www.jec.org.uk

Hello all,

I thought I should share the output of my last 2 days trying to get to the bottom of a "Check Engine" light on my 1994 6.0 V12.

A bit of background - the car is in generally good condition, 76k miles, with new (OEM) fuel / air filters, new (2017) Marelli branded ignition / HT leads / plugs but I have not been able to shift the cursed "Check Engine" light. It tended to come on as the car warmed up (but before fully warm) - or randomly once it was to temperature - on light or trailing throttle.

The car had just been to David Marks for an extensive service, replacement ABS module and new lambda sensors and David kindly pointed me in the right direction for a bit of home detective work by supplying me a copy of the workshop diagnostic flow chart for FF23.
I followed the Jaguar diagnostic logic sequence, as described below, and then went off on my own tangent!

So:

Step 1) Check Lambda sensors - all ok, as replaced by DM.
Step 2) Check EVAPP (electronically controlled purge valves in the front LH wheel arch) - I checked & cleaned the connections and determined that they were responding to the ECU as you can feel them "pulse" when the car is warm at idle. However, I could not confirm if they were sealing correctly, so I have found some New Old Stock (from Australia!) and will fit them when they arrive.
Step 3) This is where the diagnostics gets a bit vague - it basically asks you to look to see if any other fault codes come up and rectify them! I can't read the ECU for any other faults - and I don't have a trip computer fitted (so I am having to guess that I keep getting the FF23 fault identified previously) so my only other item was fuel pressure. I can't find a source of new 6.0 V12 fuel pressure regulators (3.0bar) and I don't have an inline gauge to hand to check the performance of the current one (as has been demonstrated on this forum before).

... so I did some more digging and came across this recommendation for FF23 causes on the jag-lovers forum (https://forums.jag-lovers.com/t/1993-xj ... f23/354476).

possible faults:

high fuel pressure
o2 sensor heater ground loose or engine ground loose
ECM ground loose or corroded
purge valve stuck open
High resistance in MAF connector
Incorrect MAF calibration
Vacuum leak in intake
defective o2 sensor

Now this list set up for the AJ6 / AJ16, as it describes checking the mass airflow sensor (MAF), but the 6.0 V12 I have learnt doesn't have a MAF, instead uses an intake air temp sensor (IATS) and manifold absolute pressure sensor (MAP), integral with the Marelli ECU (connected via the small red vacuum tube from the tapping on rear end of A bank intake manifold).

Sure enough - I found a small crack in the elbow of the vac tube to the MAP sensor in the ECU where it connects to the manifold - all fixed with a cable tie. I also cleaned the IATS connections to be sure.

Step 4) Driving the car from here seemed to have fixed it, but on the return home, the light came on again and the car started "surging" on trailing or closed throttle. It also seemed to be idling too high and then surging between 1500 - 1000 rpm at idle.

A bit more searching gave me: https://forums.jag-lovers.com/t/xj-s-hi ... /348262/15

Step 5) Which led to cleaning of very dirty throttle bodies (see image - A bank)
and some adjustment of the idle screw on the AAV. I think the AAV idle screw has been adjusted fully out (to compensate for all the dirt in the throttles) and the valve was also sticking on operation, hence allowing too much air in and thus acting as a leak into the intake -> hence the Check Engine light as the ECU tried to correct for it. I think someone previously had this issue and had just tweaked the idle speed to fix it!

The idle is still a bit hit and miss, so I have bitten the bullet and bought a new AAV from JLR, which I will also fit when it arrives.

Fingers crossed! I hope this tale helps others struggling with either high idle / surging or FF23 issues.

AF

Hi Andy. That's a great bit of detective work and let's hope that proves to be a lasting solution.

Another one, but one that should not apply to the 6.0 engines, is defective earthing. 6.0 cars got extra earth straps from the plates at the back of the cylinder heads to studs on the bulkhead.

Hi all,

A bit of an update on my FF23 and high idle issues. I have made many things better, but it is still not completely fixed!

I have fitted the new AAV, along with the air valve to manifold hose which connects to it. As you can see in the pictures, the original was dirty but not completely gummed up. On examining the new OEM part, the internal spring appears to be different - with a smaller coil diameter and longer free length. It looked a bit odd and twisted, so I tested both in the kitchen with boiling water and they both close correctly. So on with the new part and keeping the original as a spare.
Before trying to get the idle speed correct with the new AAV, I checked both throttles for their position at idle, having previously cleaned all the muck out of them. They were nowhere near right! So I reset them as per the workshop manual with a 20thou feeler gauge at top. I also found that the throttle bell crank linkage on A bank was sticking when hot, so lots of cleaning and WD40 on the linkage got this moving correctly. I also made sure the cruise control actuator rod was moving freely and not binding up.

I then ran a 300ml bottle of Wurth fuel Injection cleaner through about 60 litres of fuel to help clean the injectors up.

The 2 new EVAPP purge valves arrived from Australia and these I fitted, while checking that the connecting hoses were clear and not cracked.

Finally I made a concerted attempt to set the idle following the workshop manual directions of working on a warm engine at idle with no DTCs showing and with all loads turned off. Wait 2 - 3 minutes and then adjust on the AAV. Well, I seem to get to around 750 - 800rpm and the speed appears pretty stable, though the idle quality wasn't great.

