[xj] Air Filter/Hot Air vaccuum control

I’ve had the filter assembly off for a while and am about to
replace. There is a vacuum module on the backplate at the
front of the unit with two pipes - one inboard and the other
outboard. Can someone please confirm that the tube from the
manifold goes to the inboard pipe and that the outboard one
is connected to the flap actuating unit near the hot air
intake. Also is there any way to test this set up off car -
I have to confess I don’t really understand how it is
supposed to work in the first place (is it to ease starting/
cold running or is it a primitive emission control?)- and
sucking on the pipes does absolutely zilch. I assumed the
hot air flap was vacuum operated and that a thermostat/
switch/bimetal was built into the vacuum module??? I
should add that I have no reason to believe anything is
faulty - the filter assembly was off to allow access for
other work.–
Rolph XJ6 C Manual: Alicante(Spain) Tampere(Finland)
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In reply to a message from Rolph sent Sun 4 Mar 2007:

Rolph

This is difficult to describe in words but would be so much easier
in pictures! Unfortunately I swapped car locations around today and
the SII is now in the lock up some way away.

A single vac line should run from the manifold to the unit on the
engine side of the air filter housing. A second line should come
off this unit (there are or should be two little pipes) and go to
the hot air flap on the front of the air filter. (from your
description this is what you’ve done?)

The idea (as I think you’ve deduced) is that hot air off the
exhaust manifold is fed round to the intake and the bimetal in the
first bit modulates vac to the hot air flap in the second to
maintain a constant hot temp for the carbs. (Sorry can’t remember
what it is - a power destroying 40C +, I think.) The hot air flap
should move reasonably easily if you suck on the line that goes to
it, but if the diaphragm has gone perhaps won’t.

I’ve read various explanations as to why this was done but the one
that sticks in my mind was to meet emission specs at idle. By
always feeding air at a known temperature one of the variables was
taken out of the fuel metering equation.–
al mclean '93 XJS 4.0 - '79 SII XJ6 mod - '84 4.2 Daimler
Telford, United Kingdom
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In reply to a message from almcl sent Sun 4 Mar 2007:

Thanks Al - thought it might be something like that. I guess
mine does not work and probably hasn’t for a long time. I
did wonder whether it was linked in some way to the
operation of the AED but it would seem not. Anyway it gives
me some options to play with. The rather ugly cross over hot
air feed from the exhaust can go for a start - but that is
just cosmetic. More interestingly the intake for the hot air
to the air filter box can have a pipe fitted to pick up cold
air from beneath the car - which is of course what AJ6
Engineering do on their large throttle mods on the V12.
Worth playing with anyway.
You know the hot air pick up plate on the exhaust manifold?
It’s fastened with a couple of very rusty bolts. I can move
the front one fairly easily - the one with the smaller head.
The rear larger one does not want to turn with reasonable
leverage - or am I just being chicken do you know?–
The original message included these comments:

The idea (as I think you’ve deduced) is that hot air off the
exhaust manifold is fed round to the intake and the bimetal in the
first bit modulates vac to the hot air flap in the second to
maintain a constant hot temp for the carbs. (Sorry can’t remember
what it is - a power destroying 40C +, I think.) The hot air flap
should move reasonably easily if you suck on the line that goes to
it, but if the diaphragm has gone perhaps won’t.
I’ve read various explanations as to why this was done but the one
that sticks in my mind was to meet emission specs at idle. By
always feeding air at a known temperature one of the variables was
taken out of the fuel metering equation.


Rolph XJ6 C Manual: Alicante(Spain) Tampere(Finland)
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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In reply to a message from Rolph sent Mon 5 Mar 2007:

I’ve often wondered how the HIF7s would respond to some decently
cool air, let us know what your experience is?

I’ve had difficulty with that bigger bolt, too.

In fact it still has a dimple in it from where I applied the
welding torch, I absolutely couldn’t get it to move the first
time ‘cold’ at all but with heat and some copper ease on re-
assembly it’s been OK since, touch wood.–
The original message included these comments:

It’s fastened with a couple of very rusty bolts. I can move
the front one fairly easily - the one with the smaller head.
The rear larger one does not want to turn with reasonable
leverage - or am I just being chicken do you know?


al mclean '93 XJS 4.0 - '79 SII XJ6 mod - '84 4.2 Daimler
Telford, United Kingdom
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In reply to a message from Rolph sent Mon 5 Mar 2007:

OK - so I’ve taken the thing apart. If you have one of these

  • be joyful - if not be even more joyful!

http://www.jag-lovers.org/v.htm?1173114006

The bottom bit comes off by drilling the two spot welds on
the bracket. Mine sort of works - well the diaphragm is
presumably intact ( after 30 years it would be inappropriate
for bishops and all you actresses out there to comment) -
but as they say it is a little stiff and cannot as far as I
can see be replaced. The top bit is the flap arrangement -
complex in it’s construction - comes out by grinding the pin
ends off. So the whole lot can actually be put back together
if one wishes to.
An expensive piece of kit to make at the time I suppose -
what else can one say!–
Rolph XJ6 C Manual: Alicante(Spain) Tampere(Finland)
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