Idling too slow

Can’t remember if I read something before about the aav…but since I am no longer using the stock aav… is there a way to make a device that can be adjusted to control the idle without being too complicated

Bernard Embden’s solution is pretty cool:

Or add a second Extra Air Valve.
Can be switched automatically with a thermostat or manually with a switch in the cabin.
And if the idle is too high you just restrict its opening, or use a smaller valve.

What if you put a hole in the air crossover pipe… Then add an adjustable valve with a small filter on it… then you could set it where you want it… this would be ok for engines that idle too slow would it not?

Any “extra” air you introduce to the engine will raise the idle speed, so, yes, your idea should work.
Have you completely removed the AAV?

Have you recently replaced your PCV valve? I found by unclogging my PCV, my idle went up. I ended up buying a new JAG OEM PCV because the idle was not steady, and aftermarket ones that fit caused the idle to be too high.

No have not removed it… but unhooked vac line to it some time ago … it just needs a little more idle rpms. I also back then added a spigot on left air filter to pull vacuum through the distributor… The eventual replacement of various vac lines probably contributed to less air being ingested thus eventually slowing idle down… Now to find or make a adjustable valve… The filter will be a lawnmower gas filterScreenshot_20210330-131318~2

Will check pcv also…

If I plug my PCV line, the idle will usually fall a good 400rpm. It adds quite a lot.

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Unless I’m missing something here, can’t you just use the idle speed adjustment bolt on the AAV to get the (warm) idle speed you want?

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Don’t know… the aav acted up a few years ago… So i thought I was told to inactivate it… Guess I’ll have to look again… if I remember it was idling too fast…so after unhooking it and with OLD vac hoses all over …enough air leaked in to make it idle ok… since then I have replaced many vac hoses.

I guessing your AAV has either a defective wax whatever or a rusty sticking spring component. Jonas Jagimprover who frequents this website has a nice repair kit option or will even refurbish your existing one. If you go to YouTube Living with a classic Jaguar, Adam will even show you how to test your AAV.

Even if the AAV is stuck fast and won’t give you a cold fast idle, you could still use it to set the warm idle speed. Turning the adjustment bolt counterclockwise will open up an air passage to increase the idle speed, and turning the same bolt clockwise will close down the passageway to reduce the idle speed.
OTOH, if you don’t want to mess with the AAV, you could do this. If the throttle butterflies and linkage is set up properly you could turn the adjustment screws on both banks a flat at a time, slowly increasing the air-gap on each butterfly. Should be done evenly on both sides.

Thanx… I wondered about the throttle valves… Can an adjustment be made at the pedestal unit?

No. At the two throttle butterflies, one on each bank.


Another part of this conversation would be… what is the ideal idle rpms… in neutral and in gear (automatic transmission.) Mine seems to hang around 650 on my dash tach.

No adjustments at the pedastal. The throttle plates have a set adjustment to a feeler gauge fixed setting when closed. Idle is not adjusted at the plates once the fixed adjustment is made. The idle adjustment is done at the AAV only.

Of course an improper ignition advance setting will also affect idle speed.

The attached document provides complete instructions on how to set up the engine.
S58_XJS Engine Performance-752.pdf (1.5 MB)

Looks like your car is the same year as mine. If you haven’t refurbished your AAV the wax thermo bulb can get lazy (like an old thermostat) and/or the internal parts can get corroded.
I went through this last summer with my car and cleaned up my AAV I’m still going to refurbish it at some point in the near future by ordering the parts from Jonas.
I believe your engine when cold in neutral with the AC turned off should be idling at 1000 RPM. When it warms up it should be at 750 RPM . Yours seems to be a tad out.
I believe the reason for this as Kirby pointed out when I was adjusting mine is you want a certain RPM at idle to turn the mechanical cooling fan in stop and go traffic to keep the engine cool.

Not that it applies directly, my lump idles at 1000 when cold and when warm at 500. My Jeep is similar.

If I had a vote, which I do not, it would be that 650 is just fine. Criteria. Fast enough so that the engine in gear does not stall. Slow enough so that it does not creep at a stop.

As my car has Efans, fan speed is not of import. They kick in at 200F!!!


Actually I just started my car up after winter snooze and I’m probably a little off spec. For RPM.