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My 1983 Jag XJS has been running pretty good for the last few months. I have replaced some wires, fuel pump, tune up and other items as I go. I was driving it the other night and for the first time, it left me stranded. It sounded like it was running out of gas.
It turned over easily still, the fuel pump was still running and I had good spark from the coil. I trailered it home that night and dug into it. This is what works. The fuel pressure is on spec. The distributor turns and the coil wire gives a strong spark as does the few individual plug wires I tested. I pulled a plug and it was dry, is it should be when off.
I get power to the injectors but there seems to be no pulse to them. I did take the linkages off the main center throttle control and opened the throttle up quickly with the ignition on. I do get a clunking sound like the injectors are all opening for a short second as someone suggested testing to see if they were working. Now I am at the point of either the throttle potentiometer or the ECU.
I bypassed the inertia switch just to make sure but that really has no control over the injectors anyway. Still nothing. I don't have the parts to throw at it. I would like to try either another ECU or another throttle potentiometer. Like I said, good spark, good fuel and pressure and the injectors have power to them when I turn the ignition on.
Any suggestions? Anyone with a Lucas 6cu ECU that would let me borrow to try out or potentiometer? My current ECU was enhanced by AJ6 Engineering back in 2001 or 2 according to the receipts I have for it. There was a torque kit installed back then.
Turn the ignition on (do not start) the injectors should have 12v on BOTH pins.
When you turn the ignition on the injectors should click once, this is the priming pulse, then when you crank they should click once per engine cycle. If you get the priming pulse but injectors do not fire when cranking one possible cause is the shielded wire from the ignition amplifier.
Easiest way to test this is to unplug the ECU connector. Short pin 18 IN THE CONNECTOR (not the ECU) to ground, then in the engine bay measure continuity of the wire from the ignition module to ground it should be short circuit.
Well Warrjon and orangeblossom, I checked out pin 18 and there was continuity, it was a short like you said it should be. As for the resistor pack and connectors, I cleaned them too but will try maybe a bit harder tomorrow. There is a larger white wire on the positive side of the coil that does make what sounds like, an injector clicking when turn the ignition on and I connect and disconnect it. Its not like it will click as fast as I can touch and take of that white wire, but more of a delayed click from an injector or injectors. But still a no go on the engine. I will hope to check further and clean more connectors as I look around.
I appreciate the help and pictures so far. Keep up with that, I really miss driving it. It always started right up too before this problem. I made no changes except changing the fuel pump and lines in the trunk. The pump was a direct replacement for the Lucas unit and the hoses were pretty straight forward to change.
Well Warrjon and orangeblossom, I checked out pin 18 and there was continuity, it was a short like you said it should be.
You need to be a little more specific here, where did you measure the continuity.
What I said was to short pin 18 on the connector to ground then measure continuity to ground on the white wire coming from the ignition amplifier on the LH intake manifold.
If you measured continuity to ground at pin 18 of the ECU connector this is not OK. If you measured continuity from pin 18 to the white wire on the ignition amp in engine bay this is OK.
Remove the plug from the CTS (Coolant Temp Sensor) on the rear of the LH thermostat base. Join the 2 terminals INSIDE that plug with whatever is handy, I use a paper clip, then try the start sequence again.
That CTS is the weak point, as is the wiring of that plug you removed. Check all that very carefully. Remember, you have a running V12, unplug that CTS, DEAD V12.
The fact you have no injector activity when opening the throttle with the Ign ON, engine OFF, is a tad worrying, and points me to the TPS for starters.
I have never tried this with the CTS unplugged, so I really have no idea if a dud CTS would prevent this activity, but somehow I doubt it.
The actual injector loom, in the bottom of the V,is notorious now for failing, and age is the culprit, but mostly misfires are the beginning, and total shorting to earth, dead engine, soon follows, so it should really be on your To Do List. If it is starting to give issues, it will make finding the inactivity of the injectors quite difficult.
