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V12 Air Filter Boxes

Identification Features.


Rear mount air box.

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Later forward mount air box.

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Fundamentally there are two types of air filter boxes used on the V12 XJS and XJ12 5.3. The LH photo above is the rear mounted type found on all XJ12 Series 2 & 3 cars and on the XJS until the arrival of ABS in March 1988. All XJS V12s after then have the forward mounted type shown in the RH photo in which the entire assembly has been moved forward relative to the throttle orifice.

There are many other variations according to model year and market. The following list explains the significant features and differences amongst them all.


Original D Jetronic 1975-77.

Intake tubes nominally 9” long and angled inwards by approximately 25o with 11/2” bore moulded trumpets.

Air thermistor mount on LH intake tube. Fuel cooler brackets, idle air pipe stub and crankcase breather fitting on LH back-plate.

All back-plates have over-run valve pipe stubs. Some variants have a vacuum dump pipe fitting on RH back-plate. Outer covers locate in shallow folded groove at lower edge of back-plates.


Original D Jetronic 1977 onwards.

To cure a tendency for being blown open by backfires, four metal tongues were added to the lower edge of the covers to engage in four slots in the lower flange of back-plates.


5.3 10:1 c.r. XJS (also a few XJ12 saloons) 1980-81.

Different air temperature sensor stub to take larger threaded type for Lucas Digital EFI (6CU).

Some variation of pipe attachments to RH back-plate, sometimes including a hole for a solenoid air valve for some European market cars.


5.3 HE 1981-88 (saloon to 1991).

Hole in RH back-plates for solenoid extra air valve. At some stage the top edge fasteners were changed from spot welded strip type to detachable wire clip type.

Cars with air injection had a 3/4” hole in the RH filter outer cover for diverted air.


5.3 XJS with ABS & Lucas ignition March 1988 – Nov 1988.

Forward mount type with nominally 7” long intake tubes facing directly forward. Throttle hole at rear end and support bracket at forward end of back-plates.

The crankcase breather attachment was moved back along the back-plate.

Back-plates are now about 45 mm deep compared to 35 mm of previous versions.

This is the only version of the forward mount boxes with over-run valve pipe stubs in back-plates.


5.3 XJS with Marelli ignition.

All forward mount type. RH back-plate has large hole and two small screw holes for air temp Otter switch (ignition).

The air injection hole in RH cover became standard on all and sealed with a grommet when not required.


6.0 XJR-S.

Some early 6 litre cars with Lucas EFI had enlarged 21/8” tubular metal intake trumpets badged TWR.


6.0 XJR-S with Zytec engine management 1990.

Early versions had the enlarged 21/8” tubular metal intake trumpets and also had air thermistor mounts on both intake tubes facing inwards.

The RH back-plate had a pipe stub for an additional crankcase breather pipe.


6.0 XJR-S with Zytec engine management 1992.

Short 2 inch i.d. intake tubes facing forwards. Because of connection to cold intakes length of intake tubes differ in accordance with cylinder bank offset. Two air thermistor mounts and additional breather pipe stub as before.


Production 6.0 XJS 1993 onwards.

All 6.0 air boxes have 2 1/8” parallel intake tubes. RH 4” long; LH 31/4” long. Angled inwards at approximately 10o.

RH back-plate has a boss for the Marelli air temp sensor near the throttle and a hole for a solenoid air valve.

LH back-plate has a 1/4” UNF stud.

Early type have conventional breather fitting on LH back-plate with forward facing tube but with no Smith valve. Fuel cooler brackets discontinued.


6.0 XJS from 1995.25 MY.

The long established original front crankcase breather system is replaced by a 5/8” stub pipe on each back-plate. These are connected via hoses and flow restrictors, to tappings from the manifolds, running to a new breather gauze system located in the rear half of the crankcase valley.

The stud on the LH back-plate changed from 1/4” UNF to 6 mm thread.