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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. And if you are driving and releasing your gas pedal RPM drops to 500-600. - Once again NOT always (70% of time);
From Ignition 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..
GM... Has it ever run properly for you, and when? Did it prior to the work youve reported? Did you do all the work you mentioned in one shot or, was it some here some there, and the issues popped up after some of the here, or some of the there? Might have to start back tracking some...
A bit of a long shot but that's sometimes the behaviour you might see of an air leak on the engine side of the MAF.
The excess air causes the car to think it's running lean, so it increases the fuelling, revs rise, the car works out it's idling high, drops the fuelling, revs drop, and the cycle continues.
Look for an air leak after the MAF, maybe a vacuum pipe, or perhaps using a spray on the intake manifolds etc.
Paul:
Excuse my asking; but does the V12 Marelli have a MAF?
Yoopas:
Also the TPS is a definite candidate for the cause, in the V12. I have feeling that the The TPS should be set to the normal closed voltage of 0.32 on all V12 HEs in XJS cars, Marelli included.
Good point, Greg! The air intake circuit on the V12s is fairly primitive so I guess not! So maybe you can't get that idle cycling like you can on the 4 litre?
Ok. Good to know. I didn't look when hearing 1994 MY, and didn't know that it was a Marelli thing, so good to know.
I had something similar with my car 2wice... Both times it was bad and faulty wiring with 1,,, coil wires and connector at the coils,,, and 2, it was the main 12v junction where a B+ 12V source splits off (4 or 5 ways) to feed (from memory) coils, ignition amps and another 12v wire that heads towards the front of the car.
You did a lot of digging around these wire sets (coils AND this wire loom) so see that you didn't disturb anything while making changes. The wiring is old and can be fragile.
Check the condition of the plugs at the coils. Check closely, as both times I had problems the fault/failure was INSIDE of the coil connector/plug. Where the copper wire (there are 3 of them each coil) actually is crimped onto the clips that slide over the pin on the coil. The wires had broken, and only 2 or 3 strands of copper remained in the crimp.
Then, the second area to take a look at is the major junction. You can find that "junction" just on the side of the fuel rail on the LH side. Wiring loom there. The junction (on my 1990) was roughly between the 4th and 5th injector plugs. It's taped and covered. Remove the tape and get eyes in it.
Also, make sure that when you plugged the coil plugs INTO the coils that the PINS in the coil(s) didn't get pushed off to the side, not making it into the plug, and are bent out of position.
While you're there check the condition of the dreaded white co-ax wire. Same exact neighborhood.
Back to basics and no 'I know that is OK' statements. You absolutely can get idle hunting as described on the V12, and it is generally air related.
Step 1 confirm the throttles are correctly set up before you go one step further, nothing has slipped and the linkage bushes etc are OK, springs in place etc etc.
Step 2 check for air leaks - brake clean works well but be careful and discriminate with it - if you spray in a location other than the air box intake trumpet and the revs increase for sure there's a leak in that area.
Step 3 make sure all the 'emissions' stuff is functional - the solenoids and vacuum delays and such, don't assume that a connected hose is OK
Step 4 verify the operation of any fuel enrichment devices - I'm not sure what is on your car - aux air valve etc etc
but there is a known 'modification' for erratic idle that can be sorted easily and really cheaply
you need to ground both sides of the inlet manifolds to the firewall , by way of a ' fly lead' , this ensures the buildup of induced voltage does not send erroneous signals to the ecu that tries to constantly compensate, the new extra ground wires are shown in red here, and where they are connected to,,
Good point, Greg! The air intake circuit on the V12s is fairly primitive so I guess not! So maybe you can't get that idle cycling like you can on the 4 litre?
Guys, thank you so much for such response! I'm staggered
All the work mentioned above in my thread was done because of this unstable idle. First i've started looking for an obvious vacuum leak, but the further you go the more you change.. Jag's.
After changing all of mentioned above - cat runs really smoothly - Coin test - passed. Unfortunately Idle itself - different story.
I'm excluding these possibilities:
Crank position sensor (front of engine) and the rear position sensor (by the flywheel)? - With these Faulty sensors car would run (even drive) noticeably incorrectly + error code would be generated;
Vacuum leaks - absolutely all hoses have been changed - there are literally no old hoses left.. And in general Idle position (Before drop down) looks good - around 800-900 RPM's.
Injectors connectors - if these would not work correctly i would immediately feel non working cylinder and temperature of manifold there would be way lower - it is not.
Both manifolds already has ground's connected to firewall (Factory recall);
Emission stuff - connected - question how to check if solenoids are really functional..? Interesting thing here - many many and i mean many different possible connection diagrams
TPS sensor - tried today to look for a voltage as suggested connected Red/Yellow wires - got 0,19 Wolts. But after trying to move speed cable - Voltage never changed. - i'm assuming either incorrect wires where measured or something is not correct with facelifted ones. - + in case of incorrect readings error code would be generated.
So.. i'm confused
Current consumption is around 30L/100km or ±9MPG - Pure city ride - i assume it's perfectly normal for 6L V12. - So not so rich. - And my foot is not so sensitive with this cat
Find an appropriate point to connect a T in the vacuum system and check the value.
I don't know what that value should be but I guess somewhere when running around 14" Hg.
Check as many points in the vacuum system as you can.
See my earlier post for TPS voltage balance.
With a DMM or old school meter with needle, one probe in the yellow, one in the red - TPS still plugged in (essentially back probed), key in the ON position, car not started, you should read between something like .32 to .36v.... with a manual hand turn of the the throttle wheel at capstan,,, voltage should smooooothly increase to top out at around 5v...
I didn't see anything about the wiring to the coils and the "major" junction I referred to... Worth a look,,, I'm tell'n ya.
Current consumption is around 30L/100km or ±9MPG - Pure city ride - i assume it's perfectly normal for 6L V12. - So not so rich. - And my foot is not so sensitive with this cat
That consumption is massively too high. Even all city driving in a HE you should get more like 15 per 100 = 14 mpg
I recently posted that on the JagLovers forum where you asked the same question.
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.
I recently posted that on the JagLovers forum where you asked the same question.
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.
What is (or is there) a base low voltage? A max in the other direction?
08-12-2022, 02:50 AM
Unsolvable V12 Idle issues.. Help from Pro's is needed!
