Jag start/tickover probs

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    • #8821

        Hello All,

        Just wondered if any other soft top member can help with a problem I’ve been having over the last couple of months…

        My 1985 3.6 manual Cabriolet refused to tick over when first started. When it warmed a little it was fine & ran/started well all day. The following day no tick over again. I changed the Water Temp Sensor & the Air Temp Sensor. No joy. I bought a new cold start injector, again no joy. I spoke to the head garage guy at Jag Motors, Wigan, he had a look, said I done right up to then. In his opinion, the next item to replace was the Auxillery Air Valve. I bought one & replaced the original, which when tested in hot water, proved to be jammed. After setting the timing with the new valve, all was well for a fortnight, then again no tick over at start & a new problem of having to crank engine over continually to get it to start at all.

        So any ideas you or other members may come up with, will be very gladly acted upon.

        Regards, David Jones.

      • #8824
        Laurence Jones

          Hi Dave

          I had exactly the symptons you have described with my own 3.6 Cabriolet last year – Slow running seemed to be a random thing that the car either decided it wanted to do or not on a whim. Made driving it very hard and unpredicatble – sometimes needing to be sat at traffic lights bliping the accelerator like a boy racer to keep it going and others the car being perfectly behaved.

          We swapped a the same sensors as yourself, with similar results to your own – it would work for a while then start messing aroudn again. Then one day the car wouldn’t start at all -dead 🙁

          Subsequently we HAD to change the ECU to get the car going and borrowed one – and all the random yet persistent slow running issues went away. Replacing the ECU was not on the list of things to try, but I wish it had had been right at the start.

          I’ve met a couple of other folk who have had similar problems and it does seem to be that if you have this random slow running issue that its cause by the ECU – when it’s a consistent and persistent slow running problem then it seems changing the sensors does fix the problem.

          I’d be interested to know if others in the Club have had the same experience.

          Hope this helps,


        • #8852

            Hello Dave Jones Just a thought, when I had some issues with my 1987 3.6 cab starting/running years back I tried the usual sensors without much impact I then bought a replacement ECU which cured the problem.
            After examining the original I found quite a lot of surface corrosion on the terminals, two sets of multi-connectors from memory, I cleaned up the terminals with alcohol/cleaner and tried refitting original just to see..
            Now worked perfectly!
            So maybe before you buy a replacement ECU might be worth spending half an hour just removing your existing ECU and giving the contacts and connectors a clean. So many ECU inputs are based on changing resistance of sensors/components than any corrosion can upset the apparent ohms detected.
            As you probably know it’s mounted under the passenger foot well carpet on the 3.6 models, an area that can get neglected, even damp. (do disconnect battery first)
            As I say just a thought!

            Regards, Robin

          • #8884

              Hello Robin

              First off thanks for the reply. I thought the fuel management unit was the box in the boot bolted on the drivers side rear wheel arch? This unit I’ve had off & cleaned & will be checking any results on Tuesday (out till then). I thought the box under the carpet beneath the passenger side seat was the amp for radio/tape? Have I been looking at the wrong unit then & does the seat have to come out to check it or can it be removed leaving the seat in situ?

              Thanks again, Dave.

            • #8922

                Hi Dave Jones
                “A little knowledge is a dangerous thing” – just got both the parts books out and the w/shop manual too..

                Mine is a 1987 Model VIN 139052+ and it is indeed different to your 1985 which as you say has ECU in the boot same as all the V12’s do.. I thought all the 3.6l Cabs had ECU in footwell but obviously incorrect.

                I should have kept quiet as many items are different on my late model 3.6 cabriolet which was why I bought it originally, though I particularly liked the veneered ski slope.
                Mine has it under passenger toe-board next to slot for the cruise ECU. FYI the std fit Jag/Clarion radio cassette won’t have a separate power amp but happy to be proved wrong!

                If you have already had your ECU off and cleaned the contacts then that’s really all I was suggesting. It might just work especially if car has stood unused at some point. Mine had cobwebs!

                kind regards, Robin
                PS I’ll try and check parts manuals for vin changes before I make assumptions next time..

              • #8924

                  My car has never been laid up & all the ECU connector blades looked in very clean & bright condition so I’m not really hope full as to this being the problem, see how it goes 2morrow.


                • #9157

                    Here again Robin, first off, checked the new auxillery air valve was working with a can & how water, all fine. Fitted to engine, very hard to start, continual cranking, then when going, revs all over the place. Been in touch with Heywood Jaguar in Rochdale who told me to get ECU checked/sorted & if no better get back to them. So this is my plan now unless any other member has any idea’s ?????????

                  • #9636

                      Hi again Robin, just to let you know I took my ECU off & sent it away for inspection/repair but it came back with a full 2 page diagnostic report which just meant “no problems here”. I spoke to the guy who sent it off & have now put it in the hands of Camco diagnostics ltd in Warrington, who are going over the cars many fuel associated parts to see if they can find the problem then hopefully rectify it. The Saga continues…David

                    • #9651

                        Most frustrating when you cannot pinpoint cause of the problem. Hopefully better brains than us will find solution soon..
                        It can be a benefit of the fault codes generated in the later cars such as the XK8 onwards – my 1987 Cab does have some codes (Lucas 9CU ECU) but literally about seven, read via the trip computer. I think the later 4.0litre had about 30 codes in the 15CU ECU..

                        Best of luck, Robin

                      • #10827
                        Gerard Harings

                          …..i do have the same prob. i was thinking about changing al of the sensors and  all thing wiht a cable thads atatcht to the engine….98000 km on the odometer…

                        • #12492
                          Steven Foster

                            One sensor that no one has mentioned is the thermotime sensor.  This sensor runs the cold start injector system.   I too am experiencing cold start issues but shes fine when run for a minute.  I have changed the sensors like yo all have said and also have a modified aj6 engineering ecu fitted.  I am currently awaiting the arrival of a thermotime switch to swap in….. We shall see what happens then ….

                          • #12544
                            Norman Thomas

                              Hi Dave,

                              Just to add my input into the discussion – you have checked a lot of the peripherals and I’m just trying to establish from the chat what you have done with the ECU – I would recommend that you get the ECU checked out by Roger Bywater at AJ6 Engineering – I don’t imaging that there is anyone else around who has half the knowledge of these early injection systems as he does, and I can’t really imagine anyone in any current repair shop understanding this (antique) system either. Check the AJ6 Engineering website for info.

                              I’m speaking from experience as well – my car was running fine (1986 3.6 manual cab), then started to develop erratic starting problems till it finally gave up the ghost. Roger at AJ6 discovered a lot of dry joints in the ECU (seems like the most common cause of any ECU problem) – upon refitting the repaired ECU, the engine was a different beast altogether – smoother, more responsive, and even more fuel efficient!

                              I think we try to imaging ECU problems as the last resort but if your car has the original ECU, then at over 30 yrs old, its likely to be suspect.

                              Just one other check first – does your car have the vacuum pipe connected to the ECU – make sure this is OK (not leaking) as well – the correct function of this has a big impact on the engine operation.

                              Hope this helps, all the best, Norman.

                            • #14406
                              Gerard Harings

                                ….a brohter of me told me to replace al the vacuum hoses and conections….after al they are 32 years old….so i disconect the inlet magnefolt and now work on it to change al the hoses…keep u posted…


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