1983 Jaguar XJS 5.3 HE idles but cannot accelerate

Hi Everyone,

I am a new member of the Forum but have been perusing topics here for some time as I try to sort out my XJS. Thanks to everyone for the plethora of information you have provided! Previously, I have never worked on a Jaguar and I don’t know how anyone could work on one without such information!

I found a 1983 XJS 5.3 HE which sat in an airplane hanger in Wyoming for many years. It is cosmetically pristine and has obviously been stored indoors its entire life and not driven in inclement weather. It was not running, so I towed it home.

After much preparation, I got the car running but it barely idles at 500 rpm and cannot accelerate even the slightest without stalling. Here is what I’ve done so far, but nothing improved how the car runs and I’m about out of ideas. Can anyone offer suggestions of what else I can do?

1983 Jaguar XJS 5.3 V12 47K miles


Weak, charged then maintained with trickle-charger


Antifreeze, oil, oil filter and air filters look fresh, will wait to change them after engine is running properly

Fuel system:

Bad fuel, dirty tank and sump: several pounds of rust and varnish removed!

Drained and power washed both, new gasket on sump

Bad fuel sender and gasket replaced, tested, works

Replaced fuel neck o-ring

Plugged sump and main filters, replaced with standard main, aftermarket stainless-steel sump

Stuck, dirty fuel pump, freed, cleaned internal filter, pressure and flow tested

Dirty fuel system, applied 15” vacuum to pressure regulators and forced a quart of isopropyl alcohol at 40 psi through entire system from trunk back to trunk to clean it

Injectors cleaned by running injector cleaner through each on bench, verified pattern and flow. New filters, pintle caps, hoses and clamps installed, rail thoroughly cleaned

Ignition and electrical:

Replaced spark plugs, verified ignition wire firing order, tested ignition wires, all cylinders firing

Lucas ignition module looks new inside, didn’t replace HEI module, replaced crumbling connector and other connectors with glue/heatshrink crimp connectors

Checked, cleaned and tightened all GNDS in engine bay and trunk, but haven’t checked the one under the engine yet

Checked and cleaned all fuses in engine bay and under left and right dash

Throttle bodies:

Replaced throttle linkage end bushings, throttle body gaps were 0.002, adjusted throttle linkage, cleaned throttle bodies, gaskets looked good


Full-load vacuum switch tested, vacuum hose replaced

Interior air controls line holds vacuum, interior controls not tested yet

Brake booster and hose hold vacuum

Vacuum solenoid wires disconnected, tested and reconnected them

Tested full throttle solenoid, throttle switches, vac solenoid, regulator, air control, vacuum dump, capsule, delay valve, manifold vacuum line holds vacuum

Replaced all emissions vacuum lines on A side and fuel regulators according to stickers on hood

Auto trans vacuum line holds vacuum

Cruise control and associated line hold vacuum

Engine prep before starting:

2 tsp of oil squirted into each cylinder while spark plugs were out, hand-cranked by front pulley nut clockwise to ensure engine was free and turned smoothly prior to starting


Starter would not turn over and fuel pump would not run

Removed and tested main relay, fuel pump relay, starter relay, inhibit relay, replaced fuel pump relay anyway to ensure reliable operation.

Open connection found after inertia switch, suspect immobilizer somewhere behind radio (could not find anywhere else). Transmitter for it found under dash near right fuses, all magnetic door reed switches removed, installed fresh battery in transmitter and tested it, placed in glove box. With reed switches removed (Normally Open when doors are closed) the receiver should work properly but connection remained open

Bypassed inertia switch in engine bay by connecting coil 12V to inhibit relay pin 86, can easily revert to normal connection if open eventually found.

Fuel runs for two seconds when key in RUN and continuously when engine is running

Fuel pressure regulators both work and hold vacuum, rail pressure holds at 36 psi, pressure slowly drains when engine is off, maybe 4 psi per day.

