• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

90 XJS V-12 High Idle

Share this page

John;

As suggested, I installed the pressure guage in the trunk at the inlet to the fuel filter. PSI was not better. The original pump was a Lucas 4FP P/N 73351A. The replacement I was able to obtain for that P/N is a Pierburg pump P/N 7.21287.53.0. Pressure at the inlet to the fuel filter after 18 inches of hose from the pump is 38 PSI with the car off, but ignition on and 38 with the car idling at 1200 rpm and it does increase up to 42 PSI at about 2500 RPM but the engine is very rough, but with smooth idle at 1200. The AAV is still screwed in all the way.

The ECU is a 16CU P/N DAC 6337, with a Y in a circle followed by 84787B. The date code is 4788. Is this a fuel pressure regulator issue? It appears that the pump can produce higher pressure if it receives a signal from the ECU to do so as was indicated at the higher pressure at 2500. With the RPM increased to 3000 RPM, the roughness continued , but the pressure never rose above 42 PSI. The car was not moving during this.

If you think something else will be learned, I will install the pressure guage where you think we can find out something about the pressure and drive the car to record pressures, but it does not run too smoothly.
 
PS:

John, I reread what you told me to do and found I had not checked the pump outlet directly. I used a C-Clamp to squeeze the hose at the fuel filter inlet. I turned on the ignition and the PSI increased to 94 PSI.
 
You have a healthy fuel pump (it is not signaled by the ECU, it only responds to 12 volts supply). Take the drive test with the gage at fuel rail pressure. All indications are that it will be OK but it is worth checking it for about 40 psi under all conditions of vehicle performance. Beyond that about the only thing left is to change out the ECU to verify its performance. Are you certain that the ignition system is OK? The Marelli distributor cap has a reputation of unreliability but it usually results in an entire bank of the V12 not getting ignition rather than erratic performance. Can you create this rough running with the car standing still? I am thinking of other tests to isolate an ignition problem. I think the pointer is getting pretty close to the ECU.
 
John;

The roughness is with the car standing still and with anything above 1500 RPM.

The Distibutor cap was just installed before the last round of tests when I changed the plugs and wires.

I have located a used ECU and will negotiate a purchase. It will be a while before it arrives.

I will go ahead and do the drive test and report back what was found.

More to come.
 
PS

I have the regulators that were on the car when I purchased it. I had a fuel richness problem that was associated with the ECU which was reprogramed as part of the trans kit installation.

I'm also going to change the second (l/h)regulator to see if the replacement is causing the fuel rail pressure to be low. I will do a before and after drive test.
 
PSS;

One other question. Can the Marelli ignition ECU be acting up? Is there a simple test to see if it is sending the right spark? Both amplifiers that are mounted on top of the radiator are new and both coils are new.

I had a problem where while driving the engine would simply shut off. After the car cooled, it would start right up again. The only part of the igniton system that has not been replaced is the ignition ECU. Thjis all took place about six months before I had to do the engine rebuild.
 
Michael, During this entire time that we have spent on the problem, I haved had a premonition that the ECU was modified in conjunction with the transmission conversion. Now you have confirmed it; I wish I had known this from the onset. When you get the substitute ECU, I am certain that this problem set goes away. That is not to say that another irregularity might be there as a result of the transmission but it probably wont be as severe.
Do the test drive with the gage on the fuel rail under a good range of driving conditions. The pressure should stay within 5 psi of 40 psi. If this is not the case you have a fuel delivery problem that we can solve. If it remains within spec, then the ECU reprogramming is the culprit. You will have to work this out with whoever reprogrammed it. I think it was ill advised or in error.
This problem is not a Marelli ignition problem. The ECU controls ignition timing; the Marelli system only produces the trigger to the coil. If you are getting a spark to the plugs, Marelli is doing its job. The ECU also controls the injector timing and dwell. My guess is that an unmodified ECU will solve most of your driveability problem. It may not be optimum for performance or economy. These computer based cars are very tricky when it comes to changing the factory ECU settings.
 
