[v12-engine] Idle too high (cold and warm)

V12-Engine
1

Hello all

I�m currently in the last throes of restoring a 1988 XJS
V12. Euro spec, High-CR (12,5:1) non-cat HE engine.
I�ve had the engine apart down to the tappet blocks (to make
them oil-tight), had the complete cooling system apart,
radiator out, new waterpump, overhauled all cooling
passages, epansion tank, had the injection apart, changed
all hoses, all fuel hoses, all gaskets (injector, throttle
plate etc), overhauled all wiring, the complete fuel tank,
sump-tank, hoses, filters, the lot. Even eliminated the
half-moon seals., Full overhaul of all chassis rubers,
springs, shocks, gearbox mounts, subframe mounts, radius arms…
In short I went through Mr. Kirbies book from a to z (thank
you VERY much Mr. Kirby).

Now, everything is put back together and after some intial
problems (sticking injectors), the engine is running fine.
It�s oiltight, doesn�t overheat (if I keep the coolant
fiters cleaned) and seems to be running fine on all 12. Has
power, smoothness etc. all is well. Except:

I have a high idle.
With a cold engine as well as with a warm one. When fully
warm the engine idles at about 1100 rpm. It�s not the normal
idle cicuit. I�ve turned the idle control screw fully in,
even if I block the hole in the left air filter where the
aav draws from, the idle does not go down (exept of course
when the engine is cold and the aav is working).

The electrically opened secondary air valve is currently
disconnected, I�v plugged up all vacuum lines except for
those going to the fuel pressure regulators, the gearbox and
the ecu (I�ve even plugged up the connection to the gearbox
for testing while stationary).

I�ve retorqued the nuts on the intake manifolds (new gaskets
under the manifolds of corse, the thick cardbord type).

The only way to get the idle down to about 800 is to block
the PCV. The engine seems to be drawing a lot of air through
the PCV. When I pull that from the left air filter housing
and cover it, the idle goes down. but still not as far as
one would expect. I�ve changed the pvc for a new one
(original jag part $$$), no change.

The other vacuum lines are currently all disconnected, the
distributor vacc advance is currently directly connected to
the butterfly housing (upper connection) so it sees no
advance at idle (but full advance at cruise).

Any help and suggestions would be greatly apreciated.–
Tierfreund
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2

In reply to a message from Tierfreund sent Thu 27 Dec 2012:

I would check the settings on the butterflies. See if the
linkage is allowing the butterflies to close properly, and
check the actual clearance of the butterflies (IIRC 0.002’’)–
Maynard 94 XJS V12 Coupe 91XJS(RIP) 86XJ6 78MGB 67MGB
Niles, IL, United States
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3

In reply to a message from Tierfreund sent Thu 27 Dec 2012:

Thanks, I will. I don�t think they�ve been disturbed and the
issue was not there (or not as noticable) before the
overhaul but it�s certainly worth checking. I have checked
the linkage and it�s fine, it allows the butterflies to
fully return to their stopscrews and the stopscrews have
definitely not been altered (they�re not loose either), but
I didn�t check the butterfly gap with a gauge yet.

I also checked the dump valves today (because of another
post in the XJS-forum), and there�s no scution from the
manifold coming thorugh the dump valves to the filter house
either.–
Tierfreund
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4

At 10:09 PM 27/12/12, you wrote:>Hello all

I�m currently in the last throes of restoring a 1988 XJS
V12.

I have a high idle.
With a cold engine as well as with a warm one. When fully
warm the engine idles at about 1100 rpm. It�s not the normal
idle cicuit. I�ve turned the idle control screw fully in,
even if I block the hole in the left air filter where the
aav draws from, the idle does not go down (exept of course
when the engine is cold and the aav is working).

Tierfreund

Try blocking the inlet to the air filter on both sides.
Then if idle is high you know for sure it is extra air coming in from
some place it usually does not.
That narrows the number of places to start looking for the problem.
There are lots of openings into the inlet manifolds that can be a
source of extra air.
You only need 2 square cm of an opening to cause a big increase in idle RPM.

