V12 erratic idling issue

Next test, Jean…

…you have spark at the coil center lead, right - 1/4", strong and blue? Now connect a spare spark plug (triple gapped) to any plug lead (ensure the plug body is grounded) and ‘crank’ - you should have spark, again strong and blue. Using the center lead bypasses the distributor - this test verifies that the rotor connects coil center lead to the plug wires. There is a spring loaded connection (usually carbon) between the lid’s center and the rotor - this may fail and there is no connection between coil and plugs. Or the rotor itself may have failed…

The main point is to have strong blue spark at the plug(s) - this verifies that your work inside the dist has not disturbed anything vital, and that the pick-up and amplifier work as they should. If the spark is weak; the air gap between the pick-up and timing rotor may be incorrect - it’s specified to 0.020 to 0.022".

If all checks out; the ignition works as it should, and I take the verification of the #1A being at firing stroke is double checked. In which case; the ign should not be the cause of the non-start…

For ‘fun’ you can back up tests with the usual EI static test; verify which plug lead the rotor is pointing to - and connect a spare spark plug to that lead. Loosen the dist and ‘wriggle’ the dist slightly back and forth (ign ‘on’) - this should trigger the ign amp to fire, and the plug should spark…

All this is about as much you can do to verify ignition…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Today :

yellow/pink wire atthe ECU with 5,2 V
And 5V to the regulator throttle sensor.
I can’t really see if the spark plug is wet with fuel…

I have a spark at the spark plug, strong but not sure of the color…
I have a spark at the coil, 1/4", strong. I can’t be sure it’s blue. For me is not blue.
I made a video, and I take some pictures from the video. You can see the spark of the coil. I’m not sure of the color.

I don’t have problems with the distributor cap.
I checked the air gap, near 0,0205
I checked 1A, firing stroke OK

And now, today, when I take out the spark plug 6A, and check for the spark, the engine starts…
I stopped the engine. I put back the spark plug 6A, and the engine didn’t start…

After that, I checked the air gap, I put the damper on 0° and rotor arm on the sign of the plastic shield. The engine has started for 15 seconds and stopped alone.

I thought that I don’t have fuel any more (I put 4 liters at the begining of the tests, 4 days ago…). I put again 5 liters. I tried to start the engine, nothing. Then I tried again all position of adjuster, nothing.

I am literally desperate and this is not a joke :disappointed_relieved:

I would stop playing with the distributor; that the engine ran indicates that the timing is close. I think the yellow/orange spark is weak, but seems to be able to jump a wide gap- possibly a weak or intermittent coil. You need to verify fuel at injectors and correct pressure- tap in a gauge to the rail (usually easiest at the 2 front injector hoses) and leave it connected until you have it running reliably. That way you can see if pressure is correct and steady.
It surely sounds like a fuel delivery issue.
Back in my British motorcycle days, you could clear a fouled plug by holding the coil wire a small distance from the spark plug- theory was you were forcing the coil to produce maximum voltage that would make the plug spark even when fouled. Those coils were notoriously weak…maybe yours is also.

You must be joking, Jean - starting every time if removing 1A plug…!?

With the engine set at damper TDC; you cannot ‘move’ the rotor arm - did you turn the distributor to align the rotor to ‘the sign’ of the plastic shield’ and by how much? In which case you alter the ign timing…

I think Robert is right; a fuel issue must be pursued - the spark seems adequate, and if the engine runs normally for 15 seconds in whatever configuration; ignition would not be a non-start issue. While supervising fuel pressure, as Robert suggests, is the best way of addressing a fuel issue - you should also check other fuel factors…

While 4 + 5 liters of fuel into an empty tank should be adequate; does the fuel pump run for the prescribed 2 second when the ign is turned ‘on’? With the gear lever i ‘D’ - does the fuel pump run constantly with ign key to ‘crank’, each and every time? Have you tried running on the other tank - a first step with fuel issues? And certainly, while cranking, the injectors should click regularly…

This could be a double fault issue - giving the bewildering symptoms. And while changing the gearbox in itself should not be an issue; the ‘fiddle factor’ may be in play - you may inadvertently have touched some tender spot…

xj6 85 Sov Europe (UK/NZ)

It is very frustrating indeed, but you will get there eventually.

