Smooth start up Rough Idle 1988 XJ-s HE V12 CEI ignition (Australian Delivery)

Good afternoon Team,
After months of testing and fault finding on my '88 XJ-S HE V12 the problem of rough idle is getting worse.

After I had completed the rear cage rebuild I had the car running around quite well, merely having a ‘miss’ whilst at highway speeds. I had the car dyno’d and the result was 150HP and running lean.
Based on that I sleuthed around the fuel system, including:
a. having all injectors tested (replaced gauze filters etc),
b. sump tank out (clean as a whistle);
c. fuel pressure tested, good at 39 psi;
d. Fuel regulators checked, LHS was U/S and has been replaced (I thought that was the problem).
e. TPS set to correct voltages;
f. Throttle bodies cleaned and set to 0.002", rods and stops aligned to ensure uniform opening;
g. ECU vacuum feed is good;
h. New fuel filter;
i. Fuel lines blown through, from front to rear.

Car, at that stage started and warmed up perfectly, once warm up is complete, the engine reverts to stumbling, missing condition although, with reasonable power.

So onto timing.
Checked the distributor weights freedom, good,
disconnected the vacuum advance,
got the car to 3000 rpm, and advanced timing to 18 BTDC, as per manual/various XJ-S websites…

This had made the performance worse? Stumble worse, car is harder to start, although once started warms up perfectly until warm… then rough idle.

I have since checked for air leaks, plugs are dry sooty with a grey electrode (according to NGK this is too rich or too far advanced).

Are you sure your coolant temp sensor and wire feeds are working properly? Could be over fuelling if ecu thinks engine is still cold.

G’day Greg,
I did check that by: stopping the engine and short cct of the plug after the engine had warmed up; unfortunately no difference to the idle. I have also taken off the air cleaners , and checked, as best I can, the vacuum lines and various components against Roger Bywaters diagram on the AJ6 website, referenced from Kirby’s book.

This problem has been with me since I had the car into a Darwin Workshop to replace the rear main crankshaft seal. One of the workers there turned the engine when the distributor was out, and it hasn’t run properly since. I have this horrible feeling that I have a false reading for advance and somewhere the engine is out of sync with the distributor?

I have been under the car yesterday, and confirmed the harmonic balancer mark is in fact the right one, whether it is in the right place is something I am going to confirm this afternoon when I take the dissy cap off, to recheck the vacuum advance is working.
Thanks for your comments all are very welcome.
Cheers, Chris

What happens if you just disconnect it?

That’s a clue isn’t it?

One might think so, but as long as it went back in and timed properly it shouldn’t make any difference.

It sounds like a vacuum leak, but could be weak spark, though weak spark normally manifests itself with poor cold starting.

It would be worth checking the Gnd connection to the ignition system is sound and recheck the fuel mixture, or better still, perform an intake smoke test.

Good afternoon Aristides,
It is a clue, but as Diesleman commented I think they also messed with the cold start/warm run vacuum system on the RHS of the car, amongst other things. I have tried to compare my system with the Roger Bywater diagram in ‘the book’ and it is definitely similar to the ‘B’ system, but missing No 2 (a 2 way vacuum solenoid), and No 6 (another 2 way vacuum solenoid).

I lifted the car on the hoist yesterday afternoon and checked rotor position with harmonic balancer marking, and they are close: when I say close the rotor isn’t quite pointing at ‘1’ and the timing mark is a little past the ‘0’ position, but close enough for government work. I have recently timed the car at 3000rpm to 18 BTDC, and checked and set the other variables as detailed above, leaving possibly a vacuum system issue from the apparently incomplete system on my car, as per Dieselman ideas?

I have also been viewing an Australian Series 3 V12 owner on You Tube: Casper Graham, and he has completely removed the cold start system on his car. I lieu of trying to sleuth through the components (various) comprising this system I am considering removing it, and testing the start/idle cycle without the components in the loop.
Again my sincere thanks for everyone’s input it is really beneficial to bounce symptoms around amongst people who have seen these faults before.
Kind regards,

If you suspect vacuum issues try running the vacuum advance directly from the top ported vacuum.

Go back to basics.
The engine was warm when running on the dyno, so this isn’t a cold start issue and vacuum problems manifest themselves to a greater degree once the engine warms up, due to the intake air restriction becoming greater to reduce idle revs.

