It's been like a science experiment trying to figure out why all 12 cylinders are flooding with gasoline. I eliminated the possibility that any of the injector wires had grounded in the engine bay or otherwise. To prevent filling the cyl's and sump with gas I disconnected the injector plugs and rigged up 4 dash light bulbs to plug into the injector harness (one bulb for each leg so that the lamps light when the injector would be open). In all instances, when my test lights come on, they all come on. Sometimes when I turn the key to ON the lights light and stay lit until I turn the key off. On a different day they may not come on when the key is on. This morning the lights came on for about 30 seconds then flickered a few times before going out. Yesterday it was frosty in the morning and the lights wouldn't come on with the key. I removed the computer, took the covers off and warmed it with a heat gun to about 80 degrees F. Reinstalled but nothing changed. I kept trying it every couple of hours. Nothing changed until early afternoon when the temp. and sun angle warmed the engine bay and the lamps did come on with the key. The pc boards look ok in the computer. Frustration is setting in. I'm running out of good ideas. Help!

 

Have you ruled out the fuel pressure regulators?

 

How have you eliminated the possibility that the injector harness is the problem?
Greg

 

I haven't actually measured the fuel pressure, but those lamps that I've wired in to indicate when the injectors would be open, are on when the key is on even when the engine isn't cranking. Thanks

 

Yes, I put an ohm meter between all the injector wires and engine manifold, then wiggled the harness in every way I could imagine and there was never even a blink of continuity. Also I checked for continuity going back from the injector harness plug, with the computer disconnected; also no continuity. I reasoned from that outcome that either the computer was faulty or the computer is getting some bad information from somewhere that tells the transistor drivers to ground. Thanks

 

I'm not clear on how you have your test lights wired, but, in case you didn't know.....

The injectors *will* show 12v on both sides with the key "on". That doesn't indicate a fault. With the engine cranking or running the ECU will ground one side of the injector to operate the little internal solenoid

Cheers
DD

 

If you wire the light across the injector, the injector may not fire but the light will come on when the ECU fires the injectors.

If you have wired the lights from the injectors to ground then the light will be ON when the injectors are off.

If you wire the light on the ECU side of the injector then it will be OFF when the injectors fire.

A very common cause of the problem you have described is broken insulation on the injector harness.

As Doug said the injectors have 12v on BOTH sides with key on, you need to check this.

Forget continuity testing unless you understand what you are measuring it WILL lead you in the wrong direction, always better to measure voltages.

 

Jumper the coolant temp sensor plug to take it out of the equation.

Fuel pressure off spec won't usually cause flooding. Would require 120psi to just double std flow rate.

With a auto stethoscope or a long screwdriver touching the plastic injector body, listen to the injectors. Have someone switch the car to pos 2 ( dash lights on ) any click from injector? Then twist the throttle open. Do you get a single click from injector? Check all 12.

What year and engine do you have?

 

Thank you all for your replies. I never thought to jumper any of the sensor wires; only to unplug them. I'll try that later.
I should add a bit more information: The car is an 8th month 1984 yet the wiring diagram in Mitchell manual for 1986 is the closest to accurate. My car has 35 pins on the Lucas computer while the 1985 diagram that I saw had far fewer.
The way that I wired in my little test lamps was such that each one replaced one injector (insulated from ground). The way it is now I've unplugged all 12 injectors, then plugged in each of my 4 test lamps into the harnesses that lead to cyl's 1A,2A,1B,2B. From my diagram it shows that for example 1A harness is in parallel with 3A and 5A so if my 1A test lamp lights, that tells me that 1,3,5A injectors would open. Similarly the others are factory wired in groups of 3. Since all 4 test lamps light, flicker, or go out simultaneously, this would mean all of the injectors would be getting the signal to open (if they were plugged in). The test lamps make it easier to see when the injectors are being called to open (and safer). In my earlier stages of trouble shooting fuel actually filled the air cleaners with gasoline and was dripping on the ground.
From past experience with other cars (not Jaguar), I'm reluctant to buy another computer only to have it do the same thing. I've been fooled by that a few times.

 

Couple of things I have had:

Does your car have cold start injectors?. If so, remove them electrically and mechanically from the system. they are reknown to leak fuel, and Jag fitted, not fitted, fitted, etc throughout the V12 life.

If when turning ON the ignition, the injectors (lights etc) click, then the TPS may be AWOL, or waaaaaaay out of range. SO, unplug the TPS, try again. If NO click, problem found. If it still clicks, then the loom is highly suspect.

The injectors should NOT click when ign goes to ON, but should click ONCE when the throttle is opened rapidly with ign ON, engine OFF.

Tucked down by the rearside of the RH headlamp bowl, in the engine compartment, is the EFI resistor pack, silver box thingy, with a multi pin plug at the bottom, work that plug out and clean all the gunge out of it and the mating socket. That sucker will reek havoc.

