No injector clicking, sometimes works after disconnecting battery

With no injector clicking at all, it is either a dead ignition (the signal goes to the ECU from the coil, there’s a bullet connector on the water rail, should produce a click every third time with the ignition on).
Do you have spark while cranking?
Or it is an ECU problem. I wonder why it would click on full throttle, since the injectors click, even if the AFM flap is closed. Fuel pump running doesn’t have anything to do with injectors. The injectors can get disabled with the throttle closed, but ONLY above a certain rpm, above cold idle speed.

Does anyone know whether the starter relay (oblong silver can on the bulkhead) has any say in ECU operation? I know that it powers the pump and it might do more than that…

At least it’s still daylight, I know the gas station feeling. My fuel pump had disconnected itself and we pushed it out of the way in the dark. Good luck!

It’s alive! After touching the cables together it now starts!
Thanks for the help!

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Wow ! Not on;ly great news. but, a great lesson. Off to a good staart. learned something!!!

Somewhere i heard of that to revive my once smart VIZIO TV. it wored!

Actually, I may have dumped an older VIZIO that the capacitor dump thig might have revived…

Oh well, this one has a larger screen…


I always put some cleaner for the injectors every 6 months and have never
any problems.

The ECU does take a “cranking signal” from the starter relay. How this signal is used, I’m not positive.

It’s written the the ECU slightly enriches the mixture whenever the engine is cranked, regardless of any other conditions or inputs. I’ve always assumed that’s how the cranking signal was used.


I wrote the above with a Series III XJ6 in mind. I forgot that Rasmus has a V12.

The cranking signal to the ECU would trigger the fuel pump relay ground, I assume. And according to the S58 manual, does fatten the mixture


Dir you check ignition with a spare spark plug, Rasmus - always carry a spare plug for ignition quicktest…

Both the ignition and injection is triggered by the ign amplifier - but the ignition is more sensitive to amp failure. You may have a heat sensitive amp - and waiting for it to cool down may indeed restore function.

And while the V12 has a hybrid digital/analogue ECU; I consider the usual method of restoring computers by disconnecting power to restore the ECU as pure hoodoo…:slight_smile:

In addition; cranking WOT is unlikely to start the engine even with the injectors clicking - it bewilders the ECU. Certainly; the book advises WOT cranking, stating ‘if the engine then starts - no further action is required’.

But as you have this as a recurring problem - pursue a possible ign amp failure?

xj6 85 Sov Europe (UK/NZ)

I personally think that disconnecting the battery had nothing to do with solving your problem.
It’s not like modern cars, the ECU in our cars has no memory, no adaptive features and no reset.
When you turn the ignition off all power to the ECU is cut off.
I think it was the time factor, something cooled down and started working again.

Unfortunately, first and outmost culprit is the ECU.
Had the same problem but opposite, my ECU would work only when warmed up, impossible to start but when it did it run absolutely perfect.

Ignition module or the coax wire could also cause such problems as it communicates with the ECU via the coax wire.

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The cranking signal plus WOT cuts fueling totally - being the cure for flooding - at least on AJ6 and subsequent. The X300 had a glitch whereby a brief start such as rolling out of the garage without full warm-up could flood the engine if attempting to start it again shortly afterwards.

The cure was to crank at WOT, since the ECU recognized that full throttle was never required for a dead engine crank so it cut fueling to remedy flooding.

During cranking the excess fuel was cleared and during the process at some point there would be an approximately correct amount of residual mixture and the engine would stumble into life if the accelerator was lifted (thus restoring injector fueling as WOT was abandoned).

If the engine did not catch, the drying out continued for as long as WOT was held. When, after 10-20 seconds or whatever the starter was rested, the now dry engine would start as usual with foot off the accelerator and fuelling resumed.

The same issue has occured once before, it was after I accidentally bumped the car into 1st gear from 2nd, which revved the engine to over 6000rpm. When trying to start the car again nothing would happen, so I had to have it towed (over 500 euro) and when it arrived home I tried starting it, nothing, unplugged battery and waited 30 minutes afterwhich it worked just fine.
When listening to the injectors they were not clicking, but I have no idea why.

As Aristides writes I don’t believe the ECU has any protection features like modern cars, but it still wouldn’t allow the car to start after it revved so high.

Causation versus correlation can sometimes be difficult to sort out.

Neither the ECU nor the engine should be fazed by 6000 rpm.

My gut intuition…which I’m not asserting to be foolproof…is that the ECU has come obscure fault; a weak solder joint, loose/corroded connector pin; something along those lines. Diagnosis by substitution might be in your future.


Yeah, I have a second thread going where a rought idle is being discussed, the cause of which is unclear. The plan is to get injectors cleaned this spring, and go from there, checking compression etc.
Someone has been back there before, when I first inspected the ECU to check if there is any vacuum there part of the hose seemed to have been replaced, and the cover was missing screws. There are so many untold stories with this car, but so few explanations…

We have far more explanations than answers! Is it a manual conversion or OEM? Later cars had different ECUs for manuals to cure stalling/vacuum issues. Does sound like a dry joint.

It’s an OEM, I’ll have to check the ECU this spring

Full and fair explanation, Peter - a hidden, now revealed, feature of the ECU.

The same was also relevant for carbs - drying out a flooded engine. And I guess the feature was also incorporated in the V12?

xj6 85 Sov Europe (UK/NZ)

Can’t see why not, Rasmus - if the engine stood it without bursting a gut; I can’t see overreving causing any ECU complication…

A common feature of your problem is ‘waiting’ to cure the non-start. This will equally cool down an overheated ign amp/coil, and/or dry out a flooded engine.

Injector action is caused by ign amp, as is ignition. You may have injectors without effective sparking - but the other way around is less likely - check ign with a non-start…

xj6 85 Sov Europe (UK/NZ)

So, just took the car out for a spin and there seems to be a new issue.

When under load or with no acceleration the car is smooth, but when I give it just a tiny bit of gas the car violently jerks, like it is shifting into 2nd gear or something. I tried cruising in 2nd but the problem still persists. I remember reading about a similar issue on an XJS which, if I remember correctly, was caused by a bad ECU, but I can’t for the life of me find the thread.

There are only two ways to diagnose a bad ECU, put in a known good one and see if the problems go away, or send them to Roger Bywater (or a specialized ECU repair shop) for testing.
I opted for the first and I was lucky, I got a very good 16cu ECU at a very reasonable price.

I trust there was no actual downshift indicated by the tacho, Rasmus…?

Any sign of smoking from the exhaust when this occurs - ie is the misbehavior due to too much fuel - or too little? Does the engine then recover as rpms increase without further throttle movement?

The throttle pot moves with the throttle - the manifold vacuum drops at opening, the recovers as rpms rise. So the ECU may get the wrong gen from the respective sensors - or the ECU fails to interpret correctly. Take your pick…

Theoretically; this may be an ignition reaction - too much advance for the rpms. A bit far-fetched - the usual reaction is just pinking…

xj6 85 Sov Europe (UK/NZ)

The ‘could be’ list might get a little long.

Years ago, with my XJS V12, I had a problem where the engine behaved perfectly under very gentle throttle applications but bucked violently otherwise. Long story short, it was the A-bank (inlet) fuel pressure regulator. The cure was to eliminate it entirely…somewhat common in V12 circles.