V12 engine Problems

Hi all im a newbie here and live in the uk. I recently acquired a 1978 xjs which was running very badly and obviously not on 12 cylinders. I have owned v12 jags in the past but dnt have to do any work on them apart from servicing as they ran fine. After checking the usual, plug leads on correct plugs, all plugs sparking, cylnder compressions all ok. The wiring harness for the injectors was in a terrible state having being repaired more than once and was very brittle due its location in the v. I made a new harness and fitted it and the engine seemed to run worse than before. I checked the wiring I had done several times and all seemed ok. I saw on a site that one connector of the injector plug should show 12 volts and the other 0 volt,Iis this correct ?. Every one of my plugs showed 12 volts on each connector. The engine is firing on cylinders 2 and 5 on A bank and 1-2-4-6 on B bank. I have a inkling that the problem lies with the ecu which lies on the front crossmember above the radiator.
I don`t want to spend a small fortune on a replacement ecu if it is not the problem.
I would gratefull if anyone could shed any light on my problem.
Thank you , Chris.

With ignition on and not running, they will show 12V on both terminals. This is because 12V is fed to one terminal and the injector is energized by the ECU grounding the other. As long as it is not grounded, the second terminal will register 12V simply coming through the coil from the other terminal.

Your '78 should have D Jetronic EFI, which fires three injectors on one bank and three injectors on the other bank all together as a group of six. To get the arrangement of firing you claim would require a screwed up injector harness, I think.

And the ECU isn’t on the crossmember over the radiator unless someone moved it. There is a MAP sensor in there somewhere, although I expected it to be on one of the fender wells. The MAP sensors are often a problem, as the diaphragm can get cracked and leak.

Have you tried simply changing the spark plugs?

Hi Kirbert, Thanks for your quick reply. My car has indeed got the D jetronic EFI as you said.I may have got mixed up saying the ECU instead of 2PA power amplifier on the crossmember. There is a MAP sensor on the inner wing or fender as it is called on your side of the pond but I dont think it would cause the problems I have. I may be wrong. I have swapped the injector from a non firing cylinder with a firing one but its just the same. As I said i have checked the harness I built over and over and it seems fine. I have also swapped the spark plugs which appear fairly new but again, all the same. Could the trigger board or injection rotor inside the dizzy be at fault ? I did a lot of work on Jags in the 70s and 80s, mainly MK2 and XJ6 but never had the opportunity to work on injection cars so I am picking things up as I go along on the internet and can only go with what I find there. Any help would be much appreciated. Best regards to all, Chris.

How did you find which cylinders are firing ?
It looks like a strange combination.

Hi Richard, thanks for your interest. I checked which cylinders were firing by pulling off the spark plug leads one at a time and double checked by pulling off the injector plugs one at a time. The spark plugs from the non firing cylinders are very wet but are still sparking. I can`t figure out if it is a ignition fault or injection fault. I have been trying to solve this for weeks and at this point I would be pulling my hair out if I had some.
Regards, Chris.

That’s kind of a contradiction…
If the said plugs are indeed sparking they should not be wet, unless the cylinder gets massive amounts of fuel of course…

Christopher,
I have done a basic test of fuel injectors in situ that may be useful to you. I have done this test on my fuel injected XJ6s and well as our 1990 XJS convertible (5.3L V12 with Marelli ignition) as a relativity quick test that all the fuel injectors were firing and the repairs that I made to fuel injector harnesses were good.

I first installed a fuel pressure gauge on the fuel rail by installing a “T” fitting. Then I pressurized the fuel rail by turning the ignition on, but then turning the ignition switch off once the fuel rail was charged. Then I pulled apart the fuel injector harness plug to expose the pins. Finally I used a 12V battery to apply power to the correct pins to fire each fuel injector group one by one. If the harness and the fuel injectors work as they should you will hear the fuel injectors click and see a drop in the fuel pressure on the gauge. The details of what color wires goes to which fuel injector is found in the Jaguar XJ-S Repair Operations Manual (ROM) and the S57 Electrical Guide.

In addition to just listening to the fuel injectors you can use a mechanics stethoscope to independently verify that each fuel injectors is clicking when it should since they fire in groups.

You have an earlier version of the fuel rail so I am not certain of where you might connect the fuel pressure gauge on you engine.

This was the method that I used to discover that the recently serviced fuel injectors that I installed during the engine bay restoration of my wife’s 1990 XJ-S convertible a few years ago had all gotten “stuck” and we’re not firing. The results of this test positivly pointed to a fuel injector problem and ruled out an ignition problem.

I returned the fuel injectors for reservicing and after I got them back the engine started and ran perfectly. I had not idea that there was a shelf life on serviced fuel injectors. In my case they sat for a few months before installation.

Paul

Could be the trigger wheel itself. It’s a plastic wheel with 12 ferrite inserts. Some have found one or more ferrite inserts have gone missing, each of which disables one cylinder.

