I have a 1977 V12 Jaguar and have a weak mixture problem.
The car runs, just, on light throttle but as soon as you
give it gas it coughs back. I have cleaned the injectors
checked the fuel pressure… However, I checked the voltage
at the injectors … Its 0.3v, now I thought it should be
about 2.5v, am I right? What could be causing this?
Injection Amp? I was told they ‘never go wrong’… ECU?
Can anyone out there help, please!–
Ian427
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Depending on the model, many voltage testers need to be zeroed manually. At
that low end of the scale, I’d want to make sure mine was working properly.
I have a 1977 V12 Jaguar and have a weak mixture problem.
The car runs, just, on light throttle but as soon as you
give it gas it coughs back. I have cleaned the injectors
checked the fuel pressure… However, I checked the voltage
at the injectors … Its 0.3v, now I thought it should be
about 2.5v, am I right?
What were you using to measure? About the only way I know to look at
injector voltage is with an oscilloscope. Also, you have to be sure you have
the probe on the right terminal of the injector since one side will be low.
What could be causing this?
Injection Amp? I was told they ‘never go wrong’… ECU?
I fought with a “spitting” problem like that for years. While I can’t put my
finger on exactly what “fixed” it, by gradually fixing or replacing nearly
everything it’s finally under control. I changed out the power amp with
another used one but as nearly as I could tell that didn’t help. I also sent
the ECU to AJ6 Engineering and had it refurbished/replaced and fitted with
an off-idle fueling pot.
Couple things I’d recommend. One, have your injectors cleaned and tested.
Put in a fuel pressure test port so you can be sure the rail pressure is
right. Also, run a tank full of fuel with a good valve and cylinder carbon
build-up additive.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
5
Ian427 wrote:
I have a 1977 V12 Jaguar and have a weak mixture problem.
The car runs, just, on light throttle but as soon as you
give it gas it coughs back. I have cleaned the injectors
checked the fuel pressure… However, I checked the voltage
at the injectors … Its 0.3v, now I thought it should be
about 2.5v, am I right?
Gee, I think it’s supposed to be 12V! Just exactly how are you
measuring it?
I’m more familiar with the later Digital P. On those, 12V is fed to
the injectors. The other terminal on each injector is grounded
through the ECU, either through or bypassing the resistor pack.
Hence, if you measure from either injector terminal to ground with
the ignition on and the engine not running, you should see 12V.
Measuring across the terminals, you should see 0V.
Your car presumably has the D Jetronic, but I’ve been led to believe
that the power to the injectors is largely similar other than the
injectors are grouped differently than they are on the Digital P.
In reply to a message from Kirbert sent Sun 29 Aug 2010:
Thanks Ed and Kirbert,
I put my multi-meter on a micro-volt setting and measured
across the terminals on the injector. I did not want to
measure to ground for fear of popping something in the
ECU… :-)… I will try that if its safe to do so!–
Ian427
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// please trim quoted text to context only
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
7
Ian427 wrote:
I put my multi-meter on a micro-volt setting and measured
across the terminals on the injector. I did not want to
measure to ground for fear of popping something in the
ECU… :-)… I will try that if its safe to do so!
Most things that operate at 12V can’t be harmed by connecting 12V to
them. I keep hearing about people worried about burning up ignition
modules or whatnot, wonder where they get these ideas.
There is one notable exception, though, and that’s the injectors
themselves. Connected to 12V for more than a few seconds will burn
them up. They are intended to only see 12V for thosandths of a
second at a time, and be cooled by fuel while they’re at it. But the
only way you’re gonna apply 12V to an injector is by grounding it.
You’re not going to accomplish it by using a VOM to measure voltages.
He’s probably measuring across the injector while connected. It will be different when running.
Geoff Green
I have a 1977 V12 Jaguar and have a weak mixture
problem.
The car runs, just, on light throttle but as soon as
you
give it gas it coughs back. I have cleaned the
injectors
checked the fuel pressure… However, I checked the
voltage
at the injectors … Its 0.3v, now I thought it should
be
about 2.5v, am I right?