When you said that it felt like your Car was running out of gas, I was starting to wonder if you also changed the Fuel Filter, when you replaced the Fuel Pump.
How did you measure the Fuel Pressure?
Did you undo the Pipe from the FPR on 'B' Bank?
Did you Turn on the ignition (without starting the Car)
And see a Jet of Petrol come out like a Fire Hose right over the 'A' Bank front wing/fender.
(Taking precautions obviously) King wire disconnected on the Coil and Fire extinguisher handy just in case.
Remove the plug from the CTS (Coolant Temp Sensor) on the rear of the LH thermostat base. Join the 2 terminals INSIDE that plug with whatever is handy, I use a paper clip, then try the start sequence again.
That CTS is the weak point, as is the wiring of that plug you removed. Check all that very carefully. Remember, you have a running V12, unplug that CTS, DEAD V12.
I recently saw a short YOU TUBE video on this. I didn't know but do now. I am absolutely certain that no roadside car service mechanic would have diagnosed that. So with no hesitation I added a spare coolant sensor as part of my travelling emergency kit.
This got me to thinking. Is there a list of most common Effect or Symptoms/ Cause/ Remedy for the XJS? For example: Symptom: Car stops running. Re-starts after a bit and has cooled down. Stops running again shortly after getting back on the road. Cause: 1) Coolant Sensor failure, 2) ?........Remedy: Replace coolant sensor
A do know about the cts. I unplugged it after installing and new one and the engine quit immediately.
I rigged a T fitting in the fuel line that runs to the left hand back of the fuel return line before the last regulator and it falls within necessary pressure.
I have fuel and spark. I need to know what controls the opening and closing of the injectors. I feel it could be the throttle position sensor as it does effect the pulse under different throttle openings. But I am not sure if the ECU has control of it too. What bothers me is that I have no way of finding out unless someone has the tools to check the TPS and also the ECU.
So, that means throwing those parts at it to see if I can eliminate them or if one or the other fixes it. Finding these parts seems to be almost impossible.
That is why I was somehow hoping that someone here on the forum has extras for just that purpose. I am not sure how to go about getting the widest voice to them here but this is a start. I sure hate having a 4000 lb boat anchor. I have just reached my limit as to checking parts.
With your help I have eliminated many items but those leave the things hardest to test. Do you know of anyone that still has these capabilities?
Unless you have access to a scope forget testing the ECU. Everything else can be tested with a multmeter.
Once you have ruled out everything else what is left is the faulty part. Fixing these cars requires a disciplined methodical approach. Work on ONE system (like fuel) until you find a problem or rule it out.
The ECU pulses the injectors once when the ignition is turned on, this is to prime so there is fuel when the engine cranks, you may need to pull one injector and fire it into a container to verify it is squirting, or you could get a noid light, or just a 12v bulb will do.
No Injector pulse when ignition is turned on.
The culprits are
Inertia switch - this removes power to the fuel pump and ECU.
Wiring - injector harness
Ignition switch and associated wiring/relays
Power resistors
CTS
TPS
ECU
Injectors
Start at one end of the list checking voltages.
DO NOT CHECK CONTINUITY as this can confuse the problem.
Ok...lol. I follow you and the procedures. I did connect with a guy name Roger Bywaters from AJ6 Engineering. He said he could still test my ECU. What I am wondering though is that the 16ECU is a better unit and will plug right into my 6ecu. One is analog and the other digital.
One item we have talked about is the white wire coming off the ign amplifier on the left hand intake. The small white wire plugs into a longer and larger coaxial wire. The outer sheathing grounded to the right intake manifold but he inner wire going to the ECU pin 18. I have had suggestions that this could be a problem, that coax being broken or shorted out against the out sheathing of that wire.
If that is the case, how does one replace a coaxial wire? You can't splice it and it runs from the engine to the ECU! And where could I even find a coax wire like the one used in the car. It does show continuity from pin 18 to the end that plugs into the small white wire from the ign amp. But if it is shorted or the inner wire touching the outer sheathing it might still show continuity but then the signal would be interrupted. How is it possible to replace that coax?
