[xj] V-12 engine idle

XJ
1

The engine in question is an HE from a mid-1980’s XJ-S. I’ve
changed the spark plugs, gapped them at 0.25, put new plug
wires, distributor cap, rotor. I set the butterfly valves at
0.002 inches, checked all of the vacuum connections, plugged
the leaks. I have closed the idle setting on the AAV as much
as possible. My problem is that the it still idles high…
about 900 rpm. It’s better than it was at 1100 rpm, but I’m
not sure what to do next. I don’t have any specialized
equipment for checking settings on the ECU. Is there
something else to check?? Also, what should the ignition
timing be set for? Is 18 deg BTDC correct?

Thanks,
Bill–
Bill Cooper, S-3 VDP 5spd, XJ12-C (2), XJ6-C, S-1 XJ12 (2)
Charlotte, NC, United States
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2

In reply to a message from drbill sent Sun 14 Mar 2010:

I forgot to mention… there are no over run valves and no
TTS.

Bill–
Bill Cooper, S-3 VDP 5spd, XJ12-C (2), XJ6-C, S-1 XJ12 (2)
Charlotte, NC, United States
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3

I can think of a couple things…

If the AAV can’t bring the idle down lower than 900 rpm you probably have an
air leak…somewhere…that is overpowering, so to speak, the amount of air
being admitted by the AAV. Intake gaskets, perhaps? Try snugging the nuts.
Hard to say on a V12 as there are so many potential areas for a leak to
occur.

Also, try blocking off the AAV entirely and see if anything changes. The
inlet for the AAV is in the back of the LH air filter housing, behind the
filter itself. Just block it off with a cork or something

Also check the throttle rod bushings at the bell cranks located at the rear
of each intake manifold. Worn or missing bushings can cause a bind in the
linkage and hold the throttles open a wee bit. Just had a V12 in my garage
yesterday, in fact, doing this exact thing. Jiggling the throttle changed
the idle speed !

For the timing, yes, 18� BTDC…at 3000 rpm.

Cheers
Doug Dwyer
Longview Washington USA
1995 XJRFrom: “drbill” coopdvm@bellsouth.net

The engine in question is an HE from a mid-1980’s XJ-S. I’ve
changed the spark plugs, gapped them at 0.25, put new plug
wires, distributor cap, rotor. I set the butterfly valves at
0.002 inches, checked all of the vacuum connections, plugged
the leaks. I have closed the idle setting on the AAV as much
as possible. My problem is that the it still idles high…
about 900 rpm. It’s better than it was at 1100 rpm, but I’m
not sure what to do next. I don’t have any specialized
equipment for checking settings on the ECU. Is there
something else to check?? Also, what should the ignition
timing be set for? Is 18 deg BTDC correct?

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4

The AAV is like a thermostat. There is a wax bulb that expands with heat
and that moves a piston to cover the air port. As it cools a spring
returns the piston. pretty simple. However, its an aluminium piston in
an aluminium bore and it corrodes and sticks. Then you have high or
erratic idle problems. Sometimes the idle will return to where it should
be, and other times it will not. The solution is a new or rebuilt AAV.
There is a place in Australia that sells rebuild kits, and it works
well. I can’t remember the name, but I’m sure Google will turn it up.
When I ordered it a few yars ago I had it by mail in a week. It was also
1/6 the cost of a new AAV.

Timing is 18 degrees at 3000 rpm, which works out to about 0 degrees at
750 idle, with the vacuum advance disconnected and plugged.

Craig

drbill wrote:

I have closed the idle setting on the AAV as much
as possible. My problem is that the it still idles high…
about 900 rpm. It’s better than it was at 1100 rpm, but I’m
not sure what to do next.

