Ok, so my 1986 XJS will at least start now, reliably. But the idle is rough and erratic. I've already sorted through the ignition system and made sure it is up to par. So now I'm starting to look into fueling issues.


Here is the strange thing:
While the car was idling (roughly), I unplugged the power resistor completely. That made no difference ! From what I have read, the car should not be able to run at all with the power resistor pack unplugged ?


How can that be ?
I'm thinking it has something to do with the main relay and/or adjacent fuel pump relay in the trunk.


The wiring on the car has not been altered, except for one thing. When I bought the car a few years ago, I remember some jumper wires in place of either the main relay or fuel pump relay in the trunk (I don't remember which). That may have been the previous owner's way of working around a fuel pump control problem with the 6CU. At one point, I replaced the 6CU with a 16CU and installed two normal relays which I had removed from my 1983 XJS (I had put brand new relays on the '83).


I tried swapping the 16CU and put the old 6CU back in. The car would start and idle smooth for two seconds and then die. I found that indeed I did have to ground the orange connector on the fuel pump relay to make it run with the old 6CU. Once I did that, though, the idle was just as rough and erratic with the 6CU installed as it was with the 16CU installed.


How it is that the car can even start and run (albeit badly) with the power resistor pack unplugged is beyond me. I do still hear injectors clicking. But I have a hard time determining which ones are working and if any are not.


PS:
While the car was idling with the power resistor unplugged, I tested the voltage in the plug leading to the power resistor. On the wires corresponding to the four pathways through the resistor, the voltages on one side measured about 13 volts, and on the other side the voltage of both pathways fluctuated from about 2.5 to 5.0 volts. Like this:


A=B=C=D=
=E=F=G=H


A-D voltage: fluctuates between 2.5 and 5.0 .
B-C voltage: fluctuates between 2.5 and 5.0 .
F-G voltage: about 13.0 .
E-H voltage: about 13.0 .

 

A few months ago I bought an '86 that had not run in 10 years. I tried to make the engine run "as-is" but the fuel injector harness was bad and the injectors would not fire in the correct order. I could get the engine to run but just barely and it was very rough.

After cleaning the fuel injectors and replacing the harness the engine was still running rough. I replaced the computer and it ran perfectly.

I suspect that the faulty harness damaged the computer. Replacing it resolved the issue.

Mark

 

It is possible that both of the computers I tried today are bad. But a few weeks ago when I first started having problems with the car I also tried a different 16CU that functions fine in my '83. That made no difference.


A fault in the injector wiring harness is certainly possible.


EDIT:
After some more reading, it appears that these cars can idle with the power resistor unplugged. But if the engine is revved up for any length of time with the power resistor unplugged, damage to the diodes in the computer may result.


Tomorrow I will try and determine if the engine is running overly rich or overly lean, in both banks. Today I did try disconnecting the vacuum line to the 16CU while the car was running. The result was that idle speed dropped a little bit, and the exhaust had a slightly black sooty look from both tailpipes (but not smoky). When the vacuum line was reconnected, the idle speed increased and the exhaust cleared up completely (I noticed a fair amount of water vapor and condensed water dripping out).
.

 

All correct, the PR is there to protect the ECU. If all the ignition (plugs, wires, dizzy cap, rotor, amp) is up to snuff, then rough idle can be, in no particular order:
TPS set badly/failing
Injectors not 100%, injector loom not 100%
Shielded wire from amp to ECU failing
Odd loom deficiencies from Engine temp sender to ECU
Coils going home
Fuel pressure irregular, poor B abn FPR
and so on.


Will the car rev cleanly, or not?
Greg

 

NEVER unplug the resistor pack with the car running.

The resistors are there to limit the current drawn by the low resistance injectors.

The injectors are activated by the ECU switching one side to ground. So with the ignition on (car not started/running) each injector should have 12vdc on both pins in the connector. Check this if one side is 0v or low you have a problem in your harness.

 

As mentioned the power resistor is essential.

