After an extensive clean up and preventive maintenance
project, I finally got the XJ running quite well. For about
20 miles over several runs everything seemed fine, Started
eaisely, ran excellent and fuel pressure stayed up after
shutdown. Then, while out again, I thought I detected a
slight miss. I was 5 miles from home and by the time I got
back it was barely running and I heard several slight
backfires. I quickly tried to listen to the injectors before
it died. Several sounded all right with that faint click,
click, click, but most didn’t sound right. I have done
several tests to try and find the problem.
Injector winding resistance checks OK.
Wire leads to injectors all checked 9-10 volts across
terminals. ROM says this should be 3-4 volts. What can this
indicate?
Checked resistance on 4 wire trigger board. One side
checked OK with resistance jumping from high to low as rotor
turned. Other side only measured high resistance as rotor
turned. How would this affect running?
Fuel pressure now does not stay up after shut down. Leaks
down immediately. Stayed up before.
Any suggestions on what else to check or what these symptoms
might indicate is wrong?
Thanks for any help. Dick Kress–
Dkressk
Massillon,OH, United States
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In reply to a message from Dkressk sent Thu 17 Mar 2016:
this one sounds a bit messy but first…the fuel pressure
problem.the regulator if I remember correctly is vacuum
operated on all v12’s.if the backfiring has resulted in the
engine firing back into the intake then-and because age can
be a factor-the diaphragm could have ruptured and the
pressure will bleed right off as a result.
about resistance levels and over-voltage conditions:
if the car has one of those four point resistors connected
to the injector harness feeds it could have failed but for a
more accurate evaluation on the earlier version of the v12
you have I would suggest guidance from kirbert.he’ll have a
better handle on that situation than I will have as the
facelift model is the one that I have more experience
with.definitely sounds like a regulation problem though.
=dok=–
thewytchdoktor=v12 fun!/94 xjs 6 litre/ s=k.log w
Winchester Virginia, United States
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In reply to a message from wytchdoktor1 sent Thu 17 Mar 2016:
Boy that sure sound’s like a case of plugged fuel filters,
or just as likely, a fouled inlet screen on the surge tank
(don’t know if your XJ has this, but on my XJS, the fuel
tank feeds to a smaller ‘surge tank’ that on my car is
located under the battery in the boot).
The fuel pump draws from the surge tank, and the inlet on
the end of the hose or pipe is fitted to a screen/filter.
Fuel then goes to a regular fuel filter on its way to the
rails.
Anyway, on a car that has been in need of PM, and at this
age, I have to suggest a good going over of the fuel filter
setup.
What happens is the crud that comes from the tank settles
in the bottom of the surge tank. As you drive, this crud
gets picked up on that surge tank filter, and eventually
starves the engine of fuel. When you shut the car off, all
this junk falls off the filter to the bottom of the tank.
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In reply to a message from Dkressk sent Thu 17 Mar 2016:
Thanks for the quick reply. I would be very surprised if
anything in the fuel supply system is causing the problem. The
PO had replaced both fuel tanks and all hoses and components
in the trunk just before I bought the car last Summer. Since
then I have replaced everything else in the fuel system.
Filter on suction from tanks, filter before fuel rail, both
FPR replaced, all injectors sent to be tested and cleaned, and
all hoses and clamps in engine compartment replaced. In
addition PO had replaced the injector harness. When this
happened I thought maybe some of the injectors stayed open
letting the pressure go down.–
Dkressk
Massillon,OH, United States
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In reply to a message from Dkressk sent Thu 17 Mar 2016:
Does the car run, now?
Or, does it do a repeat of the behavior?
It sounds like a very comprehensive run through of the fuel
system was done.
Have a look at the injector and ignition harness ground
points, and also at the ground strap that ties the engine
to the frame, too. Were any of these items fussed with on
the way to getting this car running once more?
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In reply to a message from Dkressk sent Thu 17 Mar 2016:
Dick,
If you think the injectors or regulator is leaking, you
might be able to isolate the individual rails to see if they
hold pressure by using two long nose vice grips to clamp the
supply and return hoses at each fuel rail. You would
definitely need to be sure all of the hoses you clamp are in
good condition, otherwise you might crack one or more.
Although, if one does crack, it needed replacing anyway. New
hose will be soft and pliable. Old hose will be hard and
brittle.
If you do this, be sure to wrap the end of each plier with
some tape to help prevent damage to the hose from the teeth
on the pliers.
The above ALSO depends on where you have your pressure gauge
fitted.
I believe if you turn the key switch to the ON (not start)
position, the fuel pump will energize and pressurize the
lines, rail and injectors. Then apply the clamps and
remember to turn the key switch off. Then check the gauge
for any pressure leak down.
SD Faircloth–
The original message included these comments:
addition PO had replaced the injector harness. When this
happened I thought maybe some of the injectors stayed open
letting the pressure go down.
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In reply to a message from Dkressk sent Thu 17 Mar 2016:
Update:1. Loss of fuel pressure after shutdown was caused by
a leaking anti return one way check valve. This was not
causing the not starting problem. Fuel pressure is good and
fuel is circulating properly.
2. Have replaced the trigger board and verified that
ignition and spark firing is all OK.
3. The problem seems to be that there is no fuel getting
into the manifold from injectors.
4. When trying to start it will fire momentarly but when
starter is released it wont continue to run. Seems like the
fuel injected from cold start injectors lets it fire for
only a moment. Regular injectors should then start putting
fuel to cylinders but aren’t doing so.
