On a 78 XJ6 ser.2
How to test an AAV?
What are the symptoms of problems with them?
As Tom says, Walter - it’s unlikely to be the AAV…
The crude test of the AAV is simply to verify, by removing the air hose to
it, that slide inside is about half open with the engine cold - and closed
with the engine hot. A more sophisticated test involves removing the AAV and
heating/cooling it while watching the slide evenly opens and closes with
temp variations…
However, if the idle does not respond when adjusting the idle screw -
something else than the AAV is involved. Engine rpm is regulated by the
amount of air entering it. In idle, air is regulated by throttle gap and
idle screw - varied by the AAV during warm-up.
Idle should only be set with the engine hot, normally to 800 - 900; when
cold, the AAV will increase idle by some 200 - 400 rpms. So in any case;
check the AAV - if the slide is open (engine hot) the AAV is malfunctioning,
and will give high idle rpms even if the idle screw is closed and the
throttle gap correctly set. However, turning the idle screw should still
change rpms…check - and also check AAV slide, quickly done. As an aside;
are you sure the idle screw seat is now correct? A failed seat will of
course render the idle screw ineffectual…
Another cause of no/improper idle screw response is an air leak - seek and
ye shall find…:). While a disconnected vacuum hose will increase idle by
some 50 rpms - if the hose on the underside of the AAV is disconnected, it
is a major air leak, rendering the engine rather unresponsive to the idle
screw…
While 1300 rpms is too high for a hot engine in neutral (AAV closed) - idle
drop is of course natural, but with this high idle; why do you want a
further increase? The quirk with automatic transmission is that the
converter’s resistance increase with its rpms - idle tends toward some 750
rpms in gear irrespective of neutral idle setting. To some extent the
difference reflects engine power - an air leak will reduce engine power;
in-gear idle slower than expected…
Another check of engine power is idle vacuum. Connect a vacuum gauge to the
fuel pressure regulator’s vacuum hose - normal idle vacuum is around 18" Hg.
While a vacuum leak will cause low idle vacuum, it is not the only possible
cause; engine wear, misfiring fuelling faults and engine timing will also
influence engine power - but a vacuum gauge is still a worthwhile diagnostic
tool…
No simple answers without further checks - but again; why do you want higher
idle rpms?
Frank
xj6 85 Sov Europe (UK/NZ)
I have a problem of not able to increase idle
speed, have 1300 rpm in neutral and drops to 700
when in Drive.
Checked the .002 inch at the throttle.
Checked the throttle plate switch.
Replaced the ‘‘O’’ ring on the mixture screw.-----Original Message-----
From: W. Schuster
Sent: Friday, November 18, 2016 10:28 AM
To: xj@jag-lovers.org
Subject: [xj] Anybody have knowledge how to test a AAV.
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The archives and FAQ will answer many queries on the XJ series…
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