There are several threads on here about repairing the coax. Most just splice in a piece of regular wire back to the firewall or wherever the coax becomes flexible again. Works fine. No need to replace the entire coax. Just make sure that the braided part of the coax is grounded either at the ECU, or where you splice in the new wire.
I have a 6cu that I replaced with a 16cu. The 6cu was operating intermittantly. Run fine one second and quit or run very rough the next. I opened it up and resoldered a ton of connections. Worked ok after that, but I didn't trust it as I never found an obvious cracked solder joint. I can ship it to you if you want to try it in place of yours. I live in Alabama. But first take yours out, unplug the connector, give the ECU a couple of good smacks with the palm of your hand, plug it back in and try it. Doing that is how I diagnosed mine was intermittant in the first place.
About a couple of months ago I was in the very same position as yourself but if anything my situation was worse, as my Car hadn't been Started in over 16 years!
And in order to get her going, I had to go through everything one step at a time eliminating every reason why She wouldn't Start until She did.
So I would suggest you retrace your Steps and re-check everything.
You said you replaced some wires, so what were they?
Could a faulty connection with what ever you did have caused a problem?
Why did you take the Throttle Rods off the Capstan?
Did you put them back before you tried to Start her?
One of the things you could try is to get a long length of Coax Aerial TV Wire then run it round the outside of the Car and connect it up.
Just like it would be if it were inside the Car.
If She Starts then that's your problem sorted and if She doesn't you will know that its not that.
It is highly unlikely the inner conductor is shorted to the shield, in 30 years of repairing electronics I have never seen this. Most likely the connection in the engine bay or at the ECU connector.
Leave testing the ECU until last, assume it is good until you have verified everything else that is easier to test. I do realise the 6CU is a know problem, but you need to discount everything else first.
Go through the procedures Grant posted before doing anything else.
If the problem is only the white wire you would have one injector pulse when you turn the ignition on.
The wires I am talking about that I replaced or the ones leading to a solenoid that allows the fuel tank vapors to vent through the charcoal canister. when I bought the car, the guy I bought if from replaced the wires from each of the injectors to the plug in front of the resistor pack on the right hand inner fender. Those were done well but they were not plugged into the right injectors and he had the spark plugs wires out of sequence. I was an easy fix and I got the car running fairly quickly.
I removed the throttle rods just to adjust the butterfly openings and to turn the throttle to feel the smoothness of it turning without the rods. I replaced the temp control switch and replaced the auxillary air valve. This car had been upgraded with a torque kit from AJ6 Engineering. So bigger butterflies, tuned intakes and an enhanced ECU. the 6cu that is in it.
The car was running great for many weeks and I had just installed a radio and got it working fine. I was a half mile from an intersection and it just started running poorly. In fact it ran like it did when I ran it out of gas purposely in my garage when I first got the car after getting it running. It ran rough for a bit, slowed down of course then just died. The same thing that night.
It does sound curiously odd I admit but it felt like it ran out of fuel. The pump was still running, it turned over, I had spark. Vapor lock? Well no, because it still won't start.
Thus my dilemma, why did it start to fail in a slow fashion like it did. A friend of mine has some equipment that can check some of the electronics and will hope for the best.
I would like to know though if anyone has changed the 6cu over to the 16ecu unit. If so then I found one online that might be my best option to having either a spare are a good ECU.
Thanks for all the help and suggestions. Its still down so keep them coming. I have spark, air and fuel but no fuel getting into the cylinders.
The 16CU will work as long as it is pre-Marelli. I don't think the Marelli 16CU will work reliably with the Lucas tach signal from the ignition amplifier.
So if you do buy a 16CU make sure it comes from a pre 1989 XJS.
02-12-2017, 04:56 PM
Fuel injectors not opening