Engine Idles at 500 rpm with 8” of vacuum, stalls when throttle opened, engine sounds tight, makes no strange noises and does not smoke, cannot hear any vacuum leaks, mixture seems lean

Exhaust flow feels smooth on both pipes when engine running


Calibrated to 0.34V, wiper voltage increased smoothly through range to approx. 5V, however, did not have an analog meter to check for spikes

Ohmed TPS connections to ECU (each was about 1.2 ohms), measured between wiper and +5/GND at TPS and ECU, each was about 200 ohms higher at ECU than at TPS even after thoroughly cleaning the TPS connectors

Took the TPS that was on the car apart and the carbon was worn off for perhaps the first 1/3 of rotation. A second TPS looked very clean externally, like it was original. It calibrated easily to 0.32V but no change in engine operation

Replaced the TPS with new red version with adaptor. Had to finely file two holes to get it to fit. Calibrated to 0.32V via red and yellow wires, no change in engine operation


DAC2597 6CU HE CEI 11.5:1 O2 Sensors Catalyst

A spare ECU in the trunk marked “Bad” works the same as the one currently installed. Opened that and it is in pristine condition with no evidence of burns, repairs, or bad solder connections.

Both ECUs hold vacuum, the vacuum line is clear and registers the same vacuum as at the engine

Found this firm that repairs ECUs, are they reputable?

Unit 3 Brookmarsh Industrial Estate
70 Norman Road, Greenwich
London, SE10 9QE
United Kingdom

If selected, I would send the ECU marked BAD to them as it is probably the original and like the rest of the car, is in nice cosmetic condition. They supposedly test, repair and retest to ensure robustness

A more reputable place from the Jaguar forums:

I emailed AJ6 but did not receive a response so far
AJ6 Engineering,
60, Henshall Road,
SK10 5DN,

Phone: 0044 (0)1625 573556 Text messages cannot be received on this number.

Business Hours: 9am – 6pm Mon Fri (GMT)


Additional items:

Activating the full-load switches to bypass ECU feedback did not make engine sound different

Cleaned gas cap gasket and adjusted both tabs so that the gasket seats evenly

Did not replace B-side charcoal canister lines, however, they looked fine

Did not test temperature vacuum valve as I thought it risky to remove

Could not find a charcoal canister purge-valve, looks like a direct connection?

Did not test supplementary air valve

Did not adjust the base fuel map

Red plastic plug with small cut found on B side air filter housing blocking Idle Air Valve intake, removal didn’t affect engine operation, may be indicative of sticking/stuck IAV?

Alternator has been replaced with GM version, overcharge lamp on dash is on but have not looked into this yet, could overvoltage have damaged the ECUs?

Did not test 45 second timer, could not hear a difference after 45 seconds of running

Have not checked timing and have not replaced the distributer guts, however, visual inspection looked clean and in good shape, verified all cylinders firing by connecting timing light to the coil ignition wire and watching light for misfires

Have not looked at pulses from the Lucas module to the ECU, did not verify all wires continuity to the ECU

Thanks in advance for any help you can provide and I apologize for anything I may have missed in other topics.


Wow, a very comprehensive first post, welcome Ken.
I am afraid I am no help as I have never had a V12, came close once with the 6 litre XJ40.
Hopefully some others on the forum will come up with more ideas.
WRT Mr Bywater he’s always very busy but also getting on in years I believe and possibly(?) looking to ease off on the workload.

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Thanks for the welcome and information. I will try to call Roger instead. I heard that he does not respond well to emails.

Hi Ken and welcome.
A very long list indeed! Good work.

I would change it regardless (get only the GM/AC Delco), good reassurance for a small price.
They don’t like the heat and deteriorate with time.

It’s a clue but it does not rule out 100% that the ECU is not bad.
Roger Bywater at AJ6 Engineering is the man you want, but he is very busy and will take a very long time.
Testing with a known working ECU is the fastest/easiest.

If not working it will kill the vacuum advance.

You should try that.
Most important you would see if the ECU responds to the O2 sensors signal.
If you think it’s running lean try a couple clocks richer.

You didn’t mention anything about the Coolant Temp Sensor, extremely important.
Before anything check resistance vs temperature.
Unplug it to see if it makes a difference (infinitely cold)
If not bridge the plug (infinitely warm).

You should check also if you get the same resistance values at the ECU plug.
Same with the Air Temp Sensor, albeit to a lesser effect.

It’s caput. Any positive trigger relay will do the job.

