It stops making a spark. Probably both of them stop! But carefully check the wiring and the connectors; there can be breaks in the copper beneath the insulation just where the cable enters the connector.

 

Greg, I’m not sure we’re talking about the same problem. I have the AJ16 in-line six cylinder with coil packs on each cylinder. My question is why one failing coil pack would cause the engine to cut out rather than just causing a single cylinder miss. If I disconnect one of the coil packs, I get a single cylinder miss, not a dead engine.

 

Sorry Steve, I did not realise your car was a six.
Maybe the amplifier (or whatever mechanism triggers the coils) was getting stressed by failing coils so it shut down on getting hot. I would consider renewing that part too (but as I know nothing about the sixes, this suggestion might be quite wrong too!).
On the V12s, there is a LOT of excess heat generated by the amplifer and if it cannot be dissipated properly via the heat sink mechanism, the amp fails.

 

There’s no amplifier. Best I can get you is the EMS Power Relay.

So each coil pack has two wires feeding it.

One is a shared White / Brown wire that is also shared with:
(a) each of the four oxygen sensors
(b) secondary air injection relay
(c) EMS Power Relay
(d) Fuel Pump Relay
(e) Low Power ECU Terminal

the other is a wire that goes right back to an unshared high power ECU terminal.

I’m not sure why 1 bad coil caused a shutdown….unless maybe something’s shorting over from that high power ECU wire to the white/brown shared wire and the ECU didn’t like that?

 

As mentioned when I suggested new coils, I believe the issue is the coils expand with heat until they reach a point that they arc or short out (IOW the high energy impulse that is supposed to go to the spark plug, instead is directed elsewhere, and creates a confusing or unrecognized signal to the ECU. ECU gets that signal, and cuts the ground to the Fuel Pump Relay. The ECU signals are very low voltage, and very discrete, so any high energy 'dirty electricity' in close proximity to the ECU wires can create interference.

At least that is the speculation I recall from the X300 forum regarding the AJ16 being sensitive to failing coils, but it's consistent with what I've seen in other electronic systems where high power 'leakage' can interfere with low power signals.

 

Steve,

I recall reading two reasons why faulty coils can "spike" an ecu.

The first is as Mac has described. The ECU is operating at c12v. The high energy side of the coil is operating at 000s of volts. If the coil fails and the energy escapes back to the wiring circuit back to the ecu, it can spike it. In theory, the ecu has the means to protect itself from back voltage but I think the high energy of the coil can overcome that.

The second reason is that the coil operates at a particular impedance. If it heats up and fails, it can suddenly draw a lot more current from the ecu which can cause the ecu to malfunction.


Just as an aside, you mentioned something above which I hadn't appreciated. You said that when the car stalls, you can restart it immediately, then it stalls again straight away. Does that happen repeatedly? ie can you start the car momentarily repeatedly eg 20+ times, after the stall?

Also, when you said it ran longer today, did it then stall? Or did it basically stay running until you turned it off? What exactly were the parts you changed which elicited this change?

Cheers

Paul

 

Paul et al, thanks for all the additional thoughts. I'll try to take your points in sequence in hopes of leaving a trail that is helpful to other AJ16 troubleshooters.

1. "...and creates a confusing or unrecognized signal to the ECU. ECU gets that signal, and cuts the ground to the Fuel Pump Relay." All sensible up to the point where power is cut to the fuel pump. I reported above that the fuel pump continues to run while the engine dies until the CKPS reports no RPMs. The fuel pump relay shuts off as soon as the engine is at a standstill. I also reported that there is still spark - at least on Cylinder #1 - while the engine is dying. I could hear the spark jumping the gap more and more slowly as the engine RPMs drop to zero. Hence my confusion - how is the engine dying?
2. "Also, when you said it ran longer today, did it then stall?" No, it never stalled. I turned off the ignition key after an hour. I can't say that I ever tried 20+ times to restart it (someone might think I'm insane), but it would consistently restart briefly many times after the initial cut out.
3. The last thing I changed, and it was the ONLY thing before this test, was installing six new coil packs.

 

@95XJSteve it's been over 6 months since this post, can you confirm a set of coils was your long-term fix or do you still have problems with your XJS?

 

Thanks for the follow up. I can confirm that I have not had further problems with the engine cutting out when warm. I have not tested it on a very long drive, though, just around town, since the reported test drive. I'm optimistic that the problem has gone away, but still not convinced that replacing the coils was the cure. This car seems to have a mind of its own!