Is it possible the valve timing has been messed with, if not take it as read to be OK, but check the crank pulley marks by removing No.1 plug and using a follower to ensure the piston is at TDC on the firing stroke.
You should be able to see the distributor is also set correctly and use a timing light to set the Ign timing to spec.
Next use a vacuum gauge to monitor intake vacuum, record your results.
If vacuum is low, plug, or clamp any vacuum pipes to see which one makes the vacuum increase.
Perform snap throttle test to see if the engine can produce maximum vacuum on over-run.

A quick and dirty vacuum test is with the engine running, block the idle bypass hose. This should starve the engine of air and make it idle very slowly, or even stall.

I would investigate further and rule out the Coolant Temp Sensor. You should be getting these readings. Make sure you’re getting the same readings at the ECU.

0 degrees C (32F) 5900 ohms
10 degrees C (50F) 3700 ohms
30 degrees C (86F) 1700 ohms
50 degrees C (122F) 840 ohms
70 degrees C (158F) 435 ohms
90 degrees C (194F) 250 ohms

Good morning team,
Apologies for my slow response I have been acting on your siggestions which took some time.
a. Checked the coolant temp sensor at 30 degrees 2500 Ohms; 50 degrees 920 Ohms; and 80 degrees 540 Ohms. So within spec I would suggest?
b. Am conducting vacuum testing this afternoon.
c. Engine stops the moment I unplug the temp sensor;
d. The air sensor has little to no effect open or short on anything (as you might suspect). In other posts it has been described as a fine tuning input to the equation, not a mass air flow style contributor. I don’t believe we have a mass air flow sensor on this engine set up.

Interestingly, having taken off both air cleaners the start idle has rocketed to 2000 rpm? I will be reassembling them this afternoon to ascertain if the various RHS vacuum system components need the system to be closed up to work properly.

As a check I also took the AAV out and tested it whilst I had the air cleaners off to check opening and closing: all good.

SITREP to follow after this afternoons work.
Kind regards,

The resistance is a bit high.
It would not harm changing the sensor.

Now that’s weird, unless it’s a coincidence.
What is the state of the air filters?

From the photo it seems that the opening it hasn’t closed completely.
Is that the case?
You could adjust it by tapping the top part to insert it further in.
It should be fully closed at around 70°C.

Good evening Aristides, All,
Well another interesting day in the workshop. I put the AAV back in (yes it was fully closed at 70 C), re-checked the butterflys at 0.002", and then put the aircleaners back on: it sounds easy when you say it quickly! Started the car.

Starts and idles cold beautifully, back to 750 - 800 rpm on warm up. The idle when warm wasn’t great but it ran ok, so I took it for a test drive around the block. No acceleration/power missing badly on acceleration.
Drove back to the workshop, and thought I’d have a tweak at the timing on the vernier: the idle picked up, and the miss improved markedly, to the extent that it was very small/minimal (no worse than my old Daimler V12). However, idle increased to 900-1000 rpm. I have run out of vernier adjustment on the distributor, so I don’t know if more advance will improve performance.
So another test drive: plenty of acceleration and power, ran quite well, although Wendy said it smelt a bit rich as I zoomed up the driveway out onto the street.
Back to the workshop, tried to reduce the idle at the AAV; best I could get was 900 rpm with the bolt fully wound in? Re-checked the CTS voltages as the engine cooled 201 Ohms at 90 C which is again a bit out according to Greg’s table (more than 10%). I also checked the TPS as it is easy to get at it with a hot engine, and it had risen to 0.82V ignition on/engine off.
a. Reset TPS,
b. Check actual timing advance using current settings @3000rpm;
c. Check vacuum in the inlet manifold as per Dieselman’s advice
d. I have no advance left on the distributor as the allen head screw slots are at their limit, and the vernier is also at max advance.
Kind regards,

Hi Christopher,

You could remove the distributor and move it one tooth backwards.
If your harmonic balancer is slipping, or if someone messed with the markings plate your advance reading will not be accurate.
Clearly the engine needed more advance.

Only way to determine this is via the Feedback Monitor Socket, details in the Book.
Could be also the fact that your CTS is lazy.
Check also the resistance at the Air Temp Sensor, it should have some effect when you disconnect it.

Air Temp Sensor.pdf (8.3 KB)

Vacuum leaks… Block the AAV to see if there is a change.