Extremely important, seized mechanical advance and broken vacuum advance are very common problems.
You should definitely remove and rehaul the distributor, a very big possibility that it’s the cause of the problem.

Battery light indicates that the battery is not charging.
With no good healthy 12v nothing will work…

Good luck and keep us posted.


It’ll take some time to digest your post.

For the moment…

The stalling when throttles are opened certainly suggests a fuel starvation problem…and that’s what I would focus on, personally.

And both throttle blades operate in synch? I suspect you’ve checked this but I have to ask!

I don’t know the max voltage that an ECU can tolerate but suspect it would be pretty high.

What is your actual voltage now, with engine running?

The overcharge light is triggered by its own control module. This would merit some research at some point but probably isn’t related to the problem at hand

More later


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Bear in mind that the XJS has both a conventional ignition/charging/battery warning lamp and an overcharge warning lamp.

The overcharge lamp, if working as intended, indicates excessive voltage.



Stalling when acceleration is called for could be fuel related, but could also be a weak spark which is being blown out under any load.

Check the timing and HT voltage, then go for using a separate source of fuel to fumigate the intake while attempting to rev the engine.

Injection systems are pretty simple once you understand how they function.

  1. Pressurised fuel in the rail.
  2. Injectors are pulsed by the Ecu to alter the duration. You can monitor this with an oscilloscope, or Noid lamps.

I suspect weak spark, low fuel pressure, no ignition speed pulse from the distributor to the Efi Ecu.

Check all Efi components are feeding correct signals back to the Ecu and check it has power and Gnd.
Is 0.34v the correct setting for the throttle pot?

You should be able to monitor the Lambda sensors to see what the mixture strength is. The range is 0-1v, which is normally switching between the two states, but an average of 0.45v Dc is stoichiometric.

If the alternator is overcharging it could well shut the Ecu down, or cause the coil to fail to generate sufficient voltage.
Disconnect the alternator and run the engine from the battery only.

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Good point.

Weak coil, possibly? or poor connections at same.

Coil resistance should be about .8 ohm primary and 7k-8k ohm secondary

Just my opinion but I think the actual problem at hand is something more fundamental.

The primary mixture control is the baro sensor in the ECU. The Oxy sensors trim the mixture. They can be dead or disconnected and you can drive the car normally with no stalling.

Sorta the same story for centrifugal and vacuum advance, and emission timers, and the like. Lord knows how many V12s are running around with these systems weak or dead and the owners are not even aware of it !

Good idea.

And check the condition of the connector!

Also, on general principles, check condition of that (very delicate) shielded white wire (“the dreaded white wire”) at the ignition amplifier!


Absolutely agree.
My point about monitoring the Lambda sensor was really to check the mixture strength is OK once the engine is running properly and also to point out that no special test equipment is required.
An oscilloscope is ideal, but a voltmeter will be ok to determine if the fuelling is switching between rich and lean. Another test is to quickly force the throttle pot to WOT without opening the throttle and check the Lambda voltage responds to the rich mixture.

What is so far unclear is whether the engine runs OK at idle then stumbles to a stall as the throttle is opened, or simply shuts off immediately.

I wonder if the Ecu is running the engine at idle as the throttle closed switch is made, but once opened there is a missing sensor input, such as engine revs, so the Ecu shuts down.

Back to the O/p.
Remember these cars don’t use an engine management Ecu, but have two separate, but interconnected, systems for Ignition and fuel.
Ensure the ignition system is functioning before moving to the fuel system. The distributor feeds a speed pulse to the fuel injection Ecu so it increases the fuelling as engine revs rise…the dreaded (thin, fragile) white wire.

As a test, once you are sure the ignition system is functioning properly dribble petrol into the intake to substitute the fuel injection system.

Each component of the fuel injection system can be tested without special equipment, either just a test lamp, or Dmm.

I must admit that is an impressive list for sure and congrats on your XJ-S and the list of things you just went through and verified . When I first got my XJ-S I was like a deer in the headlights.
A couple of things I’m thinking based on your post is, your idle seems a little low at 500 RPM. I’m wondering if you should replace your fuel pump as it’s getting on in years anyway. Might want to replace your ignition module. I got one from my local GM dealership for $45. Maybe service your distributor, check spring loaded carbon thingy at top of cap. Replace rotor and regrease , and oil etc. I’m almost thinking when my car behaved this way I had a faulty ignition coil and went with the single coil set up.
Good luck, a lot of these items don’t cost a lot of money and at least you will be able to strike them off your list. Also download Kirby’s book a lot of great trouble shooting advice in there.

