The Jag starts fine and goes into idle then revs up and dies!
New fuel pump and new relays. If I open the cap and hold the fuel pump relay pressed, it runs fine.

I am getting power but it seems to indicate the FCU - mine is a FCU 8CU - 84488a DAC 3831. I opened it to inspect solder joints and noticed it has been worked on.

Do you guys know if it is compatible with any other FCU's?

PS: I found a great article if you guys feel like reading.

LUCAS EFI / AJ6 Engineering

 

Mmmmm, probably a 6CU, but I would have thought a 16CU on that year model in that market???.

No matter.

GOOD diagnosis, thanks heaps, it really does help more than most believe.

It sounds like the ECU fuel relay control circuit is hissy, very common on the 6CU, hence the 16CU upgrade, which is a plug and play replacement.

Look at the fuel pump relay base, the black one, and you will spy an Orange wire, found it, good. That is the earth wire for that relay and is controlled by the ECU timer circuit etc. Ground that Orange wire, the fuel pump will now work whenever the Ign is ON, SO BE CAREFUL PLEASE.

If that solves your problem then the ECU is most certainly your fault.

 

FCU 8CU
84488A
DAC 3831
Is it compatible with any other FCU's? Or need to send it for repair?
Anyone know how much to repair them?

 

I gotta drink MORE.

MY HUMBLE APOLOGIES.

Yours is a 3.6, Bugga, I reckon what I said is so wrong to be downright silly. I dont think the 3.6 had all that extra stuff in the boot, never seen one, they did not come here.

HOWEVER, the fuel pump relay you appear to be pushing the contacts closed on to maintain engine running, is still being "controlled" by that ECU, and it may well be the Orange wire same as the V12's.

I dont know of repair people in your part of the world, but down here Bosch EFI Repair Specialist Agents can do that task. I use Petro-jet when required.

 

Thanks Grant. I appreciate the effort.
Anyone else might know if I can use the upgraded 16CU?
I've read that it all has to do with the engine's compression?

 

As I understand it the 16CU is V12 only....not sure if/how it could work on the 3.6 six cylinder.

Cheers
DD

 

OK, the 6CU and 16CU are V12 specific, so cross them off the list.

Been thinking all day, haha, strange but true.

Since the early XJ40 had the same set up, and are way more common than your configuration, may I suggest asking this over there. I know double posting is frowned on, but I reckon you will be OK in this case.

Our 3.6 XJ40 had similar issues, among other things, and simply replacing the fuel pump relay fixed it. I did this at the time coz the fuel pump relay on them was deemed by all I spoke to, as a real PITA.

 

"The ECU part number DAC 3831 was specific to the 3.6 XJS.
They are all now getting towards 30 years old and having spent their lives in the boot of a car subjected to damp and vibration it is hardly surprising that do go wrong sometimes. We repair lots of them but if you can tell yours has been repaired it probably wasn’t done by us.

Failure of the pump drive circuit is quite common, but often there will be other issues that may be less apparent. We can repair that sort of thing. A repair is likely to cost around £200 - £225 depending on the problem(s), postage and VAT not included.

There is another problem that all those cars suffer from and it is described in detail here on our website:- A COMMON PROBLEM WITH THE EARLY 3.6 XJS (1983-87) / AJ6 Engineering

The price of that is now £225 plus postage and VAT (if applicable). If done at the same time as a repair the total can usually be reduced by £100 or so by running the two jobs together.

The 8CU system is similar to the V12 6CU system and is descibed on our website here:- LUCAS EFI / AJ6 Engineering

Best regards,

Roger Bywater.

 

Since I started working on this car AFTER the fact that it wasn't running, I could not tell if the fuel pump runs ALL THE TIME or it shuts off periodically when it senses the fuel pressure has been satisfied.
Just a concern, but grounding the orange wire will keep it running when the car is on. Is that something you guys do to it?
I bought new relays and tried but still get the same problem. The engine will start and run for a few seconds and die.

 

I replaced mine to a 16CU and it fixed a TON of problems, including yours. Often times mine would even leave the injectors WIDE OPEN and flood the hell out of the engine. Definitely change that junk out.

EDIT: Nvm, didnt notice you had the 3.6 and not the V12

 

OK, reading all this again, more careful this time, and coffee only at teh moment.

IF, and I mean IF the 3.6 has the same relay set up as the V12, and it possibly does, as in 2 relays on the panel up alongside the battery, then the relay in the black base is the fuel pump relay, and you will have that Orange wire going to the ECU.

Your issue is that relay is not staying energised, and that is the ECU.

Earthing that Orange wire is common on the V12 cars for diagnostic purposes, it takes ECU control of the fuel away. The fuel pump runs all the time,the ign is ON, so mostly so does the engine, GOOD.

I cannot find a "swap out" ECU for the 3.6, so what you got, you got.

I would earth that wire and drive the car, only CAUTION, is if you sit with the Ign ON, and the engine NOT running the pump will run constantly, but who does that anyway, intentionally. The inertia switch will still kill that pump if you tip the car on its lid, so safety is still basically there. As I said, if the engine stops (stalls) then the pump will continue running as long as the ign is ON, but most people switch the ign OFF as soon as it stalls, in my opinion.

My V12 ran for 2 years with that wire earthed, until a 16CU popped up, no harm to me or the car.

Also note that the pump runs constantly whenever the engine is running, and the unused fuel is returned to the tank, NORMAL. It is NOT a pump cycling system as you suggest.

 

Thank you Grant. I think you just solved my problem. All I wanted was for it to run without having to "Mickey mouse" the relay with a clamp to keep the pump running.
I agree with you, that nobody normal would leave the ignition on for any period without starting the car.
I will ground the orange wire until such time as I can find another FCU 8cu at the used car yard.
I took the FCU apart and saw that some work or soldering had been done in the past. Someone used solder and wires to create or jump PCB points, due to the board's pathways peeling off or burning out!!! It didn't look that bad but it is noticeable.
Problem solved!
Now I need to find the real value for this car.
It was made in GERMANY to test on the autobahn. There were only 25 made.