5. Any suggestions on what to check to find out why
injectors aren’t working?–
Dkressk
Massillon,OH, United States
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In reply to a message from Dkressk sent Fri 25 Mar 2016:
All 12 injectors or some not firing ? On a 77 V12…when is
the last time you pulled apart all the electrical connectors
on the wiring harnesses and also at the ECU ?
The first step in any diagnosis to to make sure all the
electrical contacts are clean and bright.
Do that and report back.
SD Faircloth–
The original message included these comments:
Any suggestions on what to check to find out why
injectors aren’t working?
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When trying to start it will fire momentarly but when starter is
released it wont continue to run.
Your focus on fueling may be misplaced. This is symptomatic of a failed ballast resistor pack. It has two separate paths, one for start and the other for run. When the run resistor fails it will cause what you are reporting.
The testing procedure is in the ROM. If you don’t have access to it let me know and I’ll send instructions.
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In reply to a message from SD Faircloth sent Fri 25 Mar 2016:
More update.
Pulled all fuel injection plugs apart and cleaned. They
really didn’t look bad. The injector harness is relatively
new. All injectors were just cleaned and tested. No
difference in running.
Bypassed original ballast resister with an external one.
Car did start after doing this. Running was very, very poor
and was missing badly as before when I was able to get it
running.
Tested all injectors with a 2.5V light hooked to each
injector harness connection. All tested good. So injectors
are getting a good signal. I guess my theory of injectors
not working was wrong.
4.After getting it to run little I pulled several plugs.They
were all very, very black & sooty. Not wet.
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4.After getting it to run little I pulled several plugs.They
were all very, very black & sooty. Not wet.
5. Where to next?
Dkressk
Massillon,OH, United States
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In reply to a message from Ed Sowell sent Sun 27 Mar 2016:
Have removed the plugs several times. They get sooted up
within a few minutes of poor running. No sense of pulling
again until I determine what is causing them to soot up.
I have a Crane XR700 ignition system installed with a
matched Crane coil. I installed the resistor that came with
the coil instead of the factory ballast resistor.
The solution seems to be to determine what is going on in
all the combustion chambers to soot up all the plugs in a
few minutes of rough running. Any theorys.–
Dkressk
Massillon,OH, United States
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I have a Crane XR700 ignition system installed with a matched Crane
coil. I installed the resistor that came with the coil instead of the
factory ballast resistor.
Has it ever run in this configuration? Not saying it won’t… Just don’t know I stayed with the OE resistor pack when I installed the XR700. Works fine.
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In reply to a message from Ed Sowell sent Mon 28 Mar 2016:
Engine ran fine, before developing this problem,with the XR700
and Crane coil hooked to the factory ballast resistor. I only
tried using the crane supplied resistor when it was suggested
that the factory ballast resistor may be faulty. Problem is
still there with no significant difference with either
resistor being used.
Only component I haven’t tested, besides the ECU,is the
throttle switch. Is there anything there that would cause this
terrible sooting on the plugs?–
Dkressk
Massillon,OH, United States
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Engine ran fine, before developing this problem, with the XR700 and Crane
coil hooked to the factory ballast resistor.
Not to harp, but I would suggest that you test the OE resistor pack and if the resistances are in range put it back on and look elsewhere. If they are in range, try to find a replacement. IOW, I think it’s important to have the correct ballast resistance in Start, and the correct ballast resistance in Run.
In reply to a message from Ed Sowell sent Mon 28 Mar 2016:
Engine ran fine, before developing this problem,with the XR700 and Crane
coil hooked to the factory ballast resistor. I only tried using the
crane supplied resistor when it was suggested that the factory ballast
resistor may be faulty. Problem is still there with no significant
difference with either resistor being used.
Only component I haven’t tested, besides the ECU,is the throttle switch.
Is there anything there that would cause this terrible sooting on the
plugs?
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On some there is a ballast resistor “pack” for the injectors (on the older
pre-HE?) which allowed a “peak and hold” arrangement. If those aren’t doing
their job, the injectors won’t fire. (They may click, with the “peak” phase
still operating, but wouldn’t “hold” open.) This would result in an extreme
lean condition??
It is a very crude arrangement that dissipates heat in the resistor pack,
rather than the injectors, but it achieves a fast “snap open” of the
injectors for better fuel control, especially at low opening duration.
My recollection is that they are commoned together in groups of 3. So if you
have lost all injectors, that would point to ECU or earthing. If you have
only lost 3 injectors, that would point to the resistor pack wiring.
Rgds
Mark
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I don’t know this particular engine all that well, but, aren’t you guys
talking about two different ballast resistor packs? One on the ignition, and a large pack for the injection?
I sure we are talking about the ignition ballast resistor pack, strapped to the throttle pedestal.
I don’t know
if this model’s fuel ECU needed them, but the later 16CU ECUs needed
them.
The PreHE has an EFI power amplifier mounted on the radiator top rail and a 3CU in the boot.
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On some there is a ballast resistor “pack” for the injectors (on the
older
pre-HE?) which allowed a “peak and hold” arrangement. If those aren’t
doing their job, the injectors won’t fire. (They may click, with the
“peak” phase still operating, but wouldn’t “hold” open.) This would
result in an extreme lean condition??
There is only one PreHE EFI set up as far as I know: a 3CU in the boot which is based on the same Bosch ECU used in the Porsche 914, and a power amplifier on the radiator top rail. The latter has 4 power transistors that are switched by pulses from the 3CU. Each fires 3 injectors. This is all very solid stuff, rarely causing problems. If it’s not working it’s probably because of harness problems.
On grounding, I would say that more than once the ground strap to rear of the right intake manifold has been left off after work in the area. I may run by finding other weak paths to ground, but barely.
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