I have just purchased a 1989 Vanden Plas XJ12.
The car was not able to start when I purchased it.
Changed out the old 6CU computer to a 16CU and it started but the engine knocked under acceleration. I tried to set timming but it killed the engine. I suspect the pulley with the timing marks had twisted on the shift rendering them useless. So as a short term fix I just retarded the timing until the knocking went away.

All was good for about a month.

The car started to randomly act up, not wanting to start, or run rough. What I suspect was over fueling. Also the knocking returned.
When it acts up and does not want to start the fuel pump will run continously when the ignition is turned on (not just prime for a couple seconds).
Seems like the computer thinks it is running when it is not. ??? ? ?
I suspected the shelded wire going to the ignition amplifier. If I wiggled it the fuel pump would stop. So I changed a section of it and the problem went away for a couple of days but it now has returned and is permanent....which I guess is better than an intermittent problem.

Things I have checked or tried:

Checked for continuity at ECM connector between pin 18 and ground - No continuity

Completely disconnected ignition amplifier - pump still runs

Removed injector resistor pack - pump still runs

Unpluged Injector harness - pump still runs

Reinstalled old ECU - no change

Removed fuel pump relay - pump stoped obviously

Checked that there was power supplied to relay - yes

I'm at a loss. Thinking maybe the ECM is acting up?
Not sure but I think the old one was doing the same thing.

Sorry for the long post. Thanks for reading!

 

Oh one other thing. I also changed the coolant temperature sensor and checked the resistance reading at the ECM connector. It was within specifications.

 

A little weird.

The circuit of the ECU which grounds the fuel pump relay is known to give problems....but usually in the form of being outright inoperative. Your problem is the opposite: always operative. Strange things happen but two ECUs giving the same rather unusual fault? I dunno. Bit of a stretch.

The ECU also takes a 'start' signal, from the starter relay, on Pin 26. A problem here might have the ECU believing the starter is engaged and turning on the fuel pump in response? Bit of a stretch, again, but might merit checking. Pin 26 should show 12v "+" only with the key turned to 'start'. Frankly, I'm not entirely sure that the 'start' signal is intended to operate the fuel pump, but it seems plausible.

Any sign of wire tampering by the previous owner? That seems more likely, IMO.

I'll do s'more mulling.

Cheers
DD

 

As Doug has mentioned.

AND

Look at the wires at the base of the fuel pump relay, and you will see an Orange wire. That is the EARTH for that relay, via the ECU timer, and then the ECU circuits once Ignition pulse is recognised by said ECU.

A quick diagnostic "fix" is to ground that Orange wire seperately, thus allowing the pump to run whenever the Ign is ON. Should be returned to spec once the Gremlin is sorted.

Next is to sort if the thing has spark.

Setting the timing at a static 10deg BTDC before any further attempts to start would be a top idea to save detonation damage.

Being a Sedan, you will still be a Lucas system, which is sooooo simple to sort.

 

I just might add one other point.

the first post suggests that the marks on the front pulley are no longer indicative of the position of the pistons. An elastomeric coupling has detiorated?

So, find the "real" firing TDC of number 1. Set static as Grant says.

Carl

 

Thanks for the reply guys.
I'll check your suggestions and get back to you.

It does have spark. It will start and drive if I keep the throttle open but run roughly and it uses A LOT of fuel. On my limp home it used one of the tanks of fuel (45kms)

One other thing worth noting if I remove and connect the wire from the ignition amplifier to negative terminal on the coil it will fire the right side of injectors (with ignition on).

Same will happen with the shielded signal wire if I disconnect and reconnect it.

That's why I think the ECU thinks the engine is running. I think the fuel pump is doing what it is told but I will check.

Wiring seems to be not too messed with. Looks like a couple repairs is all but I would like to refresh it all. Much of the wiring under the hood is getting hard and brittle.

 

Checked and it is operating properly on my car.

 

[QUOTE=Grant Francis;1925638]
A quick diagnostic "fix" is to ground that Orange wire seperately, thus allowing the pump to run whenever the Ign is ON. Should be returned to spec once the Gremlin is sorted.
/QUOTE]

Ran a jumper wire as you suggested and it made no difference to the cars operation but the fuel pump was already running when ignition is on.

 

I suspect so or I have read that there is a key on the pulley that often fails. I have not had the time yet to sort that issue but need to shortly.