Also, what should the ignition timing be set for? Is 18 deg BTDC correct?>

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5

In reply to a message from Craig Talbot sent Sun 14 Mar 2010:

The DIY fix for a dragging piston in the AAV is to remove the AAV,
lock it down on a bench vise, run a long screw a couple of threads
into one of the holes in the piston, grab the head of the screw
with pliers, and work the piston up and down to ‘‘polish’’ the bore
and the outer circumference of the piston. The spring is pretty
stiff, so it requires a bit of effort, and you don’t want the
pliers to slip of the screw head, and end up with a broken tooth.

Obviously, this doesn’t work if the bulb is faulty.

SD Faircloth–
The original message included these comments:

erratic idle problems. Sometimes the idle will return to where it should
be, and other times it will not. The solution is a new or rebuilt AAV.

–

Jacksonville, Florida, United States
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6

drbill wrote:

The engine in question is an HE from a mid-1980’s XJ-S. I’ve
changed the spark plugs, gapped them at 0.25, put new plug
wires, distributor cap, rotor. I set the butterfly valves at
0.002 inches, checked all of the vacuum connections, plugged
the leaks. I have closed the idle setting on the AAV as much
as possible. My problem is that the it still idles high…
about 900 rpm. It’s better than it was at 1100 rpm, but I’m
not sure what to do next. I don’t have any specialized
equipment for checking settings on the ECU.

The ECU has no bearing on excessive idle, Bill…

Engine rpms are regulated by the amount of air the engine is allowed to
draw. A vacuum leak may increase or lower rpms - it depends on if there
is enough fuel to use the extra air. Certainly; as mentioned by others;
if the AAV failes to close the engine will draw air and fuel as
appropriate - and idle will increase. And so will a sticking gas pedal
linkage…

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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7

Bill: your AAV is stuck and this is the typical symptom: adjustment of that
bolt will no longer effect a change in idle speed. Solution: take it apart
and clean it (sometimes successful), buy the Australian repair kit, or buy a
new one. The pre-HE AAV looks slightly different, fits and works perfectly
well, and is cheaper.

Gregory,
Victoria, British Columbia, Canada
1966 3.8 Mk 2, Pale Primrose
1992 Series III V12 Vanden Plas, Black Cherry,
2002 X-Type, 5 sp. manual, Anthracite
2004 XJ8, Ebony-----Original Message-----
From: owner-xj@jag-lovers.org [mailto:owner-xj@jag-lovers.org] On Behalf Of
drbill
Sent: March-14-10 8:07 PM
To: xj@jag-lovers.org
Subject: [xj] V-12 engine idle

The engine in question is an HE from a mid-1980’s XJ-S. I’ve
changed the spark plugs, gapped them at 0.25, put new plug
wires, distributor cap, rotor. I set the butterfly valves at
0.002 inches, checked all of the vacuum connections, plugged
the leaks. I have closed the idle setting on the AAV as much
as possible. My problem is that the it still idles high…
about 900 rpm. It’s better than it was at 1100 rpm, but I’m
not sure what to do next

===================================================
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8

In reply to a message from Dr. Gregory Andrachuk sent Mon 15 Mar 2010:

Thanks, fellows. I will snug the bolts on the intake manifolds and
double check for leaks.

The AAV adjustment: the idle will increase a bit if I loosen the
bolt.
I will block it off like Doug suggested… if the idle comes down,
I’ll assume the AAV needs replacement/rebuilding.

I’ll report back when done.
Again, Cheers.–
The original message included these comments:

Bill: your AAV is stuck and this is the typical symptom: adjustment of that
bolt will no longer effect a change in idle speed. Solution: take it apart
and clean it (sometimes successful), buy the Australian repair kit, or buy a
new one. The pre-HE AAV looks slightly different, fits and works perfectly
well, and is cheaper.