I had extra air coming in through the PCV valve or purge line on the
coupe after installing an HE engine it the car.
Other items took my attention so I never investigated.
That problem seems to have disappeared.

Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350 3.5L V8

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5

In reply to a message from Richard Dowling sent Thu 27 Dec 2012:

That�s a very neat idea. Will do today and report back.–
The original message included these comments:

Try blocking the inlet to the air filter on both sides.
Then if idle is high you know for sure it is extra air coming in from
some place it usually does not.
That narrows the number of places to start looking for the problem.

–
Tierfreund
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6

In reply to a message from Tierfreund sent Thu 27 Dec 2012:

I don’t know if this is the same problem or not but this is what
happened to my 71 coupe.
We rebuilt the carbs and set the linkage up to syncro the 4 carbs
and we had a popping in the exhaust and the occasional popping out
of the RS carbs, when that side was turned up a bit the popping
went away and the idle also went up to the 1100 range so we turned
that side back down and the popping came back. When I refer to the
up and down what we were doing was adjusting the linkage so the
right side was getting a higher idle, similar to the LH side. We
found a happy medium with the right side, still a bit of popping
but less than when we started and the idle was still high - 900-
1000 range, we made a small adjustment to the distributor and all
the popping went away, a bit more dicking with the right side carb
linkage and we were able to get the idle down to 650-700, the car
has never run better. If we would have been wearing lipstick we
both would have had red marks on our ear loobs, hot or cold all you
have to do is reach in the window and turn the key, fires instantly
and idles smooth.
When I refer to the popping I’m talking about backfire, more in the
exhaust, no flames or anything like that out of the carbs - just
that popping in the exhaust and before the timing adjustment
standing behind the car was terrible, the fumes were eye watering
bad, after the adj. it was way better.
I don’t know if this will help but it worked for us.
Check the linkage to all your carbs to make sure that at idle you
don’t have a carb that is above the linkage idle setting, advanced
on the throttle is what I’m talking about, 1 carb getting more fuel
will really affect the idle.
I don’t remember if we had the vacum to the distributor hooked up
or not when we were doing this but I heard from people on this
board that my car is a vacum retard system and not vacum advance.
Good luck.
Finn–
Finn
Vancouver B.C., Canada
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7

In reply to a message from fsm57 sent Fri 28 Dec 2012:

Thanks, Finn.

but the car is a 88 XJS HE V12. No carbs. relatively simple
Throttles. One per side, one linkage to connect the two.

Both throttle stops readjusted and linkage checked and
readjusted.

It�s not a synching issue, I�m pretty sure I�ve got a leak.
It�s just quite difficult to find.–
The original message included these comments:

I don’t know if this is the same problem or not but this is what
happened to my 71 coupe.
We rebuilt the carbs and set the linkage up to syncro the 4 carbs

–
Tierfreund
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8

In reply to a message from Tierfreund sent Thu 27 Dec 2012:

Done some more fiddling today:
I�ve blocked the air filter entries today. Hardly noticable
(when the engine should have all but stopped.

Also: A bank is pulling slightly more through the air filter
at idle than b bank.

When I block the entry to the air filter of B bank nothing
much happens, when I block the entry to A bank air filter
the idle speed actually rises a little.

Also when the engine is idling after warmup I have a slight
missfire. More on the A bank than on B bank. All injectors
are working and since the ignition is fully overhauled (yes,
Mr. Kirby I of course did overhaul the distributor) and
theres no misfire while warming up, I�m guessing the mixture
is not right either.

Remember, the car has no oxygen sensors and no cats so no
feedback loop to the ecu.

I�d say: massive leak, but I�m having a really hard time
finding it.

Any help and suggestions are appreciated.–
Tierfreund
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9

Any help and suggestions are appreciated.

Tierfreund------------------------------------------------

Take off the air filter bodies, they block access to parts of the
manifolds you need to see or feel…
Make sure you replace the 4 bolts holding them to the throttle body,
or you have the mother of all leaks - and I forgot that once.

Lots of places for leaks.
A stethoscope or similar to listen for leaks might help.

