1983 Series 3 cold idle rpm too low

Me again with what I hope is the final issue with the engine. I have noticed that when starting cold the engine will start and then slowly die out within 5 seconds. If I feather the throttle a little for about a 30 seconds it will not stall when I release the pedal and the RPMs will slowly rise to hot idle RPMs as the engine warms up. But my cold idle is 500 rpm or lower, while my hot idle is perfectly on spec at 800 rpm ± 25. I suspected the AAV but it appears that it is open when cold and shut when hot. The AAV is only open 40-50 percent when cold though. The mixture is also a little richer like its supposed to be. I also checked the throttle body and it is dead on 0.002". I am kind of stumped, The only thing I am not sure of is when I probed the plug on the AAV I found that the plugs have continuity (connected together) when cold is this right?

Any ideas would be welcome. the problem isn’t bad it’s just annoying having to do manual fast idle with your foot.

I found a note in the archives. If this helps.

I’m thinking it may.

Thank you but the V12 version is different than the XK’s but I am still going to read it over to see if I cant gleem any info.

I will look in my stacks of manuals on this. I have dealer training material on the EFI for the Series III’s

Ok cool, thank you! I feel like it has to be the AAV but it seems to function normally.

I think you’re right it certainly sounds like the AAV. The only other thing that I can think of would be the mass airflow it’s not a sensor it’s a brick. I’ll check it out and report back.

Just some FYI stuff. I have had multiple intakes backfires before due to my error. so maybe I have damaged the “door” on the AFM slightly. I do however happen to have another AFM on hand from a junkyard car.

EDIT: forgot to mention that I had the AAV in my hand while still attached and it was sucking air and when I plugged it with my finger the idle fell significantly. so the AAV is getting air to the engine. Just not enough of it maybe?

The AAV is temperature sensitive. There is or should be coolant circulating around it. If I can get outlook un frozen, I will send you the email i drafted that talks about the 3 places air can by pass the mas(sive) air flow sensor.

There are 3 places where air can bet pass the throttle valve.

  1. Idle speed control is provided by a screw adjusted by-pass situated in the “Air Distribution block” It controls the by-ass airflow from the throttle body to the plenum, providing a fine adjustment to the engine idle speed after the butterfly valve is set to .002”
  2. The overrun valve situated inside the air distribution block, which is mounted on the plenum connected to the throttle body.

Air flow is provided by the overrun valve (or gulp valve) during overrun to ensure good combustion. It’s necessary as very low pressure during rapid deceleration cause any residual fuel on the manifold and plenum chamber walls to evaporate and create an over-rich mixture. The valve has a nut adjustment controlling the spring tension. It is a pre-set at the factory and shouldn’t be altered.

  1. The AAV consists of a variable orifice controlled by a bi-metal element. The unit is mounted on the water rail and also responds to coolant temperature as a heater is fitted around the element. The heater is connected in parallel with the fuel pump and so is energized as the engine is running.

You had the AAV in your hand, William - unbolted from the inlet manifold?

To amplify; the AAV is connected by the upper hose to the air distribution block, a hose connected underneath the AAV is connected to the throttle body.

Your description seems to indicate that the lower hose is disconnected from the AAV…?

This is an air leak that will lean out the mixture; while cold, the mixture is fattened enough to run the engine at ‘some’ idle. But as engine heats up; the ECU gradually leans out the mixture and engine slows and dies - sometimes with lean backfiring …

Blocking the AAV alters air ingress to the engine, which influences air - directly affect engine…

The crux; are both AFM hoses connected to the AFM…?

xj6 85 Sov Europe (UK/NZ)

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When you say lower hose on the AAV I am not sure what you are talking about? Are you saying the otherside of the AAV that points down at the engine block should be connected somewhere?

Edit I just found this picture. mine is not connected is this my problem you think? the intake part is capped with a large vacuum cap.

Ok I feel a tad bit stupid after this revelation. essentially the AAV has been doing almost nothing because it’s not fking hooked up :laughing:. Atleast I can blame my father who took this all apart when we had the engine out the paint the engine bay. He must have forgotten it and I didn’t know about it. I now have the correct hose ordered. For now I’ll use some heater hose while I wait for it to come. Im so pissed it was something as simple as not hooking up the fking thing that handles cold starting :sob:

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I sure am, William…:slight_smile:

To elaborate. All air to the engine first goes through the AFM to give fuelling requirements to the ECU. The main duct goes to the throttle body - air flow restricted by the throttle butterfly. The idle screw taps air from the main duct via the air distribution block - bypassing the throttle butterfly, and controlled by the idle screw setting…

The AAV is also connected to the air distribution block (‘upper’ hose) - then connects to the throttle body (‘lower’ hose). This air flow is controlled by the AAV slide - (about half) open with engine cold, and closed (no air to the engine this way) when hot…

If the ‘lower’ hose is disconnected from the AAV; air is entering the throttle body/manifold without beaing measured by the AFM. Effectively a vacuum leak - leaning out the mixture. If, in addition, the AAV slide is open; unmeasured air will also enter the duct/air distribution block - further leaning out the mixture…

As an aside; The ‘lower’ hose may easily slip off the AAV, and particular care must be taken to prevent the hose ‘disappearing’ the AAV is removed…


xj6 85 Sov Europe (UK/NZ)

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Once I went through that logic in my head I had one of those “Duh!” moments! I am surprised that it ran as good as it did with this rather large vacuum leak. It also makes perfect sense as to why it would struggle to idle when cold and be fine when warm. Thanks again for saving my ass, with your what seems to be endless knowledge of these cars.

Ok just got done putting in a temporary pipe… drum roll… Its all fixed it idles at 1100 rpm ish when cold and comes down to 800 when warm! and it obviously doesn’t stall on cold start anymore!

Don’t feel stupid. we’ve all been there. I’ve been there on my YouTube Channel, Jag Mods, and got called out by one of the forum members. How embarrassing is that!! and to add insult to injury it’s one of my popular viewed videos. I working on a follow video to fix my screw up.

The important bit is that you figured it out! that’s all that matters. As my Grandmum would say “you’re not learning if you’re not making mistakes.” I responded, well Im really really learning then.


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Thanks! .1234567891011