FF23 and High Idle on 6.0 V12 - Solved?

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FF23 and High Idle on 6.0 V12 - Solved?

Postby AndyF » Tue Aug 13, 2019 9:37 pm

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!


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.

1994 6.0 V12 Coupe

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Joined: Thu Sep 08, 2005 8:31 pm
Location: Salisbury, Wilts

Re: FF23 and High Idle on 6.0 V12 - Solved?

Postby davidr » Tue Aug 13, 2019 10:02 pm

Hi Andy. That's a great bit of detective work and let's hope that proves to be a lasting solution.
1996 XJS Celebration Conv. (shown on the JEC stand at NEC Nov 2015)
2001 XKR Convertible

Previously enjoyed!
1989 XJ-S V12 Coupe. 2006 XJ TDVi Sovereign. 1995 XJ6 3.2. 2013 XF 2.2d. 2009 X Type 2.0d Sovereign estate

Posts: 418
Joined: Mon Jul 19, 2010 8:06 pm

Re: FF23 and High Idle on 6.0 V12 - Solved?

Postby PaulGover » Wed Aug 14, 2019 12:24 pm

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.
1981 XJ6 Series III basket case
1991 XJS V12 "facelift"
2008 XJ TDVi

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