Tell us about your trigger board. Does it have three wires or four? The early models had three wires, inside had a pair of magnetic reed switches, were notoriously unreliable. Jaguar replaced these with a 4-wire model that had Hall effect switches inside, thereby required a 12V power supply and hence the fourth wire spliced into ignition power somewhere. Needless to say, needs to be a good, solid ignition power source.

The two different trigger boards required two different distributor rotors. Both have a magnet in the tail, but the magnet is oriented differently for the reed switches than for the Hall effect switches. If you’re using the wrong rotor, it won’t run well.

The Opus ignition system is a known weak spot. Many opt to replace it with either the later HEI ignition system or an aftermarket, and some have reported that the car runs noticeably better than with the Opus even when it had been working well. There are even kits to upgrade the Opus to an HEI while keeping it looking like an Opus for those who value originality. Still, I am not sure a failing Opus system would cause a consistent misfire in particular cylinders as you have noted.

Hi Paul, thanks for your info. I removed a injector from a cylinder that wast firing and connected two wires to it with one side of the plug to positve on a twelve volt m/c battery and briefly touched the neg side of the battery with the other wire and the injector clicked and squirted out what must have remained in the injector, so it would seem that injector is working but the cylinder still does not fire. I must admit I only did this test on one injector but intend to do them all. There doest seem to be any where on my fuel rail where I can install a T piece or gauge. I followed a wiring diagram from the internet to makeup my harness but am now beginning to think this diagram may be wrong. Do you know where can I find a ROM andS57 Electrical Guide.
Regards, Chris.

Hi Kirbert, I have yet to remove the dizzy cap but my ignition system is lucas with three wires to it and the internet sites I have visited say if you change to the later trigger board with 4 wires the fourth one should go to to the pos side if the coil. I am going to be busy the next two days with hospital appointments.but I will remove the cap on Saturday and see what I find and report back. I will try to find an aftermarket ign in the uk. Thanks for your help. Regards, Chris.

Christopher,
I don’t have any pictures handy of the fuel pressure gauge installed but attached is a picture of the 5.3 L V12 engine bay in my wife’s 1990 XJ-S convertible with the hose that I “T” in the fuel pressure gauge circled in yellow. It is on the left side of the fuel rail between the fuel rail and the left side fuel pressure regulator. I cut the hose that was there the first time I installed a fuel pressure gauge and then bought a new replacement one and that is what you see in the picture.
I suppose that you could have a number of issues. If I were you I would start by testing each fuel injector in situ electrically, as I described previously, and find out what is working and what isn’t working. Removing and replacing the fuel injectors may not be necessary and may save you a lot of time. Unless those fuel hoses are old and need replacement anyway.
Search eBay and you will find several hard copies of the Jaguar XJ-S Repair Operations Manual on sale. Hard copies of the S57 Electrical Guide are a bit more difficult to find, but I got mine on eBay. It has the info from the ROM but in an easy-to-use format with wiring diagrams and equipment location info. I do most of the work on our five Jaguars and find the ROM and S57 essential to me keeping them running. Having the correct technical data has removed a lot of mystery (misery?) out of working on these cars.
BTW, I only have paper copies of these documents and others will have to chime in about digital copies.

Paul

Hi Paul, thanks for your pic, I wish my engine e bay was as clean and tidy. I did intend to test the injectors tomorrow but I have hospital appointments for the next two days but I will do it over the weekend. I will use a 12v m/c Battery and two leads after disconnecting the plugs with the engine off so I can hear the clicks better. If this is not a good idea please let me know.
Regards, Chris.

Christopher,
Thank you for the kind words. I have cleaned up the engine bays like this in four of our Jaguars so far and I find it a very rewarding effort.
You can also use one of those small rectangular 9 volt batteries used for small electronic devices. 9 volts is plenty to get the injectors to fire and easier to work with. I have found that an inexpensive mechanics stethoscope is really handy to listen to the fuel injectors.

Paul

Hi Paul, thanks for the tips. My car is actually a 1978 xjs with a paul banham fibre glass body fitted at his factory and looks nothing like a regular xjs. When I figure out how to post pics I will put it on the site.
I wish I was as good at electrics as I am at mechanics, but you can`t have everything in life and I may be too old at 70 to learn new tricks . Off to bed now as its nearing midnight here. Regards, Chris.

Christopher, above the window where you are typing there is a line of icons, the 7th from the left is where you go to upload pictures, just follow the prompts, a hint, once the scripts have loaded place a space between each photo like this; make sure you pick up the exclamation mark ! !!

We’re not talking about your ignition system. We’re talking about the trigger board for the EFI, which is inside the distributor just under the rotor.

Theoretically that’d be good, but I seem to recall some reports that that location is a bit too “noisy” and that it’s better to connect it somewhere else. Perhaps someone here has gone through it themselves and can report.

My screen doesn’t look anything like that!

Because you are likely using your email to read and respond.
The “https://forums.jag-lovers.com/” interface is the same. Might look slightly different on a Mac vs PC, but that’s about it.

BTW, if your picture is the correct format, you can now just drag and drop

Here is mine (sorry for going off-topic), where one can see the upload button

Oh, on the reply subscreen! Yeah, I see that.