Series 3 4.2 rough idle / warm start problem

Is there a procedure to test it? The Haynes is so vague in this part. I ordered a brand-new switch from an EU supplier, meanwhile.

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Locate the switch on the throttle cable mounting bracket, Patricio - it has a vacuum line an two(?) wires. I think it is white plastic body…?

Disconnect wire(s) and ohm through the switch. I don’t remember the default position, ‘make’ or ‘break’. But switch is one or the other with the engine stopped (no vacuum) - and opposite with the engine running (manifold vacuum)…

There is also some specific vacuum threshold for activation - which I have forgotten. The point is that drop in manifold vacuum indicates load - and at some stage the switch informs the ECU that full load fuelling is to be activated…:slight_smile:

Also check vacuum and vacuum connections to the switch…

Frank
xj6 85 Sov Europe (UK/NZ)
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Yes to all. If one sucks on it (hard) it should click, connect an ohmmeter or test light to see if it works. I don‘t know the specifications.
Now idea where the switch should connect, but mine is Tee‘d off at the distributor, so direct manifold vacuum and not on the throttle body.

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That is the ‘European’ set-up, David, and the switch is connected to the ECU. Low manifold vacuum induces fuel enrichment, reflecting pedal movement - while the ‘US’ throttle switch reacts directly to pedal deflection. Basically doing the same…

The difference is required as ‘US’ use ported vacuum - which does not adequately reflect engine load…

Frank
xj6 85 Sov Europe (UK/NZ)
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Now wait, Frank, I am european and was talking about the vacuum. That it is wired to the ECU is obvious; but not where the vacuum is sourced. The wiring is obvious (it is not, really. My PO connected the two wired to the full load switch to the two wired for the throttle switch…).

David

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For the ‘European’; the vacuum source is ‘manifold’ vacuum, David. Which drops to ‘low’ at high load/floored pedal - engaging ‘full throttle’ fuelling. Which the throttle switch does with high/full throttle - and it is sort of logical that the ECU can use the same wires…:slight_smile:

The point is that ‘ported’ vacuum reads 0 vacuum (or thereabouts) with the throttle closed - obviously counterproductive with the vacuum switch.

Subtly; the 'US engine management system is weighted toward emission concerns - which tends to reduce engine power. While 'Europeans went for.max power output. Philosophically, the throttle switch is adjusted to engage only at ‘full’ throttle setting - the vacuum switch may induce rich fuelling when low manifold vacuum indicates enough load to warrant richer fuelling for better engine power output…

Frank
xj6 85 Sov Europe (UK/NZ)
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Meanwhile…

I love these NOS things.

Absolutely (the Omron equivalent is €2, which is what I‘ll get). Industry standards are great.
Frank, in the beginning I wasn‘t sure whether the switch might also worked either for idle enrichment as well, or in conjunction with the throttle microswitch which as we all know isn‘t true sadly.
Patrizio, if the car should act up rolling down hill especially when the engine is cold, just give the switch a little bit more travel. Not that you get surprised and think something was badly wrong; just in case.

David

Patrizio,

sorry for getting back so late - I was quite busy for a couple of days.

David has answered your questions about the HT wires - sorry for the confusion. And David has also rightly stated that on fuel injected cars you can achieve the same result by disabling the injectors. I’m just more used to working on carbed cars - in particular the Jaguar - which is why I didn’t think of this alternative path to check the same issue: do all cylinders contribute to the engine’s running in the same way?

Finally, I’ve got no clue on the microswitch issue. Maybe it’s helpful to find yourself the famous S57 wiring pattern - Haynes may be suboptimal - and reconfirm which equipment is supposed to be on your car. While my SII is fairly standard, SIIIs are very different caused by various emission control regimes.

Good luck

Jochen

Update: I changed the microswitch and I noticed no significative improvement over the idle, maybe just a little but hard to say. I confirm it’s related to fuel cut-off because when leaving the gas pedal over an hard acceleration now the engine revs down regularly, when before it had a rev peak on the same situation.

Any change, even changing the wiper blades, somehow improves the idle quality. The switch has nothing to do with idle management; these are dumb, analogue cars. All it does is cut the injectors above 1400 rpm or so!
Maybe you want to look at the plugs after some idling and just after a harder run if you suspect the mixture to be faulty. However you said it runs smooth. Some roughness is just normal and on a hot day 5 seconds is… what can I say, it‘s pretty good as far as i‘m concerned.

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We have two microswitches, Patrizio; the vacuum operated full throttle switch, and the overrun fuel cut-off switch which is mechanically operated directly by the throttle mechanism. As David says; the latter cuts fuelling, with throttle closed, above 1400 rpms - ie, it is related to engine rpms on the overrun. With no fuel delivered the engine just driven by the road wheels and drops to the relevant rpms. Fuelling is the imperceptibly resumed as the rpms drop below 1400 - or pedal is moved out of ‘closed’…

Frank
xj6 85 Sov Europe (UK/NZ)
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Every Jag I’ve owned, beginning 22 years ago, all daily drivers, has had some sort of idle quality weirdness. The Series III XJ6 was the worst. The idle was a little weird every day.

I can’t explain it and stopped fighting it :slight_smile:

Cheers
DD

When I‘m sad and the XJ is to blame, I play this video on a loop and and up even sadder.

Has the rough idle remedies list been gone through completely? If so, it might - might - at least be a tad better.

David