Rough Idle when 1992 XJS V12 Marelli is hot

It’s kind of funny, I replaced the wiring harness, did the fuel injector service a few years back and used to gloat how I could stand a coin up on end and have a perfect smooth idle.
Last summer I thought I would go all ignition mode out.
I changed the ignition coil, that was a dud, I changed the ignition module, and I changed the spark plugs.
I’ve had a ever so slight stumble at idle ever since.
Now this sounds like a trivial thing but 90% of the reason
of buying a V-12 Jaguar was the allure of how smooth they
are and I should be able to stand a coin up on its edge
during idle (darn it) ! Go figure!

And they are. Almost turbine-like. Except at idle…where smoothness can be a hit-n-miss proposition, no pun intended

I hear ya ! :slight_smile:


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You want smoooooth? Go and find any BMW six cylinder from 86 to 06 for smooth idle, off idle. mid load or flat out.

Even the 12s used in the beautiful 850s were butter smooth. actually turbine like.

Yup! I’ve driven a number of BMWs. Impressive. I really like a number of BMW models. I’ve been sorely tempted to buy one but, at the end of the day, they’re just too mainstream for me. I’ve grown to enjoy being the oddball guy who drives the old Jags. :slight_smile:



Careful! Word is their quality has fallen off the last few years.

By far the best all-around car I owned was my 2001 M5. It looked a bit aggressive and only BMW nerds really knew what it was. It was great right up until it spun the rod bearings at 80k, which was a known weakness in the 5l v8. I will never own another BMW.
On the up side, there was a cool video showcasing the e39 m5.

I removed the fuel injectors this afternoon. Not too difficult - took about 2 hours. These are the newer ones that do not have the stub hose going to each injector. I will send them off the the Jaguar Fuel Injector service tomorrow for refurb. Looking forward to a smoother idle when i get them back…

One question that I need help on. When I inspected the fuel rail, the rubber hoses that connect the two sides of the rail are showing cracks. Is replacing these just like replacing the stub hoses on the older injectors? If so, where can i get new tubing and new ferrules?

I would say the same, I didn’t have the M5 but a 1997 Black
540i. The exact same one in gone in the newer “Gone in 60 Seconds Movie “. As soon as it went off warranty I sold
It to my optometrist. I believe it was governed at 240 kph
But that car felt very stable at high speed. You had no idea
How fast you were going without keeping a close eye on the speedometer.

I could not find any ferrule of the right size, so used some fuel hose and injection clips (wide and flat steel rings in order not to cut though the rubber
I don’t have the size I used but can dig my invoices if needed

edit : found it : 17mm stainless clips

Thanks for the help.

I tackled getting the crossover hoses off of the fuel rails this afternoon. I was able to do it and was able to save the ferrules. Here is the process that I used. (Thanks ahead of time to SD Faircloth. He gave me the tips on how he does this job whild saving the ferrules for reuse.)

I had all four ends off in about 45 minutes. I went at it by first running my box cutter parallel to the tube and cutting off the outer rubber layer in about a 1/2" wide area. Made sure to get the point of the knife under the ferrule to get the rubber off there.

Then I ran the knife parallel to the tube and cut off the woven layer down to the inner rubber tube. Again making sure to cut it off under the ferrule.

Then I gently cut through the inner rubber layer with the knife perpendicular to the tube. I then bent the tube back with a pair of pliers to open up the slit.

I used a small screw driver to clean out the rubber and fiber at the cut but under the ferrule.

Once this was clear, the hose would pull off.
Much easier than I envisioned and again thank SD Faircloth for the guidance on how you do it.

SD also gave guidance on what hose to use to replace the crossover tubes. 3/8" FI Gates hose. Look at E-Bay item # 284020578220. That is the EXACT hose you need to use. Reuse your existing 3/8" hose ferrules. The spec for that hose is SAE30R14T2. Must be imprinted on the hose. I could not find this locally so ordered from the e bay source.


thanks for sharing the refs and way to proceed
I didn’t had the opportunity to save the ferrules, as they were lost during a previous replacement The PO used some braided fuel line and jubilee clips, result was leak proof, but poor looking and very agressive when you need to get your hands in the V

If your hoses are bad, you may want to check your olives (CBC9113) for the supply and return line connections. Mine looked closer to O-rings they were so deteriorated. A couple still left on eBay.

“2 NEW Viton Olives for Fuel Feed & Return Pipes on 1992-1996 Jaguar XJS XJ12”

Thanks for the tip. I replaced the fuel feed lines a few years ago and they still look good. Had a local shop rebuild the lines after i took them off. That worked fine versus buying new ones.

I looked at the olives when i had the fuel rails off. they were still there and looked to be in ok shape. I reinstalled the fuel lines and snugged up the fittings and low and behold they did not leak! Thanks for the tip and the source info for the olives.

A new update. Once i got everything back together, I still had a rough idle. I then changed the Coolant Temperature Sensor Bosch 0280130014 and the idle smoothed out nicely. I have an intake Aire Temperature Sensor EAC2863 on order and will replace that next. Will post if it made any difference.

I put in the new Lucas Air Temperature Sensor. When I started the engine I got an immediate ff17 code along with the check engine light. I cleared the code, started the engine again and got the same code. I cleared the code, reinstalled the old ATS, and everything was fine.

I am planning to return the new ATS for credit. Before I do, any ideas why this happened - was the ATS bad or is this indicating a different problem that is not apparent with the original ATS?

Measure and compare the resistances of both ATS, it might give you an idea.

The old ATS measures 2980 ohm at 62 deg F (16.7 deg C). The new sensor measures 2590 ohm at the same temp.

Seems like both are in similar ranges and roughly at spec. What else should i check?

With a wire or a paper clip short circuit the connections at the plug and see if the problem persists.

I finally got some time to try this. Here is what i find

Measured resistance on both sensors again. Warmer today but readings are in the same ballpark 1250 ohm for new, 1050 ohm for old.

shorted with paper clip ff16 air temp sensor failure

New Lucas ATS SNB801 (replacement for EAC 2863 FF15 and FF17 No info on FF15. FF17 indicated throttle position sensor issues

Put old ATS in place no error codes

From this I believe that the new ATS is either defective or is not a replacement for the current ATS.

Do you agree?