Series 3 XJ6 ECU Upgrade

I have decided to bite the bullet and replace my stock ECU in my 84 XJ6 with a Haltech Elite 750. This is an excellent system readily available here in Australia with a good reputation for support, and I’ve heard of it being used in many older cars. I am an electronics technician with 40 years experience so will be undertaking the work myself. Plus, I have owned the car for 20 years and am very familiar with it to say the least, having fully restored it. I’m also experienced enough to ask advice from anyone who may have done this before. I’m especially interested if anyone has advice on replacing the injectors. Do I order new standard injectors or is there a good replacement that fits without too much work. Any and all advice greatly received.


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Just to summarize the function of the original ECU, Pete…

Using a standardized fuel pressure it computes how long the injectors are opened (once every engine revolution, triggered by ign amplifier) to deliver fuel/correct mixture, based on engine temp (CTS) and amount of air into the engine (AFM flap deflection). The internal circuitry includes functions like cranking enrichment and acceleration enrichment (‘acceleration pump’). Two separate external items, independent of the ECU, provides cold fast idle (AAV), and cold start enrichment, choke functions, (thermotime and CSI). The externals’ function is there to simplify the computations done by the ECU…

All these are necessary criteria for engine performance and is, in various guises, incorporated in all fuel delivery systems - carbs or EFI. And likely the case for the EFI you mention - the ultimate output for all is appropriate fuel/air mixture at all engine temps and any combination of throttle setting, load and rpms…

The injectors simply deliver an amount of fuel depending on their opening duration (pulse width) and fuel pressure. If the EFI in question does the same - the injectors fitted will work as is…

I’m not familiar with the EFI set-up you mention; the xk ECU is of course based on analogue inputs and computations - electronically complicated, and not very precise, but works. The gist of all fuel delivery system is to satisfy the engine’s requirements - which are independent of the delivery systems…

xj6 85 Sov Europe (UK/NZ)

Thanks, Frank. I’m very familiar with the L Jetronic system, Its a very crude but remarkably effective system. I studies this in detail a number of years ago, after having to re-calibrate the potential divider in the AFM. Like many of us, I inherited a car that had been fiddled with to correct a fault. My reason for upgrading is a multitude of problems. A sticking AAV, one injector doesn’t like the cold, another goes open cct when very hot, such as when idling on a summers day. My current issue is the ECU has decided to give up the ghost when it gets warm. I’ve repaired the under-bonnet wiring loom on a number of occasions, so time to renew it all and add the reliability and driveability of a modern system. I’m not chasing horsepower, I just want a reliable modern system. My question on injectors is twofold. Firstly, the impact of going from multi-point to sequential injection and the impact on duty cycle of the stock injector. Secondly, injectors have come a long way, so has someone found a modern injector that is readily available and works well in an XK engine.


Sounds fair enough, Pete - but as I don’t know how the proposed EFI works, nor the changes required, I’m somewhat handicapped. Can you enlighten…?

xj6 85 Sov Europe (UK/NZ)

I think it’s a fantastic idea to replace the 30 year old system with new tech. Definitely a way to find out just how well the old XK engine can run.

Just tossing out the distrubutor and going to Megajolt/EDIS on my tri-carbed engine made a huge improvement.

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I’m not sure it’s fair to describe it as such, based on one car that clearly has an issue or two…

You might find some interesting stuff here though:


I have all the stock injectors from like 5 plenums…I will glady pull them and send them (free)…if you need any…tell me how to test…will do that also. I have a whole (minus the engine) '86 xj6 parts car. Actually, there is a whole another xj6 that is going to be crushed next week. Keep in mind this is Phoenix…there is NO RUST

Mechanical fuel injection… very different animal, very robust indeed but much more limited than EFI.

Keep in mind that it’s 80’s technology… the very begging of Electronic Fuel Injection and a big revolution at the time. And it worked fine for +30 years…
And when sorted it still works very well.

Better start you own thread Mitch.

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Thanks all for the comments. It looks like I might be breaking fresh ground with this project so will be sure to document what I do for the benefit of others. Mister_Mitch, all I would suggest is you start a new thread to get some guidance on the diagnostic process for your woes. Like I always say to our service techs, make a diagnosis not an assumption. The L Jetronic system was designed in the 70s and could be described as a glorified electronic carburetor. Its eminently fixable if you can get good parts. When you have so many worn parts - as do I - , then you have to make the call as to whether you want to fix it or go with 21st century electronics. I’m going with coil on plug ignition, sequential injection and a wide band O2 sensor. I’ve seen it done on an 84, 5 series BMW and what a difference it made. More to come.