I agree with Robert and Frank, if you are sure your timing is correct, the problem is elsewhere…
Ignition (usually) it either works or not, and the fact that the engine started for a short time indicates that it must be more or less working.
The fact that it worked when you unplugged A6 is weird and most probably a coincidence, but to be sure, if the engine starts plug it back in to see if it stops…

You could try to see if the engine will catch with starting fluid.
If you could measure the Fuel pressure it would be very informative.
Make sure your pump is running (even bypass the fuel pump relay), that the injectors have fuel and are indeed clicking.
Measure the output voltage at the TPS (details at the Book)
Measure your Coolant temp sensor (or just put a paperclip at the plug so the ECU thinks it’s really cold).
When was the last time the fuel filter was changed?

Good luck and keep us posted.

Second that , Aristides - he has had more than his fair share of what can happen to a Jaguar owner…

As an aside; if the changeover/return set-up fails; it takes no time to transfer 7 liters from one tank to the other…

xj6 85 Sov Europe (UK/NZ)


  • I’s kind of you to put a gauge on fuel system, and “leave it connected until you have it running reliably”. First, I just want to have an engine that starts !


  • No, just starting one time with plug out. And I don’t know why

  • With the engine at TDC, I can move the rotor arm (10-15°) by hand with the correct metallic sound, and the rotor come back alone. You’re understand what I mean. It’s correct for me.

  • I put the TDC at 0° on the damper. Then (not in 1 try) I put the rotor arm on the sign of plastic shield by moving the base of the distributor, unscrewing the allen screws or whatever.

  • I will by a fuel pressure gauge (I don’t have). What must be the fuel pressure at the entrance of the rail ? This information is in the Book ?

  • I don’t know if the fuel pump run 2 seconds with gear lever in D. I tried, but alone, it’s not easy. In N position, the fuel pump is running all the time (I ear) with ignition “On”.
    Another point, the TPS, near the coil, is switch with a 3 wires link (a plastin link like an “L” with 3 wires). When I unplug this link and put the key ignition ON, the injectors became very noisy (like birds in a cage). When I plug back this link, with key in the same position, the injectors became very discreet. FYI.

  • I would like touched some tender spot, but really I don’t see which one.
    Around and upper the gearbox, nothing electrical issues.
    At the back of the engine, every electrical contacts are plugged.
    Every vacuum hoses are plugged.


  • Yes,I think my distributor, more or less is in correct position (I did this hundred of times on 4, 6 and 12 italian cylinders, why not on an english 12 cylinders ?)

  • The engine no more starts with or without unplugged A6…

  • Fuel filter changed 2 years ago / 60 kms ago. But I can see rust on the inside walls of the fuel tank.

Next work :

  • I will measure output voltage (already done a couple of months ago) at the TPS
  • I will measure the coolant temp sensor, but how

You should hear the pump running for 2sec everytime you turn the ignition on, regardless of gear position.
As I said, for troubleshooting purposes, you could bypass the fuel pump relay (middle relay behind the radiator) and the pump will run all the time.
Other solution is to wire a small light in parallel to the fuel pump and put it somewhere you can see it.

3 Bar

With TPS connected and ignition On, you should have 0.32 - 0.36V between the Red and Yellow cables.

Coolant Temperature Sensor (ECU)

0 C 5.9 kOhm
10C 3.7 kOhm
20C 2.5 kOhm
30C 1.7 kOhm
40C 1.18 kOhm
50C 840 ohm
60C 600 Ohm
70C 435 Ohm
80C 325 Ohm
90C 250 Ohm

That’s a relief, Jean - pure coincidence, not science breaking down…:slight_smile:

That’s the the centrifugal springs working as they should…

I’m uncertain of the purpose, or effect, of the action - the normal procedure is to unclamp the dist and turn the body. However, it may be subtility relating to reassembly of a dismantled dist…?

The fuel pressure from the pump in good order is 100+ psi, so the gauge should be above that. Connected directly to the fuel rail inlet hose, ‘raw’ fuel pressure, it shows state of the pump; it should read 70+ psi - below 40 psi the pump will not feed the engine properly. Connected to a CSI hose, ‘regulated’ fuel pressure, the pump should read some 28 to 30 psi.

