XJR6 4.0 SUPERCHARGD random misfire
 

Hello.
I have in our workshop this beautifull car.
The car appears with one cracked coil and the motor was misfiring. At firs attempt, we changed all the spark plugs and the coil that looks bad. The result it was better but the car still does a random misfire in cold, or when hot... it doensnt matter.
At todays we have changed:
All the 6 coils,
6 spark plugs,
The cranck sensor,
Cooler temperature sensor,
All the vacum tubes,
Intake air joint,
The fuel tank was cleaned and restored because it was rusty
The 6 fuel injector were cleaned in a proper company
New gasoline and air filter
The compression was tested and ok

Now we are thinking to remove the throttle body and try to change it, because it was also cleaned
Need some help from someone please...

Thank you.

Does the engine coolant temp get above 88 degree C for the ECU going into closed loop using the O2 sensors

O2 sensor connectors clean as well as the ground stud post

Does the misfire accrue only at Idle , High RPMs with no load , High RPMs with high load

EGR error signal

TPS signal smooth through all throttle ranges

Does the throttle cable return to the idle stop with the TPS signal resting at between 0.58 volts DC and 0.62 volts DC on the middle Green / Yellow wire on the TPS connector?

TPS connector have the connector lock egaged ?

Mass air flow sensor at idle reading 1.2 volts DC on the Green / Pink wire

ECU connectors clean ?

Does the 2nd fuel pump run ,this turns on at 4000 RPM but you can jumper it to run at all times

These cars are sensitive to both coils and spark plugs. Use the cheap plain copper plugs, nothing fancy and definitely not platinum or iridium. I use NGK BKR6ES and they work, or Champion plugs

Coils are a known weak spot. The original coils were made by Diamond in Japan and were high quality. They are no longer made unfortunately, and all the aftermarket replacement coils are of poor quality - including what you can buy from a Jaguar dealer. There are many reports here of the coils lasting days to months before failure. I'd not be surprised at all if you had bad coils right from new. If you could find a set of "known good" original coils that would be best.

One thing Motorcaran ( Bob ) suggested was to watch the coil tops in the dark and watch for arcing to the valve cover coil walls as a sign of coil degrading

This will pit the coil wells and can be checked for on coil removal

On a high engine load the spark plug has higher resistance and is more prone to find a lesser resistance path ( this would be a carbon tracking scenario without the carbon )

The large engine ground strap from the starter mount bolt can be checked for a clean connection and this is best accessed from underneath with a 13 mm on a 3 inch socket extension and a 10 mm for the car frame point

Just wondering what evidence Jagboi64 has to support his statement to definitely not use iridium or platinum spark plugs.
I have had a set of Denso IK16 iridium plugs fitted for the past 33,000Km with absolutely no problems. I might just mention that the actual Denso recommendation (obviously based on gap) is IK22 which is however the incorrect heat range. I fitted the correct heat range IK16, however they are gapped at 1.1mm and you need to very carefully regap them to 0.9mm.
Comments have previously been made on the forum that the iridium plugs may actually be kinder on the coils.
Just as a matter of interest my XJR at 190,000Km is still running the original coils, coil problems having never been an issue. I would also mention the rubber sealing the plastic coil cover has never been fitted to my car allowing decent cooling of the coil wells & escape of any hot air.
I have also never suffered idle problems which could be directed to the spark plugs.

John Herbert
1996 XJR

I think the relationship with the iridium and platinum plugs and the coil life is a current flow relationship having a lower current flow and less wire insulation heat breakdown / degradation effecting the life and voltage output performance of the coil

Jaguar may have stayed with the copper Champian's ( they gotta pick one ) as that was all they had proof on

Once the coil life has been degraded it doesn't mater what kind of plug you put in it will not restore the coil

But on the other hand if you bring the plug gap in closer you may be able to get the degraded coil to fire with enough impact to have a good combustion under any load / unloaded condition

In the opposite sense bringing the gap out in the TSB would decrees the coil current and extending coil life

I was able to find the performance spec in a Wells replacement part manual and the coils start out at 35 , 000 volts

You can set up a spark gap test rig with 2 pointed probes replacing a spark plug is a very involved equation I have not gone through

Paschen's law :

I'm pretty sure it came from XJR Engineer on this forum who said to only use the copper plugs. I know I tried some Bosch Platinum plugs and I didn't drive more than 100km before pulling them out, the engine misfired badly on them. Put plain copper back in and it ran fine.

Here is the post: https://www.jaguarforums.com/forum/x...56/#post543034
He was involved in the design of the ignition system when he worked at Jaguar so I trust his knowledge.

I drove an XJR/6 for several years as a DD.

There is just 'something' about sparkplugs and AJ16 engines. Many varied experiences have been told.

As a result of so many mysterious driveability issues over the years I replaced and experimented with many different spark plugs. I don't recall any problems with iridium plugs nor recall any change in behavior by switching from one brand or type to another. However, I DO recall at least one instance, maybe two, where replacing spark plugs solved the problem....even though the existing plugs were virtually new and didn't have any visible sign of fault or defect.

Sorry I can't remember more. This was a number of years ago and it's a bit blurry.

Cheers
DD

Andre, I am not generally an advocate of "italian tuneups" and indeed am not suggesting one here, but given the extent of the work which you have carried out, and the cars implied back story, I wonder whether a period of regular use might not be a good idea at this point. The misfire may disappear or further symptoms develop, but either way you will be better placed ( and will have had the pleasure of driving it!)

I'll just leave this here:
http://www.jagrepair.com/images/TSB/...oor%20Idle.pdf

With an XJR an Italian Tune Up is simply driving the car as the designers intended it to be driven :)

Cheers
DD