–
Bill Cooper, S-3 VDP 5spd, XJ12-C (2), XJ6-C, S-1 XJ12 (2)
Charlotte, NC, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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9

In reply to a message from Doug Dwyer sent Sun 14 Mar 2010:

Doug,
I blocked off the AAV… no real appreciable change. I know
it’s stuck open, though, because there was definitely air
being sucked in when I blocked it off. The weird thing is,
even blocking it off didn’t change the idle.
I also for good measure replaced all of the vacuum lines
coming out of the intake on the right side.
Would the throttle vacuum switch have any bearing here?
Thanks,
Bill–
The original message included these comments:

Also, try blocking off the AAV entirely and see if anything changes. The
filter itself. Just block it off with a cork or something
Also check the throttle rod bushings at the bell cranks located at the rear
of each intake manifold. Worn or missing bushings can cause a bind in the
linkage and hold the throttles open a wee bit. Just had a V12 in my garage

–
Bill Cooper, S-3 VDP 5spd, XJ12-C (2), XJ6-C, S-1 XJ12 (2)
Charlotte, NC, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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10

drbill wrote:

In reply to a message from Doug Dwyer sent Sun 14 Mar 2010:

Doug,
I blocked off the AAV…

It’s important that the AAV is also tested this way with the engine
cold, Bill - with the engie hot there should be no difference. And if
there is no difference with the engine cold either; the AAV is indeed
malfunctioning - or not partissipating in an orderly way…

Frank
xj6 85 Sov Europe (UK/NZ)> no real appreciable change. I know

it’s stuck open, though, because there was definitely air
being sucked in when I blocked it off. The weird thing is,
even blocking it off didn’t change the idle.
I also for good measure replaced all of the vacuum lines
coming out of the intake on the right side.
Would the throttle vacuum switch have any bearing here?
Thanks,

The original message included these comments:

Also, try blocking off the AAV entirely and see if anything changes. The
filter itself. Just block it off with a cork or something
Also check the throttle rod bushings at the bell cranks located at the rear
of each intake manifold. Worn or missing bushings can cause a bind in the
linkage and hold the throttles open a wee bit. Just had a V12 in my garage

===================================================
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11

In reply to a message from drbill sent Mon 15 Mar 2010:

Bill,

On the AAV. You may have a loose/cracked hose from the B bank air
housing to the AAV, such that blocking off the AAV hose opening at
the back end of the B bank air box, still allows air to enter thru
the hose.

Worth a look at that hose.

SD Faircloth–
The original message included these comments:

I blocked off the AAV… no real appreciable change. I know
it’s stuck open, though, because there was definitely air
being sucked in when I blocked it off. The weird thing is,
even blocking it off didn’t change the idle.

–

Jacksonville, Florida, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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12

In reply to a message from SD Faircloth sent Tue 16 Mar 2010:

You can use a propane torch to check for vacuum leaks. Don’t light
it, just crack the valve open to let out some propane and move it
around the idling engine. If idle picks up, you have a leak in
that location drawing the propane into the intake.–
The original message included these comments:

On the AAV. You may have a loose/cracked hose from the B bank air
housing to the AAV, such that blocking off the AAV hose opening at
the back end of the B bank air box, still allows air to enter thru
the hose.
Worth a look at that hose.

–
John Testrake 74XJ12L rhd, 84 XJ6 700R4
St.Louis, United States
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13

In reply to a message from John Testrake sent Tue 16 Mar 2010:

Propane torch… great idea! It’s hard to hear a subtle vacuum
leak with the engine running.
I’ve decided to replace the AAV… I know it’s stuck open. I
do think there is another source of air getting sucked into
the intakes, though. That hose may be a source and I’ll put
new ones on with the AAV.
Thanks.–
The original message included these comments:

You can use a propane torch to check for vacuum leaks. Don’t light
it, just crack the valve open to let out some propane and move it
around the idling engine. If idle picks up, you have a leak in
that location drawing the propane into the intake.

–
Bill Cooper, S-3 VDP 5spd, XJ12-C (2), XJ6-C, S-1 XJ12 (2)
Charlotte, NC, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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