Unsolvable V12 Idle issues.. Help from Pro's is needed!

Good morning, Help only from Pro’s is needed.
1994 Jag XJS V12. (Facelift)

As you will see from the movie below - Idle (Doesn’t matter Hot or Cold) is not stable at all. If in P or N - it jumps from 600 up to 1100 RPM. If in D or R - 750-500 RPM. And it’s always erratic.
If in driving mode - drives fine, maybe a bit hesitant to hold stable rpm’s on lower speeds.
From Iginition side - changed everything - new leads, spark plugs, cap, rotor, ignition coils, etc. AAV is cleaned and seems to be functioning properly.
ALL and i mean ALL vacuum lines where changed. Seems to be connected properly.
Cam cover gaskets and all others around where changed as well - so no leaks should be there.
Sensors - cleaned and changed as required (ATS, Coolant temperature, etc.) - wiring looms for those checked and inspected/changed.

From mechanical side i’m completely sure everything works as it should.

I was thinking maybe TPS sensor needs to be adjusted, but could’ not find any information about facelifted models…

Only proper Help… please… !

Video is Here -please PRESS

All help given on this forum would have to be regarded as proper.

Yea… especially random unnecessary part offers via DM :slight_smile:

Advice is worth what you pay for it.

Since I’m neither “pro” nor “proper” I guess I’ll just observe from a distance :slight_smile:



You might not be used to this forum Monsieur Kbakutis, but indeed we all all proper here…

To the matter:

Block it with a rag or something and see if it makes a difference.

Changing all the hoses doesn’t mean you eliminated all leaks…

Did you check:
Trans Vacuum Modulator
Vacuum Advance Capsule
Supplementary Air Valve
Extra Air Valve
Climate control (quite a few vacuum valves in there)
Brake Booster
Cruise Control Bellow and Valves
Is there an idle compensation valve?

There could all have an internal leak messing with your idle.

Not familiar with the Facelift (is it a 6lt?) but I would start with measuring it’s continuity.
And for the 5.3 it should read 3.2v at idle.

Also Centrifugal Advance and Timing?

Good luck.

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Make sure BOTH throttles are ‘in-sync’.
Unbalanced throttles can cause idle issues.

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This kind of erratic up/down idling is almost always a large vacuum leak.

I would suggest you tackle what appears to be an overheating issue (gauge is reading way too high) first.

These engines do funny things when they’re hot, your Merelli car makes fueling adjustments, retards timing, etc., as a result of it trying to maintain a happy baseline and somewhat protect itself. These ECU adjustments in turn can effect the idle.

I work on these everyday and everyday learn something new / you’re only a “pro” for maybe five minutes until the next mole is up to be whacked, so I think you’ll find it’s best to get responses from the entirety of the huge arsenal of talent within these “walls”.

Three things I see. 1) does a vacuum leak cause an up and down idle? I thought it caused a
consistently high idle.
2) The oil pressure gauge fluctuated periodically…and rapidly, not smoothly.
3) The voltage gauge fluctuated periodically…and rapidly, not smoothly.
Are these three events and the up/down tach connected ? SD Faircloth

Though it is unlikely to be the cause this is the method to adjust the TPS.
You will need two separate DVMs or a dual trace one.
The connector has six pins and the grounds are green.
You will need to establish a way to connect to the grounds and positive leads with the connector and with the TPS connected to the car.
I used thin insulated wires pushed into the connector pins.
Set the DVM to DC Volts and turn the ignition to ON.
With the TPS un-tightened but not sloppy loose rotate it to where the two voltages are balanced to withing one or two tenths of a volt.
If you can’t balance the voltages, replace the TPS.
There might be one or two TPSs in my parts bins if you need one.
This is exactly how the Jaguar PDU diagnostic computer does this.

Not sure about Marelli, but on Lucas, if the RPM gets too high at idle, the ECU cuts off fuel (like you’re cruising down a hill). So too big of a leak will cause idle to go up, ECU cuts fuel, it goes down, and then fuel comes back and up it goes again.

I was aware of the cycling and cut-off. Not sure at what rpm the cutoff is actuated, without searching again. Might very well be at the 1,100 point. But, not reaching the cutoff point, a vac leak leads to high idle. I was really more interested in the other mentioned gauges fluctuating. That is likewise not normal. It might suggest an electrical/possible ignition issue on top of the fluctuating idle. Separate issues or all arising from a common cause ? SD

Most of the time I’ve solved an issue on my V12, it was always twofold. Seems these cars like to fail in two ways at the same time. Good to keep in mind when trying to diagnose.

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Thank you everyone for your help. Issue solved and was caused by:

  1. Damaged O2 Ground Wire (Caused FF44);
  2. Damaged ATS Sensor wire

Both of these made mixture waaay to rich, thus idle was incorrect and MPG where really low…
I was sure that technically it was OK - it made my search for a cause a bit easier.

Thank you.