However, fi you have two pressure adjustable regulators - there is a bit more to it…

It should run for two seconds in any gear lever position - as Aristides says; it is a characteristic of the V12. It may indicate a fault in the ECU, but may be unimportant for the problem at hand…

That the fuel pump runs with ign ‘on’ in any gear means the fuel safety circuit is bypassed - pump is only supposed to run in ‘crank’. Ie, the pump is generally not supposed to run in ‘on’ unless the engine is running - it is a fuel safety feature. In ‘D’ and ‘crank’ the actual cranking is prevented by the start inhibit switch. This is to prevent the engine starting in gear.

Neither fault should prevent the engine from starting - as along as the pump is running should deliver fuel to the rail…

Needless to say; measuring fuel pressures is the best check with a possible fuel issue - it is either ‘correct’ or ‘incorrect’…

I assume that this does not include the AAV, the fuel pressure regulator hose(s) and the vacuum hose to the ECU…

xj6 85 Sov Europe (UK/NZ)


  • The fuel pump is working all the time.

  • Voltage at TPS is 0,344 V

  • Coolant Temperature Sensor (ECU) at 25°C is 2,047 kOhm

  • I don’t have the fuel pressure tool yet, but I can’t believe that it has changed since the car is not running

I started the engine, it ran for 5 seconds and stopped, since then no start…


  • I meant that the distributor was well positioned and that the air hoses (vacuum hoses) were well connected

This is getting more and more mysterious, Jean…

Humor me - does the fuel pump also run all the time in ‘crank’? It’s not difficult to concoct faults that will explain the symptoms - but all but fuel pressure seems to have been covered…

Running means the engine has fuel and ignition…for five seconds…then losing either, and won’t start? Two extremes; too much fuel has quenched the spark - plugs will be wet. Too little air and the engine won’t start…

A fuel pressure check is really required - just to eliminate one factor…

xj6 85 Sov Europe (UK/NZ)

Did you bypass the fuel rely or it’s working all the time by it’s own?
If the later you have a problem with your ECU.

Indeed, Aristides, you have a point. The fuel relay is grounded through the ECU, running the pumps for 2 seconds while turning ign ‘on’ - the run the pump in ‘crank’ and while the engine is running. How the ECU does this is unknown; there may be two independent circuits - one for the pump and one for actual injection. In which case the pump problem is immaterial - the injection will work normally, as confirmed by regular injector ‘clacking’, by injector test lamp - or better with an oscilloscope…

xj6 85 Sov Europe (UK/NZ)

I didn’ by pass the fuel relay.

Now, today I buy and put the fuel gauge pressure at the fuel inlet in the engine (picture) . I don’t know if it’s correct what I did.

Then I put the ignition “on”, and you have a picture of what the fuel pressure gauge show (picture)

When I made the test, with ignition “on”, the injectors did not make any noise, normal I imagine since the fuel does not pass any more in the injectors, with my assembly.

I’m in the fog, what is next step ?

That test tells you that the fuel pump runs and develops adequate pressure upstream of the “A” bank regulator; problem is that you cannot run the engine with it connected in that manner. Next you need to connect the gauge to read the pressure in the rail, which is controlled by the “B” bank regulator. Easiest place to connect that I have found is to tee into the hose going to 1A injector.
The injectors will only click when the engine is turning, with a single click if the throttle capstan is turned quickly.
You need a continuous 30-36psi (2.0 bar) while the engine is cranking or running in the rail.

The measured ‘raw’ pressure from the pump is, as Robert says, adequate - indicating a pump in reasonable nick.

The next step is, as he says, to measure ‘regulated’ fuel pressure. If you have cold start injectors, you can connect the gauge to one of their hoses. You can also connect the gauge to any of the injector hoses and run the pump - in either case there is no need to crank or run the engine for the fuel pressure test. However, if you want to monitor fuel pressure with the engine running and not having CSIs; you can put in a ‘T’ either at the inlet hose or an injector. With a running engine, the fuel pressure will be lower than in a stationary test…

xj6 85 Sov Europe (UK/NZ)

Your pump should not be running constantly.
When you turn the ignition on it should run for 2sec and then stop.
Two reasons this could happen:

  1. Your fuel relay is stuck closed (but very improbable)
  2. The ECU fuel pump circuit is caput. Even though it’s a seperate circuit from the fuel injection circuit, it would indicate that your ECU is not up to scratch and could very well be the source of your troubles.

Check if the injectors click when you turn the throttle capstan with ignition on.
Check if you can hear all twelve injectors click when you crank the engine. You can use a long screwdriver as a stethoscope.