XJS uses low resistance (2.4 ohms) peak and hold injectors.
These require a high current to get them to open, but once open a much smaller current will keep them open for the few milliseconds that is required. Fuel ecu has output pins to supply full voltage direct to injectors to get them open then that pin is switched off and "hold" output pin supplies lower voltage and hence lower current via power resistor to injectors to maintain open

 

Excellent suggestion.
I just checked both pins on every injector (with ignition on, car not idling). All 24 pins measured 12.2 volts.


So either the ground switching is bad, or the problem is something totally different.


Next I'll have to see if I can find my stethoscope and listen to see if each injector is actually firing. Assuming they are, how do you tell if they are firing in the right order ?


I did check the wiring harness to make sure every injector was connected correctly (none of the connectors were swapped onto the wrong injectors).


Strange thing, though ...
I disconnected the entire left bank and that didn't change the idle quality hardly at all, compared to when all 12 were connected. Then I reconnected the left bank and disconnected the entire right bank - no change in idle quality either. Then I ran the car with the back 6 injectors (3 from each bank) disconnected. No change. With the front 6 injectors (3 from each bank) disconnected, still no change.


It seems I can disconnect any six random injectors and there is no significant change in idle quality.

 

I tried disconnecting the TPS. No difference.
Power to all injectors verified. Proper ground switching of injectors unverified.
I tested the "shielded wire". First, the shielding had good connection to ground. And there was a good connection from near the ignition module to the 16CU in trunk.
I unplugged the temperature sensor near left front of engine (simulating an infinite resistance on the temperature sender) - car died (as expected). With the plug disconnected, I jumpered the two terminals (simulating a zero resistance of the temperature sender) - car ran, but no improvement in idle quality.
New coils or original Lucas coils - no difference in idle quality.
I can hear fuel pump running, but actual fuel pressure unverified.
I disconnected the vacuum lines to both fuel pressure regulators. That made no difference in idle quality, and there was no fuel leaking out of either one.


When I "stab" the throttle, there is some hesitation (stumble) at first, and then the car will erratically rev up to higher RPM.

 

So if the power resistor was disconnected, wouldn't that just result in the injectors receiving an "open" signal, but no "hold" signal ?
Does the "open" signal bypass the power resistor ?
.

 

I agree with that loom as a maybe.

Whilst you are resting the brain, look at the multi EARTH studs alongside the battery. These are for all sorts of electrickery items, INCLUDING the ECU system. The ECU does NOT earth via the casing.

Then in the engine bay, near 5A injector area is another bunch of earth wires under a Torx headed screw. These suckers also have multiple items being earthed and they break.

Pulling the vac hoses from the FPR's increases fuel pressure to "about" 40 psi, and at idle would be seriously overfueling.

TPS out of idle range is on my list, but not too high up at this moment.

A partially clogged main ECU vac supply could cause havoc. Simplistically, LO vac = more fuel, HI vac = less fuel.

Try bridging the CTS socket with a paper clip, and see if that helps. The engine should be DEAD with the CTS unplugged, but a flaky CTS could give the ECU a fuzzy signal that it will not process correctly. This bridging will trick the ECU into thinking the engine is at operating temp.

Another thing to clear up in your mind. Ign ON, engine OFF, rotate the throttle capstan quickly (just like accelerating from the curb), and you "should" hear the injectors "click". If you do, that loom is probably OK, but not definatively. NO click indicates loom/TPS/ECU/etc issues.

Maybe this ECU pinout I did years ago, will help a little bit. I still use it when sorting engine fire damage.

XJ-S ECU Pinout table.doc

 

When I "stab" the throttle, there is some hesitation (stumble) at first, and then the car will erratically rev up to higher RPM.