There are a number of nipples under the manifolds for vacuum connections.
They might also be present on the rear of the manifold.

There are the spring loaded valves on the front of the manifolds.

There is a PCV valve and purge line on the B bank.

There is a cross over tube which might be leaking.

That extra air will be found.

Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350 3.5L V8

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10

In reply to a message from Tierfreund sent Thu 27 Dec 2012:

Had same problem, found out that when I had reinstalled my intake
manifolds I had damaged the A bank gasket and it was letting air
in. After that was fixed the idle came down.

Van
XJS 1988–
Vans
Tucson, Arizona, United States
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11

Does the 88 have the power brake tube across the firewall with big vacuum
hoses connecting it to the intakes? If so, have you replaced those hoses
Mine were cracked and the cause of my high high.

Ed Sowell
'76 XJ-S coupe, red
http://www.efsowell.us/ed/myJag.html

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12

In reply to a message from Ed Sowell sent Sun 30 Dec 2012:

Hi Ed

yes, it does but I�ve replaced those hoses. Also I�ve
disconected and plugged up the conection to the vac
reservoir in the front right wheel house, just in case that
was laeking. No difference. Disconnected and plugged up the
line to the brake booster as wehll. No change.

I found a major leak today in the form of a big vacuum
connection on the intake manifold on the A bank. Hidden on
the underside of the manifold there are two vac connections.
I�d known about and plugged up the smaller one, but I had
overseen the bigger one. I guess it�s where the connection
to the eletric idle raising valve on the A bank filter goes.
As I had eliminated that electire valve (for now) I�d
forgotten about that connection.

So I plugged that up and the engine runs much nicer now.
Smoother and with similar suction on both sides, no more
misfire etc.

BUT, the idle is still at about 950 rpm. With zero vac
advance on the distributor (plugged up) and the idle screw
utrned fully in

If I plug up the air filter inlets the idle doesn�t drop,
instead there is more hissing from somewhere (sounds like
from somewhere within the V).

Baffling and obiviously still leaking somewhere.
Never would�ve thought I�d have a Jag engine that doesn�t
leak out (it really doesn�t thanks to all the good advice on
making the V12 oiltight in ‘‘the book’’) but instead is
leaking ‘‘in’’…–
The original message included these comments:

Does the 88 have the power brake tube across the firewall with big vacuum
hoses connecting it to the intakes? If so, have you replaced those hoses

–
Tierfreund
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13

I found a major leak today in the form of a big vacuum connection on the
intake manifold on the A bank. Hidden on the underside of the manifold
there are two vac connections.
I´d known about and plugged up the smaller one, but I had overseen the
bigger one.

I believe you are talking about the connections where the balance tube
connects to the intakes, right downstream of the throttle bodies.

About the hissing in the vee… Have you perhaps had the fuel rail off? If
so, you may not have gotten the injector seals properly seated. I can tell
you lots about that if needed…

Ed Sowell
'76 XJ-S coupe, red
http://www.efsowell.us/ed/myJag.html

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14

In reply to a message from Ed Sowell sent Sun 30 Dec 2012:

Please do!

I�ve had the injector rails off, the intake manifolds off,
all taken apart, all put back together with new seals, new
hoses, new wiring etc.

I instelled the injectors with new seals in the manifold
first befor installing the hoses and the fuel rail. I didn�t
see anything difficult in installing and seating the
injectors so I really wonder what I could�ve done wrong there…

Any help very much appreciated–
The original message included these comments:

About the hissing in the vee… Have you perhaps had the fuel rail off? If
so, you may not have gotten the injector seals properly seated. I can tell
you lots about that if needed…

–
Tierfreund
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15

In reply to a message from Tierfreund sent Tue 1 Jan 2013:

Check, too, on the B-bank side where the vac hose to the dizzy runs-
there’s a section of rubber conduit there that has many taps for
vac runs, and it is not easy to see this.

If the various barbs are brittle or the hoses are in rough shape,
you will get air coming in here, too.

Keep looking- you will find the air soon enough.

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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