R. Sparks,
I believe that the problem is not likely to be very serious, even if the
Idle Speed Control Valve is faulty. A rare occurrence I think. If it is
unserviceable it shouldn’t pose you too much trouble, or be very costly, to
replace with a good used unit. An even rarer event would be a failure of
the Electronic/Engine Control Unit.
Do you have any workshop manual, Jaguar or Haynes ? If you have just joined
the list you will have missed a post, by Brett Gazdzinsky yesterday I
think, on the simple routine required to ‘initialise’ the Idle Speed
Control Valve.
There is a sequence of Ignition OFF / ISCV plugged out - wait 15 sec. &
then ISCV plugged in / Ignition ON but no engine start, etc., etc. This
sequence of operations positions / ‘initialises’ the ISCV correctly for
subsequent automatic adjustments by the ECU. I don’t recall the exact
Ignition ON/OFF - ISCV Plugged In/Out sequence as it does not apply to my
2.9L unit.
If you access the archives and search under the appropriate keywords, Idle
Setting Procedure, ISCV etc, you will raise the appropriate posts. I think
that the on-line manual, on the main jag lovers website will also have this
setting procedure.
The electrical plug and socket arrangement on the ISCV is ‘safetied’ by
means of a spring-clip. To unplug, it is necessary to depress the
spring-clip before pulling on the plugtop.
The engine / motor idle speed, is automatically adjusted by an Idle Speed
Control Valve, once the ISCV has been correctly ‘initialised’ or positioned
by a simple and easy DIY procedure.
On my '89, 2.9L engine the ISCV is located in front of and slightly
outboard of the Throttle Body. (Assuming you know what the TB is ?) If you
‘follow’ the smaller (35mm dia. approx.) rubber pipe which ‘sprouts’ from
the top of the air intake pipe, between the Mass Air Flowmeter and the
‘bellows’ supplying air to the TB, the device attached to this smaller
pipe, as it turns downwards, is the ISCV.
The device has a stepping motor which responds to a square wave signal from
the ECU. (It has an electrical connection, similar to the Throttle Position
Sensor plug and socket arrangement, below the TB) Engine idle speed air is
regulated by the position of the valve. The pipe exiting from the ISCV
returns to the intake manifold, after the TB.
The ISCV, when properly ‘initialised’, compensates for different engine
idle load conditions, A/c compressor on or off, car in neutral or in drive
with foot on brake, extra load on the alternator etc. etc. In effect, this
air supply source by-passes the main air valve / Throttle / Disc in the TB,
which, when the throttle gap is properly set (0.002"), does not allow the
engine sufficient air to idle properly when ‘closed’. For correct operation
the ISCV depends on the gap in the TB being 0.002". This gap will not
normally change once properly set initially. Wear in the relevant spindle
would cause it to reduce. Another problem which will cause the gap to re
duce is build up of an oily deposit on the walls of the Throttle Body.
Either of these circumstances might cause the ISCV to have to open to the
extremes of it’s travel to compensate. This would then prevent any further
automatic compensation by the ISCV. However I don’t recall having seen this
circumstance reported to the list. The usual symptom is engine idle speed
surging, mostly due to moderate build up of ‘gunk’ in the TB.
The ISCV itself has a bleed by-pass. This is in effect a small needle valve
in the body of the unit which by-passes the stepping valve. Adjustment of
this by-pass valve is achieved with an Allen Key The size of the Allen
‘wrench’ is somewhere in the region of 5 to 7mm, I can’t remember exactly.
The bleed valve is located towards the middle front of the ISCV and is
angled slightly outward from the main body of the device. The hexagonal
Allen head should be visible by looking downward from immediately above and
in front of the ISCV.
If you have a Jaguar or Haynes manual, I think it very likely that there is
a photograph of this device somewhere in it. There will probably be a
setting procedure in the Haynes manual.
As my car has a 2.9L engine the electronics and setting procedures are
significantly different to larger engine variants. I don’t recall the
setting procedures for the larger units.
You may have to remove the ‘bellows’ to check the condition / cleanliness
of the Throttle Body. If necessary, it can be cleaned reasonably easily.
Ideally it should be taken off the manifold to do this, but it is not
essential. If you have to clean it be sure not to spray any solvents into
the TB to moisten/dissolve any build-up of ‘gunk’, unless you take the TB
off the engine and hold it upside down. (This is assuming that the Throttle
Position Sensor is fixed to the bottom of the TB. I believe that on some
models the TPS is on top of the TB). Slurry resulting from the solvent
would run down the Throttle disc spindle and enter the TPS. This is a ‘very
bad thing’.
Please post the results of your endeavours to the list.
Kind Regards,
John Tarrant.
R. Sparks wrote;
Snip <My 92 XJ40 is giving me a Fuel Fail 48 when I press the VCM button.
The
manual indicates that this refers to:
Idle speed control valve:stepper motor grossly out of position.
Is there anybody out there who can tell me what this means? How serious is
this problem and about how much is this one going to set me back? >