The 6CU ecu is known to be flakey. Pretty sure you can upgrade to a superior 16CU from mid 80s. You can find them on Ebay for $200-$300.

AJ6 Engineering, he is very busy and semi retired.

I would replace GM module just because. They are so important to ignition, and only $45 and super easy to replace. Heatsink paste very important too! And ONLY buy genuine AC Delco. Anything else will overheat.

You’ve done a lot! Keep going, took me six months to get mine drivable, and several years just fine tuning. Once these engines are back to tip top, they are a marvel.

As Greg says, 6CUs are known to cause issues. I had one that would run fine, then start running rough, wouldn’t accelerate, all kinds of weird issues. Plugged in a 16CU, and all problems were gone instantly. I went thru my 6CU reflowing solder connections, and it seemed to fix it, but i didn’t trust it.
If you buy a 16CU, be sure to get one for the Lucas distributor. They were also made for the Marelli, but they are different. Roger Bywater has specific part numbers on his website.

Hi Aristides,

I think I will first replace the HEI module and test the white coax wire. If no change, I’ll order the proper upgraded 16CU ECU from eBay. Since everything I’ve tested has worked so far and is in such great condition, I think I’ll hold off testing the other sensors and dismantaling the distributer until after that. I did have the cap off and inspected the interior, rotor, and cap and they looked excellent. Someone has already replaced the coil and I believe the HEI, but no harm in trying another.

Hi Doug,

I agree with the starvation suggestion. The throttle bodies are super clean and well synchronized with the min gap set to 0.002. The engine does not idle well now, so I haven’t measured the voltage yet. I plan to look into this as well as ignition timing once the engine is running more predictably.

Yes, I will definitely look into it more when the engine is running better. Pretty unpredictable how long it will run now. Makes it a challenge to diagnose anything!

Hi Dieselman,

I’m going to first look at that white wire, which looks good but could have an issue, then I’ll replace the HEI. Next, I plan to get an upgraded 16CU controller from Ebay, the correct model, of course. It is pretty amazing that the 6CU doesn’t have a microprocessor or EPROM. Aside from the ADC, it is pretty much pure analog!

If those things don’t work, I’ll next remove the distributer and check it out and replace the ignition wires while at it. And if that doesn’t work, I’ll check all the sensors that I’ve missed so far.

Hi Jon,

The 16CU is high on my list. I’ll check the HEI and white cable first and then order the 16CU next. Since I have yet to find anything obviously broken, I’m guessing that the sensors, distributor, and ignition wiring are probably ok and that the ECU is my best bet. If not, then at least I’ve narrowed the list of potentials down even further!

Hi Gary,

I did remove the distributer cap and everything seemed in good order there but I did not check the vacuum or mechanical advance. I’m going to replace the HEI module first and test the white wire for shorts/opens, then try a 16CU ECU. It seems more of a fuel problem than ignition, but I guess I’ll eventually sort it out.

Hi Gary,

The fuel pump seemed to have good volume and held pressure at 36psi with no trouble, but I will keep that in mind. Definitely going to replace the HEI with GM first and will next replace the 6CU with a better 16CU of the correct variety, if I can find one. If these don’t work, I’ll replace ignition wires, cap, rotor and make sure all the advance stuff works properly. Any idea if the Hall sensors are problematic?

The car has a new single ignition coil on it already, but I will test it anyway. I have Kirby’s book and have referred to sections of it already, but I’m sure there is more that I need to look at.

Hi Dieselman,

The engine barely idles, then when the throttle is opened very carefully and slowly by hand, it may surge to 1000 rpm and immediately die, or, it will just die right away. Acts like it’s not getting the fuel it needs to match more air coming in, like the injectors are not increasing fuel in reaction to the TPS or ignition module pulses. I do have a scope to check the engine revs pulse at the ignition module, but will first replace that HEI with a new GM/ACDELCO and check the white wire.