XJ6 & XJ12 Series I, II & III 1968-1992

jumpy idle

Old 05-25-2014, 11:41 AM
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Default jumpy idle

Ii have a intermentent problem with the idle and acceleration . many times the idle will jump around and the car dies . also when the AC is on it is worse. it has new plugs , wires the air cleaner is good and fuel filter is new.
any ideas on what may be the problem ????

86 XJ6 4.2 original motor
Old 05-25-2014, 01:28 PM
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'Unstable idle' is a chronic Ser III issue with symptoms ranging from a mild tremble to outright stalling...especially with a/c on, as you mention. On their best day, when brand new, the fueling was such that the engine could just barely sustain itself....and things only get worse with age.

The worst cases....engine stalling with a/c on.....were recognized by Jaguar when the cars were still in production. The fix (not seen on USA-market cars) was an 'extra air valve' that increased idle speed whenever the a/c was on. There was a kit (long since discontinued) to retro fit the parts to those cars not originally equipped. A reasonable duplicate of the kit can be made up piece-meal, though. An alternative is to simply increase the idle speed. Personally I've yet to see a USA market Ser III that really idles properly at the specified idle speed.

This problem can be compounded by a dirty throttle body, incorrect throttle blade gap, slightly clogged/degraded fuel injectors, vacuum leaks, leaky injector mounting cups (these were early impossible to find for a number of years), etc. No single glaring fault but, usually, a 'stack up' of minor faults/slightly degraded systems/parts.

I've noticed some Ser IIIs (my own included) where the torque convertor drag seems unusually high. That is, you drop the car into gear and the trans seems to drag the engine wayyyy more than expected. I'm not really sure if it is a trans problem or if the fuel injection system....being right on the edge to begin with....simply can't cope with the load.

Same for a/c compressor load. The old A6 compressor is notoriously parasitic but some seems worse than others (on the verge of seizing up, perhaps?). Excessive compressor noise might be a clue.


Fattening the idle mixture via the adjuster in the Air Flow Meter might be worth a try. Oh, I might add that wear on the resistor track of the AFM can cause seem weird idle and/or just-off-idle problems as well.

Sending the injectors off for professional cleaning and balancing might help. Doing so has improved the idle of every car I've owned. If you have even one or two where the flow/spray pattern is weak it can be enough to cause a problem

What's your idle speed right now, by the way?

Old 05-26-2014, 07:57 AM
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the Oxygen Sensor: does your car have the 1-wire sensor or 2-wire sensor?

(the oxygen sensor is mounted on the exhaust manifold).
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