Good morning Team,
Well, some interesting developments. Moved the distributor to the limit of the allen head screws (I did have some adjustment left); put the vernier back to full retard, and tried again. If anything I think the previous setting was better so, a little less on the allen heads, and mid travel for the vernier.
Whilst there I thought I’d re do the TPS. This has got to be THE most frustrating adjustment I have ever done, and I used to work on the Naval Combat Data System on Charles F Adams destroyers, which were sensitive in the extreme. Notwithstanding whilst there I noticed that the adjustments I had made on the throttle bodies had interfered with the rod adjustment , to the extent that one was too short and holding the LHS throttle open a tad. Fixed!
That made things better still, although the TPS is still out of the 0.32 - 0.36v allowable adjustment: it is spot on when I adjust it inverted, as soon as I put the TPS back on the pedestal it changes! Still I was warned it would require patience and time…so I’m back down the workshop this morning with coffee.
I have yet to put a vacuum guage on the inlet manifold but will do so as soon as I get the TPS correct.
I also invested in a Jaguar Parts Manual, which has some great (albeit small) diagrams of the vacuum system and components to compare with my system. I have had concerns that the enthusiastic, but inexperienced blokes in Darwin had left at least two elements out of the ‘B’ fuel/idle vacuum system which is different to the North American ‘A’ set up. Apparently there are other differences between Australia, and Europe/UK…why?
I feel I’m almost there, but I have, had enough of this old fragile system, and ordered a kit from Ole Mobeck in Norway to dispense with the vacuum/solenoid technology, and use a system which dispenses with the distributor, ECU, vacuum system, and solenoids various to provide contemporary ECU control of the cars fuel/air/spark system using crank position sensor, and inlet manifold vacuum (without any vacuum hoses other than one feed to the new ECU). I just felt that it was important to get the car running perfectly before installing this Field Change, as per good Naval engineering practice.
Looking forward to reporting a successful result.
Cheers Chris

Good morning Team,
Well another frustrating day in the workshop. I have tried many positions of the distributor advance and retard with no significant improvement. Obviously advanced is worse (or maybe not so obviously), and too far advanced casues pinging on acceleration, so I have got the advance to a position which seems the least rough.
I have ordered a new TPS as I cannot get the original to give the same readings twice in a row. Even moving the throttle brings it back to a different voltage: anywhere from 0.2 -0.5v. Here’s hoping a new TPS will improve the performance.
I did check the CTS and I have alreaqdy replaced this unit about a year ago, it also seems to be very close to the readings provided by Greg. So I’m leaving that alone, other than confirming readings as the problem continues.
I did get to the vacuum reading at the inlet manifold which is steady at 15Hg/Inch at idle.
AAV is good and has control of idle when engine is warm.
I also recently bought a parts manual for cars from 1987 on, and can confirm my RHS vacuum set up is as per the diagram in the book: this has been a concern since the car was worked on in Darwin.
I have also removed the second coil that was connected to the single square unit: why it was there I am unsure. Didn’t make any difference to the problem but did remove another wire crossing the engine bay.
So I am awaiting SNG to deliver my new TPS.
Cheers, Chris

The second coil is there for a reason Christopher, now your engine will run out of spark at 4.500 - 5.000 rpm.
If you want to delete the second coil you should change the main coil with the newer single version.

Not a bad idea, they have a limited life span.

Apologies Aristides, the main coil is the newer single version. I was unsure why I had the new coil and the second coil behind the front grill.

1 Like

OK latest development: replaced the TPS with the SNG Barrat substitute unit which was surprisingly easy to install. I now have an absolutely rock steady 0.32v at the TPS…it made absolutely no differenece to the idle issue.
I’m going amplifier unit next based on the fuel symptoms. I am now blowing smoke, under high revs and it appears to be too rich…as opposed to the dyno result earlier telling me it was too lean, but that was before I had an operational fuel regulator.
Now searching for the definitive amplifier module… Grant Francis suggestes an AC Delco GM unit D1906, Car Wizard says 1985 Chevrolet K5 Blazer: I’m going to search Kirby’s Book, there must be an equivalent GM unit in there. Cheers Chris

The .32V significance is probably the cut off voltage for an NPN transistor in the ECU.
That tells the ECU the throttle is closed. It would not affect the idle.

That is the one to get. It should say AC Delco on it. I bought mine from Summit Racing. I have a spare also.

1 Like