Gee, I think it’s supposed to be 12V! Just exactly
how are you
measuring it?
Hence, if you measure from either injector terminal to
ground with
the ignition on and the engine not running, you should see
12V.
Measuring across the terminals, you should see 0V.
In reply to a message from Geoffrey Green sent Mon 30 Aug 2010:
Guys,
This may be the proverbial ‘red herring’ with an analog
meter across the terminals on the injector, with the engine
running, it pulses up to about 0.3v. I can see that a scope
is probably the correct way to measure this.
But what else could be causing the engine to run lean?
I have cleaned the injectors by filling them with carb
cleaner (overnight) and then pulsing them with a 9v battery.
I have checked the fuel pressure which seems to be 28psi on
each fuel rail (half pound below where it should be is that
significant??)
What else could influence the mixture?
I got to a weak signal from the injection amp or the ECU!
Any further thoughts?
Thanks
Ian–
Ian427
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Ptipon
(Ptipon, Sonora, CA, USA, 1990 XJS, Conv.)
11
In reply to a message from Ed Sowell sent Mon 30 Aug 2010:
I have been following this thread since the beginning and the assumption that there is not enough fuel may be
wrong.
You could have other fueling problems such as the O2 sensors, the TPS, , the ECU, etc. The problem could also
be ignition. Remember that when you vary the accelerator pedal, you only give the engine more or less air, not
fuel. The system runs on knowing what the mixture is and from that how much more or how much less gas should be
metered by the injectors.
Have you checked to see that the injectors are in fact working? This is easily done by placing the ignition key
in the running position and then abruptly twisting the throttle linkage wheel in the middle of the engine with
both throttle rods and listen for all injectors clicking. If they are working I would go on to other system
components.
I can’t remember if this is a V12 or not. If it is, placing it in the V12 blogs could get more responses.–
Ptipon
Modesto/CA, 90 XJS-V12 conv, United States
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In reply to a message from Ptipon sent Mon 30 Aug 2010:
Thanks Ptipon,
The car is a 1977 V12, but I have no idea how to get this
onto the V12 forum too…
As for the ignition, I have been back an forth on this one
too, but the deciding factor for me was when I pump the
throttle (firing the injectors) the engine stops spitting
back through the inlet manifold.
Ian–
The original message included these comments:
I have been following this thread since the beginning and the assumption that there is not enough fuel may be
wrong.
You could have other fueling problems such as the O2 sensors, the TPS, , the ECU, etc. The problem could also
be ignition. Remember that when you vary the accelerator pedal, you only give the engine more or less air, not
fuel. The system runs on knowing what the mixture is and from that how much more or how much less gas should be
metered by the injectors.
Have you checked to see that the injectors are in fact working? This is easily done by placing the ignition key
in the running position and then abruptly twisting the throttle linkage wheel in the middle of the engine with
both throttle rods and listen for all injectors clicking. If they are working I would go on to other system
components.
I can’t remember if this is a V12 or not. If it is, placing it in the V12 blogs could get more responses.
You could have other fueling problems such as the O2 sensors, the TPS, ,
the ECU, etc.
No O2 sensors on PreHE.
Have you checked to see that the injectors are in fact working? This is
easily done by placing the ignition key
in the running position and then abruptly twisting the throttle linkage
wheel in the middle of the engine with
both throttle rods and listen for all injectors clicking. If they are
working I would go on to other system
components.
Probably good advice, but if they have not been cleaned & tested it is quite
possible they are not all flowing full. The fuel passages in the PreHE
injectors are small and roger Bywater once told me they can get clogged,
especially if there has been water in the fuel since it tends to form an
emulsion.
Thanks Ptipon,
The car is a 1977 V12, but I have no idea how to get this
onto the V12 forum too…
I use the mail list rather that the Forums so it’s easy to cross post,
although it’s frowned upon. My guess is they don’t make it easy in the
Forums. You could join the V12 Engines fourm and do a copy/paste of the post
body.
As for the ignition, I have been back an forth on this one
too, but the deciding factor for me was when I pump the
throttle (firing the injectors) the engine stops spitting
back through the inlet manifold.