 

Grant, thank you for your advise. I don't have cold start injectors on mine. When the injectors were plugged in they would all click when I turned the ignition to ON. Now, with my test lamps in place of them they just light when the key is on (usually). Switching or unplugging the tps unfortunately didn't change anything. It is however a good thing to know that the tps works as sort of a primer. Thanks for that.
As for that resistor box, I checked the resistance through it a week back (all 4 about the same) but I didn't take it apart. I will tomorrow when it isn't raining. I'll also jumper the temperature sensor wires to see what happens. I'm glad I joined this forum as I'm getting ideas that I never would have thought of on my own.

 

That temp sensor is a PRIME sensor for the ECU.

Example:

Normal running V12, unplug that sensor, DEAD V12.

The fact that the injectors clicked when in the Ign ON position is 98% a loom issue.

I have never used lights etc to trace these, the click of those injectors is more than loud enough.

Here is a pinout, and a schematics I have used for years to rewire those that have burnt the engine bay.

V12 EFI schematics Australian cars.pdf

XJ-S ECU Pinout table.doc

 

Purrkitty, as Grant states above the odds on a loom problem are very high indeed, and eliminating it for certain seems the best thing. I do believe you might do well to remove the injector loom entirely, unwrap it and at the very least re-insulate each individual wire with tape, re-attach to the injectors and give it a go. This is the most certain, cheapest and whilst time consuming, probably the best thing to do.
Greg

 

Grant and Greg, I can see that you guys are very knowlegable about this problem and even though I thought I was thorough while testing between all legs of the loom and ground, I guess I should revisit that test. The injector loom is not very plyable, so scary as it sounds to me, I may have to make up a new one. Hey, I'm retired now, what else do I have to do?
Grant, the wiring diagram is similar to mine but there are a few tidbits of extra info that helps clear things up.
thanks guys,
Bob

 

Anytime Bob.

In the real world these systems are simple. Confusing, YES, but still simple.

The weakest link in the EFI system, by a very long draw, and age related, is that loom. Mine is tucked up on top of the camcovers, under the inlet manifolds, and wrapped in "Header Wrap" for heat resistance.

The old style Bourns TPS is a very close second.

Also, retired I am, and I know the time restraints we now have, ha.

 

While lying awake at 4:00 AM a coupe of mornings ago, it came to me that I should isolate the ground wire that comes out of the computer that's responsible for grounding the injectors. It turned out that the black wire (one of many) that grounds beside the battery in a group of 4, when snipped kills the injector ground and also the fuel pump relay ground. In my car this wire leads to pin 1 on the computer. Looking at the printed circuit board, pins 1,2 and 19 are all joined together adjacent the pins. After looking at the wiring diagram, I'm puzzled why the wire that goes to pin 19 wouldn't just ground at the engine rather than pass through 1 cm of pc board then back out to ground. ?? Anyway, I took the fuel pump function out of the picture by pulling the relay.
The next morning (frosty) I brought the computer (with covers removed) from my warm garage out to the car and plugged it in. With the computer ground wire 1,2,/19 alligator clipped to ground, I switched on the ignition. As usual my injector test lights came on and when I unclipped the alligator clip, the lights went out. I clipped it back on and whlie going to get a meter (thinking to test the amps between pin 1 wire and earth) I came back to the car to discover that the test lights went out. Could this be temperature related? I brought out my heat gun and held it near the computer and within 10 seconds the test lights came on. I got tired of waiting for the lights to go out so I went in the house for 20 minutes and then tried it again. This time when I turned the ign. on, my test lights flashed randomly for about a minute then stayed out for another minute, at which time I heated the computer for about 15 seconds before my test lights came on. They stayed on for a few more minutes when I turned off the ignition.
Well, what do you think is my next move? I don't have a spare computer lying around. I'm open to suggestions.
Bob

 

Hot and cold intermittent faults are usually dry joints in the solder connections or broken PCB land tracks. It can also be a component breaking down but this is less common.

Open the ECU and inspect all of the solder joints, you may need to use a magnifier depending on your eyes. What you are looking for are rings around the solder, this is usually a dry joint. If you wiggle the component normally you can see the leg on the under side of the PCB move in the solder. Re-solder these.

While you're at it check the PCB tracks for any cracks.

 

Warren,
This gives me some hope. I'll definitely need the magnifier for this; I'll give it a really close look tomorrow.
Thanks,
Bob

 

I looked closely with magnifying glasses at all parts of the pc boards and there was nothing obvious that was wrong. Decided to make up an "extension" injector test lamp that plugs into one of the plugs on the injector harness and reaches to the trunk. With the key on, the test light would light, but with tapping or squeezing of the pc board in certain ways, the light would go out. The fuel pump would also quit and my other injector test light that was also on the rt. bank. This was the first time that all 4 of my test lights didn't act together. While trying to pin point the problem, I came to realize that moving the entire computer harness forward and back would make the fuel pump stop and start. Unrelated problem?
I'm a bit of a fair weather fixer, and it was cold and windy so I left it for later.

 

You are probably making and breaking the connections at the huge multiplug when you do that. The likelihood of your introducing a circuit board fault is quite high, too. I would very carefully clean up the pins and sockets for a start.

I still think you are avoiding the obvious thing to do though!
Greg