen sensor. The engine will work perfectly without it…
**
The strange thing is if it only misbehaves in the 2000 - 2500 rpm band, Paul - a failing ign amp due to heat should misbehave at all revs…more or less…
I thought the same. However, since it is easy to replace, I will put a fresh module in the amp and a see what the result is.
Thanks so much. still scratching head on this issue. Have had several years of driving until last year when this sputtering began.
Maybe your wiring to your injectors is failing some of them old wires get brittle because of the heat where they are tucked into the engine. Could be anything. Probably something simple doubt if it’s module like others have mentioned. That would not cause it at 2500 rpms. Sounds like a fuel problem.