Off for a drive and everything did feel smoother, with engine pick up under load feeling "cleaner" and more linear than before.

Having driven the car for about 300 miles to and from Yorkshire over the August bank holiday weekend, I found that I could run for miles in a great condition and then sporadically the CEL would come on again - but usually cruising at motorway speed, constant throttle at about 2000 - 2500rpm. It would be accompanied but a couple of engine "coughs" or shunts and then the CEL would come on. Also, with the more I drive the car, the better the idle seems to become. I have read that the Marelli ECU has an idle fuel trim adaption feature which adjusts idle fueling when a series of pre-conditions are met (warm idle in P or N, no DTCs, 8 mins cumulative running at below 900rpm etc), so perhaps this is working to improve the performance.

One final check I made, thinking perhaps that the CEL could be caused by a small constant vacuum leak, was to measure the manifold vacuum pressure at idle. I bought a simple gauge (with a 3"Hg offset error!) and hooked it up to the vac port on the intake manifold at the rear of A bank, next to where the Marelli ignition ECU vac hose is connected. At idle it showed a healthy 20.5"Hg with a small flutter of the needle, which smoothed out when the throttle opened. Consulting the manual for the gauge, it suggests that this could indicate loose valve stems, but as the car does not smoke or use abnormal amount of oil (0.5 litre every 1000miles or so) then I don't feel this is a concern. The key point is that the ECU is getting a plausible vacuum signal under load -> so unlikely to be the cause of my CEL.

So that's where I am up to - definitely a lot of things improved, but not fully solved. But I have enjoyed myself enormously in getting my hands dirty and trying to work out the complexities of it...

If anyone has any further ideas on where to look, I would be very grateful to hear about them!

A few more pics.

Original AAV
A bit gummed up, but not terrible.
New and old AAV comparison
New AAV with odd spring

Purge valves going in...

Marelli vacuum tube repair
Vacuum measurement at idle - note the gauge has a ~ 3"Hg error at zero, so this is actually reading about 20.5"Hg
As I said in the main post, I am now a bit stumped as to where to go next.....

Hello all,

I though I should post the conclusion to my 2 year investigation into the engine check light (CEL) on the V12. I am happy to say that it is finally fixed - through a combination of elimination of possible faults, correction of lots of small errors and the knowledge and experience of David Marks!

My previous posts have shown how I uncovered a number of issues in the engine bay, as I tried to work systematically to remove the FF23 fault which was causing the CEL.

In addition to those actions listed in my previous posts, I replaced the following sensors:
* Temperature sensor (DBC3728)
* Intake Air Temperature Sensor - Fuel - replaced with EAC2863
* Intake Air Temperature Sensor - Ignition - replaced with EAC2863

I checked vacuum hose at PCMF ECU end - > in boot under battery - it pressure tested ok (with both engine and ECU ends disconnected and capped off). The PCMF electrical connector visual inspection was ok too. The rubber vacuum tube DAC1017321 was kinked at the end where it goes into metallic section of hose under battery/floor. So, as the original hose is NLA, I sourced some replacement vacuum hose, cut to be slightly longer than original to ensure that there were no kinks at either end.

I was also concerned about possible fuel tank corrosion - could sludge be blocking the fuel pick up, causing the stumble and illuminate the CEL due to fuel starvation? So I bought a small digital endoscope and checked inside the tank. I was pleased to see that there was no corrosion anywhere in the tank.

So this left me as a bit baffled as to where to go next. My strategy was to try to ensure that both the PMCF and PMCI got good signals for all their inputs.... so this is where it was left until this year's service and MOT with David.

We discussed the issue at length, with potentially the throttle position sensor or crankshaft speed / position sensors being the culprit. "Leave it with me" said David, and after some diagnostics with the ancient and very rare Jaguar PDU diagnostics kit, he discovered that the car was now flagging up FF44/FF45 fault codes, rather than the FF23 as before.
This led to the answer to the issue - Jaguar Technical Bulletin 18-57, dated August 2001, MIL Illuminated – HO2S – DTC 44 and/or 45 – Add Ground Straps – Service Action S494 for 1994 - 1995 MY V12 XJS and 1994 XJ12.
To quote from the Bulletin:

"Some XJS 6.0L V12 and XJ12 series vehicles ... may experience customer concerns of the ‘check engine’ malfunction indicator light illuminating with diagnostic trouble codes (DTCs) FF44 and/or FF45 stored. These DTCs show an inconsistent reading from the Heated Oxygen Sensors (HO2S). The HO2S are giving a faulty reading due to a slight degradation of the engine to body ground straps.

In order to prevent this issue occurring again, two supplementary ground straps need to be added to all vehicles within the above VIN range."

This was the very thing Paul Glover had suggested, earlier in the thread! Foolishly, I had thought that, given the full dealer and specialist service history on the car, these types of issues would have been resolved. It appears that my particular car was last serviced by a dealer in June 2000, then by a well known independent in October 2001... so this Bulletin must have been missed! This sent me off looking at other Bulletin topics, but that is a story for another time.

And finally, here are the images of the additional earth straps fitted which resolved the issue. 2 x new black earth straps per side with copper coloured fittings mounted to the back of A and B bank manifolds and down to their closest earthing points on the engine bulkhead.
I am very pleased that this one is solved.

I hope these posts are of help to others in future, who may have the same issue.

Stay Safe

Andy