Thanks for everyone's tips so far.

 

Eric,

My world of assumption screwed that up a tad.

My meaning was that sometimes mechanics INSTALL that jumper to that Orange wire, sort the issues with teh ebast, then forget to remove it and return the fuel pump controlll system to the ECU.

If that is NOT the case with your car, then the points INSIDE the fuel pump relay may well be "welded" together, rather common, and usually related to a tired pump that is drawing way too much current.

If that engine is using that much fuel, STOP, you run a very high risk of hydraulic lock up.

You may have one, or both, of the FPR's (Fuel Pressure Regulators), with fractured diaphragms. Simply pull the SMALL vac hose from each FPR, if they are DRY, all is good, MAYBE. If they are wet, or stink of fuel, they are toast, and raw fuel is being sucked into the inlet manifold, and the cylinders. The RH and LH are NOT interchangeable, and the RH can be deleted if you so desire. The LH is the ACTUAL pressure regulator, the RH is more a pulse damper, and a PITA.

 

Checked the main and fuel pump relays, both are working fine. I took them both apart, cleaned their contacts and checked function.

Also if I pull either the pump will stop running.

I had pulled the Vacume hose from regulators and didn't notice any fuel but I'll take a closer look today.

Had a fuel pressure guage attached when I pulled the vacume hose from the regulator the fuel pressure dud risr so I assumed they were functioning properly.

 

Ahhh that make sense.
I didn't see any added jumper and with the ignition off there is no continuity between the orange wire and earth.

The ECU will keep the pump running if the fuel is leaking out someplace correct?

It is my understanding that the ECU just keeps the pump running if it continues to see a trigger pulse (shieled wire from amplifier?) If there is no pulse a capacitor (in ECU?) that will provide power for two aditional seconds then the pump relay would cut out.

 

OK, the ECU has no idea if fuel is leaking or not, and will cut the earth for the pump relay after the timer has timed out, unless that Ignition pulse is sensed.

Once the ECU senses that pulse from the amp TO pin #18 of the ECU, the whole system stays alive. Loss of said signal, kills the pump circuit AND the Injector pulse signal.

The MAIN relay supplies volts TO the PUMP relay, among other things.

The TPS (Throttle Position Sensor) may be AWOL, and if its gone full scale, common enough, overfueling is the result.

 

You may have stumbled onto something.

I'm not sure if the injectors should fire as a result of pulling and reconnecting those wires. I don't think so. BUT, more curiously, why only one bank? I strongly suspect a problem with the injector harness. Very common.

One common result of a harness failure is the injectors, all or some, being grounded at all times and dumping fuel furiously.

This would account for poor running and excessive fuel use. I can't imagine it causing the strange fuel pump behavior but, then again, perhaps it could result in some weird 'back feed' situation?



And staying on top of those wiring repairs is what keeps V12 owners out of the taverns. Well, me at least .

Cheers
DD

 

If the crank pulley key has sheared and the pulley turned off index, is the crank sensor on said pulley/damper? That would make a pulse and trigger the ecu, but at the wrong time. All kinds of thing confused...

 

Ahhh. Good thought....but no crank sensors on this model !

Cheers
DD

 

When I first purchased the car I did give the injector harness a quick check and taped a couple suspect spots. I'll pull the harness and make sure it is okay. I would like to make a new one/replace all the underhood wires but I am in the middle of building my home so just do not have the time.

If I disconnect the injector harness or resistor pack the fuel pump still acts the same.

 

I'll check it out. Thanks.

 

I thought the V12's batch fired their injectors. One side then the other. Wouldn't that make some sense then? Although I noticed the wiring splits the injectors into four groups so maybe I misunderstood.

Thank you for your help.

 

You've made me stop and think, which is never a bad thing.

From the ROM:

"The injectors are operated by the ECU in two groups of six. Each if further broken down into two sub-groups of three, although each pair or subgroup is operated simultaneously to make up the two groups of six twice per engine cycle."

I interpret this as all 12 firing at once ("two groups of six twice per engine cycle")...a batch of twelve.

I do know that if you snap the throttle open with key on, engine off, all 12 injectors will fire. Excellent for cold starts....the equivalent of a squirt of fuel from the accelerator pump on an old carburetor.

Cheers
DD