3.6 Cabriolet refuses to start

[Timmi2jags]

Hi. I have a 1986 3.6 manual XJS-C that has been unused for the last five years. I planned to take it down for it's MoT but unfortunately it kind of refuses to start. On the turn of the key, it fires up with no problem but only runs for about 2 seconds and then dies, this despite pumping the accelerator. Sadly to no avail. My friend (a mechanic) has now got involved. His first assumption was fuel starvation. After much trying, she finally fired up but only by clamping off the fuel return pipe at the fuel regulator valve and continuous pumping of the accelerator. The moment you released the clamp she would die again. Only once she was warm would she run of her own free will. We swapped both the regulator valve and fuel pump with ones we knew worked but that just proved the ones on the car were fine.
Anyway, after leaving things for a couple of weeks, he brought round a fuel pressure gauge. he connected it up and I turned the key and, blow me, if she didn't start straight away and ran smooth as silk as if there were no issues. He checked the fuel pressure which read a constant 50psi. From this he believes the issue may well have been electrical rather than fuel.
Buoyed by this success we booked it in for it's MoT the next day. Sadly that day was Friday 13th because when I went to start her that morning, she adamantly refused to start again. We are sure this is an intermittent electrical issue but does anyone have any idea what might be the likely cause of it. I wondered if it might be ECU or the amplifier. The ECU in the boot is an old DAC3831 unit and at some point in it's life before my ownership the fuel pump relay has been re-wired to run constantly, but prior to this current situation, the car has run fine before and ever since I bought it in 2002. Any advice would be much appreciated as the plan was to take it to the Le Mans Classic in July.

Kind regards Tim

[dhdove]

Hi Timmi
Can't explain why it ran perfectly all of a sudden but I would check whether the fuel "swirl" bowl under the battery is full of corrosion and the fine nylon filter within it is blocked......it may well be an ignition problem but after not being used for five years I'll almost guarantee that there is a whole load of corrosion muck (it ends up like clay particles) in the swirl tank, the swirl filter always takes the biggest hit.
Best Regards
TonyS

[Timmi2jags]

Hi Tony
Thanks for your help. We've tried so many things, that one slipped my mind. We did actually check the swirl tank which incredibly was clean as a whistle, but to ensure it wasn't that we tried it with the swirl tank filter removed and then with it bypassed completely, but to no avail. I'm guessing it's a connection somewhere that's got damp and corroded. Probably an earth lead maybe.
Kind regards
Tim.

[ptjs1]

Tim,

As Tony mentioned, my first guess would be the swirl pot. However, if that's ok, have you changed the fuel filter? That's a n oft-neglected part which can cause fuel flow problems, then run clear once or twice, then block again etc.

However, your description of it running ok once hot makes me wonder if it's being under-fuelled when cold. This could be fuel flow as mentioned or it could be because of ecu conditions. I can't remember when the 3.6 changed over to the more electronic setup. However, I'm guessing your 86 uses a coolant temperature sensor to send a signal to the ecu. If that's failing or the connection at the ecu is slightly suspect it could be defaulting to a warm-engine condition when cold and causing that problem. I think(?) you'll have two temp sensors - one for the fuelling and one just for the gauge.

Hope that's of some help.

Paul

[steve_m]

It's a while since I have had the AJ6 engine but if memory serves me right the throttle bodies and butterfly are prone to getting gummed up which can lead to all sorts of running problems.
Might be worth a look?

[Timmi2jags]

Hi Paul & Steve M.
Thank you for your input into this saga.
On the subject of the fuel filter, we also bypassed that too when we bypassed the swirl pot, so we eliminated everything from the boot area as the cause of the fuel starvation.
However, this morning we have made further discoveries, hopefully towards an answer to the problem. We confirmed fuel pressure to the injector rail is good and that, if you put a feed onto the injectors, they will open freely. But a check of the spark plugs revealed they were totally dry and as Jaguars of that age tended to soak the cambers with fuel on start up, this has lead us to believe the ECU circuit is not telling the injectors to open. Indeed, listening with a screwdriver on the injector and an electrical tester on the injector feed wires seemed to confirm this. From this it was suggesting a faulty ECU but, after a bit of reading on the internet, we've established that the ECU doesn't necessarily operate the injectors but there is a power resistor that does. I located this resistor behind the drivers side headlight pod, right next to the coil. Having removed and unplugged it has revealed a reasonable amount of corrosion on the electrical pins. I am very much hoping that this could be our eureka moment. Sadly I can't tell you if this is so at this point as the car's battery is now flat and I've run out of time to do anymore work on it for today, so the battery is currently on charge and time has been set aside on Friday to continue. I will report one way or the other if this is the answer, but would be happy to hear from anyone who thinks we are barking up the wrong tree.
Kind regards
Tim

[ptjs1]

Tim,

Have the symptoms now changed? You mentioned at the outset that the car would start up when cold but then stalled. Also that it could be made to run when cold by clamping the fuel return hose. However, you've now mentioned that it won't start and that the plugs are dry? Does clamping the fuel hose as before enable the car to start?

Be careful if you disconnect the resistor. If it's like the V12 equivalent it enables the injectors to stay open by lowering the resistance. If you try and run it without the resistor connected, you could seriously mess up the ecu.

Have you run through voltage and amp readings at the injectors with just the ignition on and then with the engine running (or turning over)?

Paul

[Timmi2jags]

Hi Paul.
Yes the symptoms did change and it refused to start even with clamping the return pipe. We didn't have the right equipment to hand yesterday to measure voltages at the injectors. Simply a circuit tester to confirm they weren't stuck. Although I've removed the resistor, it was only to check the condition of the contacts. This revealed two lines of four contacts. Although one line looks ok, the other line looks very corroded. Our train of thought from some minor homework is that each line of contacts opens up three injectors and that this may account for it trying to run but not succeeding. Effectively it's trying to run on three cylinders. (Would that make sense?) Anyway, the plan is to simply clean the contacts up and reconnect the resistor before attempting to try to start it again. We hadn't planned to bypass it because we assumed it was essential to opening the injectors anyway. But thank you for the advice about potentially damaging the ECU.
The plan was to do all of this tomorrow but it may have to be put on the back burner until early next week as my XKR went a bit mental today when the alternator regulator packed up and tried to fry the battery, so my day maybe taken up with fitting a new alternator tomorrow as need that working for the JEC Cotswold Weekend at Sudeley Castle.
Hope to have an update soon.
Kind regards
Tim

[normant]

Hello Tim,
I'm just taking a (very) occasional look at the blog and have noticed your enquiry - have you fixed the problem yet?
If not, I recommend that you send the ECU to AJ6 Engineering for checking.
I had very similar problems with my 3.6 to what you describe - the engine occasionally ran fine and then not. After checking more or less every component in the FI system (which were all OK), I decided that it must be the ECU.
Turned out to be dry joints in the ECU which AJ6 fixed - the engine is now running sweeter than ever.
Just Google AJ6 Engineering for the details.
All the best,
Norman.