When you have checked and cleaned all the earths, and the other things Grant mentioned, if that does not do it, it might be a fuel pressure problem or a TPS problem. Disconnecting the TPS proves not much, it is whether it gives a clean progressive signal to the ECU that matters.
Have you cleaned through the fuel system yet? That is, sump tank, sump filter, sump tank from main tank and sump tank to pump gravity supply hoses, changed main filter behind the spare wheel, blown out the fuel line to the engine bay.
If that is all done and problem persists, fuel pressure is a good candidate. You really need to test the fuel rail pressure to be certain; otherwise it is a matter of changing the B bank (US Driver's side) fuel regulator, and then the pump, as a system that cannot deliver the correct pressure could cause it. Also a very good plan to remove the A bank regulator completely. I recently had a hot start problem, whereby the car needed far too long on the starter to fire when the engine was warmish. This turned out to be the B bank FPR which was on an intermittent basis, not holding pressure properly.
There is a recent thread that goes into the fuel system in some detail, might be worth a read.
https://www.jaguarforums.com/forum/x...-158499/page9/
see post 162 and others.
Greg

 

I think Greg has his hands crossed.

B bank is USA Drivers side. The B bank FPR is the ONE and ONLY pressure regulator.

The RH, or A bank is a pulse damper primarily. Removal of the RH FPR is a good thing.

 

That is the first time that I have heard it referred to as a pulse damper; doesn't it do the same thing as the regulator?

 

Whoops, sorry Grant, meant to write US Driver's side. Just fixed it. What with a RHD car driving on the wrong side of the road, old age and general stupidity and an azerty keyboard, it is all getting too complicated...
Greg

 

LUCAS EFI / AJ6 Engineering

See AJ6 engineering web site for detailed description of all Lucas fuel injection ecu used on XJS

 

HA.

I reckon that driving on the wrong side of the road would be scary. Thats why I dont even try when in Vegas. Worked it all out in theory ONCE, when practice came, HAHAHAHA, NO WAY.

 

Nope, not the same on the V12.

The actual Pressure Regulaotor is always on the "outlet" port of the fuel rail, on "fuel return systems" that is.

Dead end systems are very different, like the X200 S Types.

The RH "FPR" is/was used to "dampen" the injector pulses on the fuel supply side????. I never did understand it, or spend much time trying to. My involvement with Bosch EFI from a Parts point of view, told me otherwise, hence I removed my RH unit, car still ran, even ran better, and the pump was quieter, as it no longer needed to "push" past that FPR to load the fuel rail.

Later in ownership, I found documentation in Palms book stating the same.

 

Indeed later version of 5.3 V12 & 6.0 V12 only had single fuel pressure regulator located on A bank side of fuel injector rail (fuel return side). No pulse damper fitted on later models

 

Thanks for that.


I've already loosened and re-tightened the grounds near battery. Also all the grounds under the black cover on the RH fender next to washer bottle.


The torx screw on the passenger intake manifold (near back of engine) has only a single small wrapped cable leading to it, with a loop terminator under the screw. I loosened and re-tightened it - no difference.


Not much change when I remove the vacuum lines to the fuel pressure regulators. So it is possible that either they are bad, or the fuel pump is delivering less than 40 PSI.


I've already tried pulling the vacuum line off the 16CU. There is significant vacuum at that end. Once pulled off, the idle speed decreases somewhat (and is still erratic but maybe a little less erratic than before) and the exhaust has a slightly sooty appearance from both tailpipes. When reconnected, the idle speed increases a fair amount at first (then settles down a little) and the exhaust clears up with only water vapor coming out.


Already tried bridging the Coolant Temperature Sensor - no difference (see my post #8 in this thread).


I switched ignition to run position (car not idling) and rotated the capstan by hand. I definitely heard injector(s) click. Should they all click or only certain ones in the firing order ?


Things to do:


1) Check fuel pressure. I've never done that before, so I'll research how to best do it.


2) Check the amount of vacuum at the 16CU. Is it possible that it could be getting too much vacuum ?

 

Maybe I missed it so I will ask...what is the history of this car?
Was it sitting for a long time?
If not, was it running well and suddenly began running rough?
Have you cleaned the fuel injectors?

I ask because if the car has been sitting for a while or run only briefly then I would would suspect dirty/sticking fuel injectors. I have seen this on several of my cars. Cleaning and exercising the fuel injectors works wonders and restores a smooth idle.

Mark