Drivablity improvement

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    • #1593
      Laurence Jones

        Having now completed 600 miles on my newly restored Cabriolet (with only a snapped bonnet release cable causing an issue) I have some niggles to resolve.

        When the car is cruising at a constant speed (any speed), or over-running (decelerating with foot off brake and clutch), and I put my foot back down on the accelerator (reasonably gently for a smooth increase of speed), the car/engine dips for a moment before it increases in speed.

        This makes the car not very smooth to drive, in traffic or cruising on the motorway, as every time I want to increase the speed slightly this happens. If I’m accelarating heavily then this does not occur.

        Can anyone help and throw some light on this please.


      • #1595
        Mike Self

          Hi Laurence

          I have the same / similar issue with my car and have tried several things – all to no avail. Hence if anyone has any ideas in this area, then advice would be much appreciated.

          I also note that when the car is stood on tick-over, pressing the accelerator causes a slight hesitancy from the engine before it picks up. As you say, while driving, this manifests itself as the engine dipping momentarily prior to it increasing in revs.

          Kind Regards


        • #2422
          Norman Thomas

            Hi Laurence (and Mike),
            I’ve just joined the “club” so I having a look around and have noticed your question. You don’t mention which engine you have but it may not matter. From your description, I’m guessing that you have flat spot in the throttle response just as you press the accelerator.
            My 3.6 had this problem and I eventually pinned the fault down to the throttle potentiometer – I took it apart to investigate and saw that the tracking inside was completely worn out just on and after the idle position, meaning that you need a bigger application of throttle to get the throttle signal. There are two pick ups which sit on the tracks – where the tracking is worn out, then there is no signal continuity from the pot.
            Renewing the potentiometer completely cured the problem – its a bit of a fiddle adjusting the new one but quite straight forward (as usual, getting underneath the car is probably the biggest problem!).
            On this topic, I came across one web site where the chap repaired the tracking by re-soldering new tracks (there are two inside the pot) but I think that it is better to buy a new one.
            Hope this helps and well done with the site.
            Kind Regards,

          • #2488
            Laurence Jones

              Hi Norman,

              My Cabriolet still has this issue – we took it for a drive out to the Cheshire countryside yesterday and it was fine while we were cruising but a pain when starting off from traffic lights/roundabouts and changing gear. I will take a look when I’ve some time at the potentiometer – Does anyone know if this is an item I can get a new replacement for (UK).


            • #2491
              John Bleasdale

                Would suggest your first port of call Jaguar Classic Spares,


              • #2492
                Alessandro Loda


                  I’m totally new in this discussion and I’m new member also but I have similar problem with my AJ6 ENGINE 3.6 litres but only when cruising around 1500-1800 rpm, then this delay in response from engine disappears..

                  I found and interesting article at http://www.jagweb.com/aj6eng/richness.php

                  Has somebody some experience with retuning of the ECU ?

                  Thank you


                • #2908
                  Stig Moltu-Jacobsen

                    Hi! My ECU was repaired and upgraded by AJ6 Engineering. My 1985 3.6 ran too rich and soot tended to build up somewhat over time. This was visible on the plugs and on the very ends of the exhaust tail pipes. The fuel consumption was also a bit high. After the ‘operation’ no soot builds up, the plugs are clean as are the pipes. However, I still need to adjust something, but don’t know exactly what. The car runs perfect over 2000 rpms on the highway, but a little uneven when going down a hill or around town at low rpms. I will check the ignition timing, the throttle potentiometer, the fuel pressure valve and maybe also other relevant parts. Does someone else with a 3.6 have any input ?

                  • #2928
                    Norman Thomas

                      Hi everyone again – good to see the engaging discussion and Bleasie – good to see you back on the circuit and that your hard work with the original Cabriolet register lives on.

                      (PS. all of my comments refer to the 3.6 6 cyl AJ6 engine)
                      At the end of the day, there are several components in the fuel injection system and unless you are well versed in fault diagnostics (which I guess most of us are not) and if you don’t have an “expert” at hand to pin the problem down to a specific component, then fixing these problems tend to be a process of tiresome elimination – so its best to pick the cheapest/easiest things first as its easy to spend a lot of money replacing perfectly good components (I’ve got a few in the garage!).
                      Back to AJ6 engineering which Stig and Alex mentions, this is a top outfit run by Roger Bywater – what he doesn’t know about the Jaguar fuel injection systems isn’t worth knowing. I think the mod that Stig has had done is referring to an idle richness problem on the 3.6 engine and AJ6 Engineering fit a small potentiometer in the ECU as part of this mod – this pot can be adjusted to give the best idle running (instructions are supplied with the mod). I don’t think that this problem would contribute too much to the flat spot which we are discussing – I had this done when trying to sort my engine out but it was the throttle pot replacement which eventually fixed it.
                      However, it may be a good idea to send the ECU to AJ6 for checking (not particularly cheap but good for peace of mind).
                      I sent Roger my engine ECU after the engine conked out (and after checking everything else in the ignition system) and he found a load of dry joints inside – upon refitting the repaired ECU, it was like having a new engine – fabulously smooth and improved power and fuel consumption compared to before.

                      Laurence – back to your comments – I bought a replacement throttle pot from SNG Barrett, but the other Jag parts specialist probably also stock these.

                      Also worth checking is the vacuum pipe which runs from the engine to the ECU in the boot – simply remove the pipe from the side of the ECU and if the idle changes dramatically, then the pipe is OK.
                      Don’t forget the injectors – these are all old engines now and any engine will benefit from an injector clean/check – there are a few places who do these – I’ve used “Injectortune” for this – remove them, send them off and you get the same ones back completely overhauled.

                      You may have gathered by now that I’ve been completely through the injection and ignition system on my engine in the quest to get it running like a new engine – I think that I’m more or less there now.
                      Don’t forget that these engines need to be used and don’t be afraid to use the power (when you legally can) – a good “clean out” works wonders!
                      Hope that these ramblings help……..

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