84 jaguar xjs v12 fuel injectors stay open

Well I’ve been messing with this car for a little while now and I’m officially stumped. The injectors on the drivers side of the motor are open all the time and the right side pulse normally. Would this be the ecu or a short in the wire that sends the pulse to the injectors? I’ve done a bunch of testing with a continuity tester only to find out that any ground sets it off. So what should I try next or should I just give up before I’m in over my head? I got the car for free and was hoping to not put a bunch of money into it.

For most cars the ECU controls the injectors by completing a ground connection for the injectors inside the ECU. If your engine design is that way then the ECU may be causing the trouble. You may be able to prove that by checking the resistance of each of the pin connections on the ECU to the injectors with reference to ground.

I’ll second Cougar’s advice.

Yes Mr. Cougar has it right yet again…he seems to know his stuff…

The Ecu sends ground “pulses” to the injectors to open them to fire… If an entire run of injectors is having the same issue then you need to use a continuity meter to check the negative side of those injectors.

Go on the net and look up your ground wire for the injectors…or just use a 12V multi meter to see which wires are Hot…and which are NOT>… when you find this then use the multi meter on the continuity setting and test your neg wires of the injectors with one lead and ground the other lead…if you find each and every injector neg wire has continuity to ground then your may have a faulty ECU OR more likely…an abraided wire somewhere on that side…grounding all of your injectors simultaneously… Jags have abysmal electrical systems…so I am suspecting theculprit may be the ECU…go to a salvage yard to source one…OR I might be able to find you one in my U-Pull-It salvage yard. An ECU from Jag will FLOOR you with the bill…so thats out… Start the testing procedures…

You GOTTA spend some money on a repair manual. Bentley is the best (aside from factory) and they make one for your XJS:
If you want any chance of keeping this beast running you have to have a manual. It’ll be the best $100 you spend. If that’s too much money, ditch the Jag.

In addition to the excellent advice given, you might also check the wiring for shorts. The car is 27 years old, and the Price of Darkness lives under your hood. It may be the ECU, but it might also be wiring. Check first, buy later.

Hey thanks for the advice I’ve called around to all my local salvage yards and can’t seem to find an ecu for it. If you can look in yours and see what you can find I’d definitely be willing to try it. Thanks again.

Hey guys thanks again for all your help. I’d really like to get the jag running right before I give up on the v12. It’s only gonna be a cruiser so I’m not in a big hurry to get it going and have time to properly trace the wires an go from there. If it ends up being the ecu I might end up going with the small block chevy conversion. At least that way I can make it simple for me but that’s a whole other project!

On another note the car has only had 1200 miles put on it since 2001, and only has 65000 miles on it altogether. So I’m really hoping it’s a bad wire and not the ecu.

Absolutely the best resource I’ve ever found for Jaguar owners: http://www.captainjaguarscathouse.com/

Parts available, and most importantly, repair manuals online and free for download. Try this, you may find all your Jaguar answers here.

There’s a simple explanation for low miles on a Jag - and you’re experiencing it.

Just curious - why would you want to go through all the trouble of an engine swap? You end up with a heavy XJS with all the unreliable wiring and accessories still in it…several thousand dollars later…

After looking this issue over again I thought of another thing that might be causing the trouble if the problem isn’t with the ECU. There may be CAM sensor problem. I think the ECU depends on the CAM sensor signal to know when to open the injectors so making sure that signal is doing its job would be a good check. There may be a problem that is causing half of the whole signal not to be working as it should be so half the injectors aren’t firing. You will most likely need a O-scope to check that out. Another thing to do would be to check for any error codes from the ECU. If the CAM sensor was a problem there should be a code set for it but if half the signal isn’t working I’m not sure if the ECU would pick that up and set a code error.

If the ECU is the problem you should be able to get a refurbished unit from some place at a reasonable cost.

Does the ecu have to match the fuel interface relay module? The ecu is a dac 3586 and the module is a dac 2589. Could this be the problem?

read this:


I think the DAC 3586 and DAC 2589 are part numbers.