One thing that hasn’t been mentioned here is air leaks. For example, if the
intake manifold gaskets are not sealing or injectors tip seals are not
properly seated air will get sucked in, leaning the mixture. If the engine
will idle at all, try spraying WD-40 or some other combustible directly on
the injector tips and listen for engine RPM increase. Or, just examine the
tips with a mechanics mirror. Sometimes you can spot a seal that’s cocked up
on one side or something.
In reply to a message from Ed Sowell sent Mon 30 Aug 2010:
The car ran fine on Dec 19th 2009, when I took it out for
its final blast, I remember it well. This issue developed
over the intervening time (in spite of a start-up and warm
through every 4 weeks during winter), hence my suspicion
that the gunky ethanol based gasoline over here has clogged
everything up.
Maybe my ‘‘carb cleaner overnight and a 9v battery’’ clean was
not enough, but the were all click and letting carb cleaner
through.–
The original message included these comments:
One thing that hasn’t been mentioned here is air leaks. For example, if the
intake manifold gaskets are not sealing or injectors tip seals are not
properly seated air will get sucked in, leaning the mixture. If the engine
will idle at all, try spraying WD-40 or some other combustible directly on
the injector tips and listen for engine RPM increase. Or, just examine the
tips with a mechanics mirror. Sometimes you can spot a seal that’s cocked up
on one side or something.
Ed Sowell
Ptipon
(Ptipon, Sonora, CA, USA, 1990 XJS, Conv.)
16
In reply to a message from Ian427 sent Mon 30 Aug 2010:
Hi Ian,
What I am suggesting is that you eliminate your assumption and go with testing to locate the problem. There are
many other components that are factors in delivering gas to the combustion chamber.
Does the engine start fine with no difficulties whatsoever? Does the engine go thru hi-speed warmup mode into
regular idle mode? Can you slowly advance the accelerator pedal and get higher rpms when in neutral or park? Or,
are you tromping on the accelerator pedal when the engine chokes? When the engine chokes, does it go back to normal
idle when the accelerator pedal is released? Or, does the engine just die?–
Ptipon
Modesto/CA, 90 XJS-V12 conv, United States
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In reply to a message from Ptipon sent Mon 30 Aug 2010:
The engine starts fine, idles at about 1100 rpm for about 5
seconds then drops to very to a very low idle speed, a then
over the next 30 seconds the idle comes up to about 750rpm
and then as it warms over the next minute it settles to a
idle of around 900rpm.
As the engine warms it is fine sound.
Revving smoothly to 2500rpm.
As the temp needle starts to climb, it revving become harder.
If you try to hold 2000 rpm and slowly increase the revs to
2500rpm it will cough.
After this, if you release the throttle, it resumes idle
If you twitch the throttle and fire the injectors, it will
rev up freely.
Ptipon
(Ptipon, Sonora, CA, USA, 1990 XJS, Conv.)
18
In reply to a message from Ian427 sent Mon 30 Aug 2010:
Here’s what you need to do. Copy your very last post and post it on the V12 blog. Be sure that you add your year
and V12 engine to your name using the ‘Edit your profile’, brown button above.
It looks like you have a temperature problem, not an injector problem. I have a 90 V12 as my name tag indicates so
I may not be able to fully help you diagnose your problem but others there will.
Right off the bat I would say to check your temperature sender on the B bank, the left hand side as you sit in the
car. Clean both ends of the connector with a good electrical contact cleaner, do not use any kind of automotive
cleaner. While you have the connector off, measure the resistance of the sensor cold. After warming the engine to
normal, disconnect the sensor connector and measure the sensor resistance with the engine hot. I don’t know what
the resistance values are for your engine.
There are 3 other sources of fueling problems that may also need to be checked but do the above first.–
Ptipon
Modesto/CA, 90 XJS-V12 conv, United States
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In reply to a message from Ian427 sent Sun 29 Aug 2010:
Ptipon,
Thanks for your help.
I have just re-posted the issue on the V12 forum.
Cheers
Ian–
1977 XJS V12 Manual - UK Spec
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