The ignition ‘system’ is not awful, Mitch - you just have faults that defeats proper engine management functions…:slight_smile:

As Aristides says; start your own thread - while it may take some sorting, the fault may not be very serious,

xj6 85 Sov Europe (UK/NZ)

AussiePete, Do you have any update on how this conversion is progressing. Up in Brisbane and considering something similar.

Hi Mike, yes I successfully completed the installation project. It took me around 12 months, as I wanted to do it properly and have it look like it left the factory that way. I am very happy with the result. I have new Bosch Motorsport injectors on a new fuel rail, with new pressure regulator. Ignition is coil over plug, with the distributor working as a cam position sensor, in effect. The Haltech Elite 750 ECU is simply brilliant and the support from Haltech was excellent. I had it dyno tuned and the car now runs like new, with rock solid idle. Its not a project for the faint hearted and requires a lot of fabrication of brackets and making and running the wiring loom. I will try and write it up one day, but am so time poor at the moment I don’t see that happening soon. It’s well worth the effort though and turns a true classic into a reliable driver without all the drawbacks. To boot, it doesn’t affect what makes these cars so great.


Good stuff! A few piccies would be great.

I recall you from the old days of JL!

thanks for quick reply. if you can post some pictures I would greatly appreciate. A write-up of the steps and some tips / parts used when u find time would be of interest to many on the forum no doubt.

Here are a couple of pics of the mostly finished upgrade. You can see the coil on plug ign, Ignitor module on the firewall in the foregrounded and wide-band O2 controller against the firewall in the background. The Bosch idle air valve sits in the old IGN coil bracket and plumbed in quite nicely. The Haltech relay/fuse box sits on brackets in front of the aluminum ECU shield. I am going to move the ECU inside the car in the future, however, due to the heat in the engine bay. The AFM is replaced by a stainless pipe painted black with the new air sensor bung welded in. I used the old AFM mounts and bushes. The water temp sensor was the same thread as the original and I blanked off the thermotime switch with a threaded bung. You can’t see it, but there is a bosch TPS on the underside of the throttle body. I fitted the throttle spindle from the US spec XJ to do this and made a mount and spacers to fit. The distributor is now a cam position sensor, with 5 of the reluctor teeth ground off to give a “home” signal to the ECU so I can use sequential injection. There is a 36-1 toothed wheel on the harmonic balancer. I made a new bracket for the SARD fuel pressure regulator. I am running 3 bar fuel pressure on new 4-hole bosch injectors in my custom-made aluminium rail.

The whole project is fully reversible should I have some perverse idea of going back to original in the future. However, the XJ drives great now, so smooth, rock steady idle, no flat spots and tonnes more torque than before. The Dyno graph had 400Nm at the wheels from just off idle to 5000 RPM ! No more power than stock though, but I wasn’r chasing horsepower. I wanted it to look like it left the factory this way and be a reliable classic. I have loved these cars since I first saw an XJ as a kid in the seventies and have owned my 84 for 20 years.

A really worthwhile upgrade if you have the time, patience and requisite skills to do it. I will write this up one of these days.


Great job Pete…!!
I bet you are very pleased, a well worth effort.

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Well done!

I take my hat off to you Sir. It does look like a factory option. I am kidding myself if I think I could pull this journey off. u will have to make this into a kit to fund your retirement so the rest of us can benefit from your know-how. Kudos !!!

PS : check out Ole Martin Mobeck Tekniske i emailed him and he is working on an xj6 kit this year. fingers crossed… v12’s cars/kits are way past my pay-grade : )

Funny, I was all excited this post was recent, but I just realized it was 2023 not 2024. But… I am still excited.

I have been battling L-Jetronic issues for a long time and am thrilled with the Haltech info and the MobeckTekniske XJ6 kit possibility.

I have been studying AussiePetes words and pix and the Haltech web site and totally want to go this route. I plan to try to put together a step by step list of parts, costs and installation steps to get me started. I am hoping ANY others that have attempted (or even thought of) this Haltech scheme will help me out… help us all out for others wanting to do this.

And AussiePete… I know you said your time is too tight to do a total project writeup, but maybe you would be interested in being “editor” for anything I put together. And to be honest, I an VERY willing to send some cash your way for your consulting services.

Let me get started an post some sort of “draft” and we can all see where it goes from here. Regarding the XJ6 kit from Mobeck, MikeDockray… any idea if that came to being??

Thanks all - Randy in Raleigh NC USA

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Let’s see if @Mobeck responds.