Jaguar XJ6 Series 3 rough idle

Since I bought it, my '82 XJ6 4.2 has had a rough idle. It doesn’t die, even when you are stationary in drive, and the moment you give it some more rpm’s it smoothes out. Also, messing with the screw atop of the MAF sensor, I noticed something odd: It runs with this rough idle with the screw all the way in, and uncrewing the screw at all just makes it run worse and die, the “factory” setting would be 3-4 turns out, as I recall?

I have a few ideas:

  • The air filter is not original, it’s some K&N cone filter, maybe it’s causing problems there
  • There might be a vacuum leak
  • The ignition coil might be slowly dying

If someone has more ideas as to what to check, I’m all ears, I’m tired of the Jag idling like a 60s muscle car.

The response to the AFM bypass adjustment implies that the engine runs lean, Alexander - what happens if the screw is bottomed? What is the cold and hot idle?

Suggestions; remove air filter (no change?) and fractionally pus in the AFM flap - normally a minute movement will cause the engine to falter. If not; it implies lean running - either an air leak or lean setting…

Also; you need to check ignition timing. Of course a misfiring plug, for whatever reason, will cause uneven idle. As will an engine - a routine compression test is standard practice…

xj6 85 Sov Europe (UK/NZ)

He said what.
It’s not the air filter. It’s a vacuum leak or the camshafts need to be realigned. Most likely it’s just lean. 3-4 turns might be the usual value but not a precise factory setting. Search for cracks in tge big afm rubber hose and block vacuum hoses for a start.

In terms of spark plugs and compression I don’t think that’s the problem, because the whole engine was rebuilt 6 months ago from the ground up. The cold idle is smooth at 900rpm, when it warms up it starts to idle rough. The AFM response also made me think it’s running lean, so I’ll be looking for some vacuum leaks…

Yes, I will do that, let’s see what I find. Thanks a lot :slight_smile:

You do not mention hot idle rpms, Alexander…?

The idle must be set hot by the idle screw - to some 800 rpms. With the engine cold the AAV opens to raise the cold idle by some 200 to 400+ rpms.

For some reason your cold idle is far too low; either because your hot idle is not adjusted properly and/or your AAV is malfunctioning - or whatever.

Have you checked the AAV? Remove the ‘top’ hose of the AAV and look at the slide. When cold the slide should be half or so open - when hot it should be fully closed.

Have you tried adjusting hot idle? If there is no engine response to adjustments; there may be an air leak. But check the AAV first - and we’ll take it from there…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

So there does not seem to be any vacuum leak, BUT: I changed the ignition coil, now it idles at 700-800 warm nice and smooth. So i don’t know, it might have been that.

Fair enough, Alexander; lots of things happens as everything warms up - and the coil might not have stood the heat.

What is your cold idle now?

xj6 85 Sov Europe (UK/NZ)

If you have the crankcase vent tube from the top of the front of the head hooked up, be sure you have a good seal on the cam cover oil fill cap, and also the rubber ‘cup’ seal that goes on the dipstick. I noticed an immediate rough idle when I took the oil fill cap off while the engine was idling. It causes an immediate lean mixture if you have any ‘large’ leaks to the crankcase, when the vent tube is hooked up to the intake as it should be. It may not be so obvious when the engine is ‘cold’ and the mixture is richer, but as the mixture leans when the engine warms up, it will become rougher.

Cold idle is about 1100

It’s in the ballpark for a working AAV, Alexander. If there are no cold or hot starting issues, it’s good enough…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Out of the blue:

The air filter. Too small? dirty?


sloppy distributor busings? Teh shaft wobbles at low speed. inconsitent gap. As speed goes up, the shaft stabilizes. gyrosopic action.