PS, European versions of manual shift V12s exist as standard from the factory. You might have to come by one of those ECUs for a Euro application in order to properly accomodate the manual transmission if there is a problem. Good luck.
 
John;

I initially ordered a used ECU and waited a week for it to arrive. Called the supplier and had them trace the order. Turned out that used ECU on the shelf was a phantom. Ordered a rebuilt which I received this Friday. Identical P/N.

Checked the throttle pot setting with the rebuilt ECU installed, but it was right at .33v. Car runs fine at idle at 1100 RPM. Idle adj. is still screwed all the way in. Just opening the throttle results in a stumble and then rough running with white smoke. I checked the idle switch on the left bell crank for the throttle linkage. It opens right where it is supposed to. This is with the car running while parked.

I traced the spark plug wires again, just to make sure I have none crossed. Cap was new and rotor was in good shape.

There is some binding in the throttle linkage where the turn table does not want to return to the idle point. Spring tension seems to be good. Both butterflies are against their stops.

I checked the ignition vacuum line (small red plastic line that gives vacuum to module) for signs of obstruction. None found. Maybe this is an ignition module issue?

Michael J.
 
I got to wondering if I was getting vacuum back in the trunk for the ECU. I disconnected the hose where it connects to the ECU and disconnected the line where it gets its vacuum from the manifold. I hooked up a Miteyvac to the line at the manifold. The vacuum accumulator at the line right before it connects to the ECU was doing its job. I could pull a vacuum, which would bleed down in a few seconds.

I conducted a similar test at the ignition module the red plastic line supplies vacuum to the module.

Just to make sure, I checked both fittings on the manifold to make sure that the fittings were open and able to pull vacuum.

It is not a vacuum problem that is causing rough running above idle.
 
John and everyone:

Here is part of the answer:

I pulled the new distributor cap, looking for something that I might have missed. What I noticed was that the pitting from the spark jumping from the rotor to the flat pins for the plug wires was that the pitting was only half way across the pin. I checked the position of the rotor in accordance with Kirby Palm's book and deduced that the distributor was off by one tooth.

I repositioned the distributor by one tooth, set the position in accordance with the book and the rough running went away. The original cap did not give the situation away because it had been used before the engine rebuild and the spark pitting was all the way across the pins.

With the Marelli ignition, the ignition ecu adjusts the timing, so at idle, the timing was adjusted by the ignition module to be correct.

Car runs and accelerates fine.

However, I still can not get the idle under 1100 RPM.

Ideas?
 
Michael,

I've been following this thread with much interest and I'm sorry but I cannot be of any assistance other than to offer encouragement. Look on the bright side, with as much work as you have had to do and as much as you have learned in the process, your car is going to run better than new when you finally get things ironed out. Also, you will be just short of a pro at this in the future.
cheers.gif
 
Michael, Our messages crossed each other and I was not aware of your last post when I sent the message. Congratulations on your distributor cap discovery. The Marelli caps are famous for this kind of failure. Do not use an aftermarket cap to fix this problem, only use an original Jaguar part even though it is more expensive. Now tdhat you have got the problem down to the higdh idle and you are sure that you have no vacuum leaks, go over the butterfly valve adjustments with a fine tooth comb; something is not closing to make this happen. Also check the condition of the adjustment air valve to make sure it has integrity. Let me think over this and I will be back to you with more suggestions if possible. Once again, conmgratulations upon finding the distributor culprit!!
 
Michael, At this point it is a very enigmatic problem. I think you need a complete diagnostic system on the ignition system to observe what is going on with spark timing and secondary high energy voltage (relative) when the rough running occurs above idle. It does not sound like a fuel injection problem since you have a smooth idle and your injection system checked out so well out of the car. I am really grasping at straws here but another long shot is the catalytic converters. With all the problems that you have had, it is possible that they have become plugged up due to rich mixtures and subsequent abuse to the catalytic material. To follow this up you would have to unhook the exhaust system prior to the catalytic entrance and see if the problem goes away. After this, I am running out of theories. Keep me posted. Sorry it took such a long time to respond due to my Mexican trip.
 
Back
Top