I have read the Series III XJ6 does not idle that smoothly. My car otherwise drives well, but at idle it’s a little rough. Not terrible; just not smooth. I can feel it idling. Is this normal?

 

I'll be watching this with interest - my 86 XJ6 also does not have the kind of idle I'd expect from a straight 6. I don't like to accept "that's just how they are" as an answer (even when I sometimes should). There's always the possibility for some improvement if one is interested in "wasting" enough time on it, and I'm curious if this has been pursued.

 

Many before you have gone through extraordinary lengths seeking the glass smooth idle 24/7/365. No success. Improvements, yes, but not what they were after.

Start with a compression test, of course. No sense in struggling with this issue if you have uneven compression.

Having injectors professionally cleaned always seems to help. Install with new seals, of course.

Trace all vacuum leaks.

Adjust valves (PITA)

Make sure everything in the ignition system is 100%. Not 99%.

Richen idle mixture slightly.

Clean the throttle body and air distribution block.

That's off the top of my head. I'll try to find some info from 20+ years ago from a guy who made it his life's mission to get a 100% smooth idle....and almost succeeded!

Cheers
DD

 

Found it. Not as hard as I thought it would be. This is a cut-n-paste from a write-up by a Jag lover named Ted Macklin a long long time ago
Cheers
DD



1)Replaced all the seals at the fuel injector plus replace all fuel injectors that have a crack in the plastic "Top Hat".

2)Replace all rubber hoses (large and small) that connect to the intake manifold esp the aux air valve hoses. And the little rubber cap at the bottom/back of the intake manifold. Don't forget about lots of little hoses that run under the intake manifold. Don't forget the brake booster vacuum lines = the rubber parts.

3)Seal the bolt threads that secure the aux air valve and the ignition amp to the intake manifold with teflon tape.

4) Make sure the cruise control bellows is not leaking.

5)Double check all plastic "T"'s at vacuum fitting junctions for leaks.

6)Replace the rubber Bellows at the AFM and the two "rubber bands" in that same air path.

7)Replace the "0" ring in the oil filler cap, the seal in the oil dip stick, and the "Coon Skin Hat" atop the breather pipe and any other rubber in that path to the Throttle body.

8)Check the distributor diaphram for leaks.

9) Add "three wire" O2 sensor.

10)Perform compression test and do valve lash adjustment if required, go for .013 .

11)Verify that you have stopped all vacuum leaks by decelerating from 60 MPH and observing that the instant MPG read out on the trip computer jumps briefly to 99 MPG. 12) Be certain that the CAT is not plugged up. 13) Clean throttle body butterfly and confirm .002 clearance.

14)Carefully route ignition wires using OEM "wire looms" to prevent "bleed over".

15) Adjust AFM idle mixture for about 5 turns out from bottom and tweak by feel from that point.

16) Return AFM clockspring to OEM setting if you messed with it.

17) Of course new plugs (Champion) and wires and dist. cap and rotor button. Verify

18) Redress wiring harness (over water jacket) with fresh tape.

19) Install synthetic oil.

20) Be certain cooling system is healthy.

21)Install Chevron 87 Octane in the tanks.

22) Buy a TIFF vacuum leak detector to verify once and for all that you have eliminated all vacuum leaks (if you still have a rough idle).



John (JAT1127@hotmail) has brought it to my attention that there is another area in the Throttle body assy that can be a source of vacuum leaks....... While the parts book shows ONE seal only on the TPS switch side of the butterfly, Flyer94a, a former lister, says there is another seal on the opposite side of the butterfly (from the TPS). Accessing the second seal requires removal of the shaft that the butterfly mounts on. (I have yet to tear mine down as the new seals are currently b/o at CVWest so I can't verify the facts yet.) But I will keep you posted. Flyer94a also advises: Be CAREFUL removing the screws from the butterfly valve - they are split and flattened on the backside. This job will be much easier if you just remove the whole throttle body assy from the intake manifold. I believe this now makes item #11 on the long list of potential vacuum leak sources that I have compiled over the last two years. (See the CVWest homepage for the Tip of the Month if you want the full rundown.) Regards, Ted Macklin

 

After much checking, all the above issues, found my distributor upper bearing failing, letting the star wheel hit the pickup at idle.
Check clearance of pickup with pressure toward pickup, then again with pressure away from pickup.
Rgds
David

 

Is the theory that at idle the O2 sensor will give the computer an erroneous signal due to not being adequately heated? And does the computer not know to ignore the O2 sensor at idle?

 

I can't remember. Or maybe never knew. It was a long time ago. And I haven't even touched a 4.2 car in 15-16 years.

The ECU does have a "Lambda inhibit" circuit which comes into play below xxx-coolant temperature.

And the throttle switch is designed to richen the mixture at WOT and simultaneously defeats the O2 sensor signal. I can't recall what's supposed to happen at idle.

Some guys kept the single wire O2 sensor but added an extra ground and reported better running in general.

Cheers
DD

 

Hey there! If you're experiencing a slightly rough idle in your '87 XJ6, it could be due to a variety of factors. Firstly, check your spark plugs and wires to ensure they're in good condition and properly connected. Additionally, consider cleaning your throttle body and idle air control valve to improve air flow. It's also worth checking for any vacuum leaks or fuel delivery issues. If you're not comfortable diagnosing and fixing these issues yourself, consider taking your car to a mechanic for a professional inspection and adjustment. They'll be able to pinpoint the exact cause of the rough idle and make the necessary adjustments to get your XJ6 running smoothly again. And if you are a student and need looking for do my homework for you so you can go myassignmenthelp.com

 

Hi Danny…yes, the xj6 idle is most often “not smooth”. Some people have gone to great lengths to try to smooth it. Very occasionally they have success. Most often not. Both of my XJ6 cars were like this…I learned to live with it!

See you at Jaguars on the Island?

 

Thank you, Gregory. That is what I have read and I have been ignoring it. I am going to check for vacuum leaks and fuel issues, just to ensure my idle isn’t worse than it has to be.
Yes, you will most definitely see me and my XJ6 at Jaguars on the Island. It’s my favorite show and I look forward to it all year. I will also be at Jaguars on the Green and the Heritage Classic.