X300 – Fuel system

4 – Fuel System ( )


Fuel is continuously circulated by an electric pump located within the tank itself. A filter is located under the car floor, beneath the tank. A vacuum driven regulator controls the fuel pressure in the injector rail, with the injectors themselves being controlled by the main engine ECU.

Be aware the system operates at high pressure, so leave the car to stand overnight, or very cautiously relieve the pressure by opening a joint in the fuel lines a fraction at a time using a cloth to mop up spilling fuel.

The continuous circulation of fuel causes the fuel tank to act as a heat sink, keeping fuel temperatures down and preventing vapourisation. A long journey on a hot day will, however, cause fuel in the tank to become warm. This is normal and no cause for concern.


4.1 – Fuel Filter Replacement ( )

This is rarely needed as part of a normal maintenance routine, but is necessary should the fuel system be contaminated or low fuel pressure be suspected.

Unbolt the unions, and be prepared for spillage. Remove and replace the filter, then refit the unions. Use new copper washers to avoid subsequent leaks.


4.2 – Fuel Gauge Sender ( )

Early X300 senders can develop a problem that results in the gauge not registering above three-quarters and/or being generally inaccurate. This is caused by contamination of the electrical track along which the wiper arm operates. Jaguar recommends a replacement unit, though some people have fixed existing unit by cleaning the track as follows.

To avoid spillage, the fuel level in the tank needs to be as low as possible and at least below an eighth full. The sender is located at the back of the tank and can be accessed by removing the trim panel at the back of the boot/trunk.

Remove the float pivot from it’s plastic mountings. Clean the electrical track and contacts with a fibreglass pencil or similar gentle abrasive. Ensure the contacts are adequately tensioned before reassembly.

When refitting, ensure the arm movement is clear of the fuel pump wires inside the tank.


4.3 – Throttle Body Removal and Cleaning ( )

Periods of cold running, perhaps caused by a sticking thermostat, will cause the throttle body to become gummed up internally, potentially causing the throttle to stick partially open and also affecting Idle Speed Control Valve (ISCV) operation. It can also be the cause of an uneven or lumpy idle.

On the XJR, the throttle body is located under the inlet manifold so this must be removed first following the instructions above. Those with a ramps or a hoist may be able to gain sufficient access from underneath.

Release the throttle cable by carefully using a pair of long noses pliers to push off the retaining clip. Also disconnect the vacuum hose from the cruise control actuator.

Unbolt the heated part of the manifold, releasing the coolant pipes as necessary, and then unbolt the throttle body itself. The ISCV can be identified as a small nozzle in a drilling in the side of the main orifice, which controls the amount of air bypassing the main throttle butterfly.

On the bench, use carburettor cleaner or a similar solvent to clean out the body, butterfly and ISCV. Allow the cleaner to get into the ISCV drillings, but hold the body so as to avoid running down into the Throttle Position Sensor, which is contained in the plastic housing at the end of the throttle butterfly shaft.

Reassembly is simple. Make sure the cruise control actuator arm can fully actuate and release the throttle butterfly. No gaskets are used, so to avoid vacuum leaks, ensure the mating surfaces are smooth and clean. Bolt everything up and reattach the throttle cable, retaining clip and cruise control vacuum hose.


4.4 – Throttle Return Spring ( )

Early models may develop a problem whereby the engine rpm will not drop to the correct idle speed. This can be caused by a dirty throttle body causing the throttle to stick ( see above ), a weak throttle return spring, or a combination of both.

Jaguar did issue a bulletin (tsb) to replace the springs on earlier models and the spring is available from dealers. If the problem persists after cleaning the throttle body, or you wish to replace it anyway, the newer part is available from the dealers.
These fitting instructions are based on the XJR model with cruise control, and so details may vary on other vehicles.

On the XJR, the throttle body is located under the inlet manifold so this must be removed first following the instructions above. Those with a ramps or a hoist may be able to gain sufficient access from underneath.

The throttle return spring is attached to the bell crank actuated by the throttle cable and is now simple to remove and replace. Ensure throttle action is smooth and that it now closes fully by pushing gently on the bell crank to ensure no further movement. Refit the inlet manifold.


4.5 – Throttle Position Sensor Replacement ( )

Gain access to the throttle body, as described in 4.3 above, and locate the TPS. It is a square black plastic unit, mounted on the end of the throttle shaft with a three wire electrical multi-plug.

Disconnect the multi-plug, undo the securing screws starting with the one at the front of the engine, remove the retaining plate and then the sensor itself.

When refitting, ensure the sensor engages correctly on the spindle. The sensor is spring loaded and must be turned slightly to then refit the retaining plate and fit the securing screw towards the rear of the engine. Don’t fully tighten. Fit the front securing screw, then tighten the rear screw. Refit the connector.

At this stage, the Jaguar workshop manual says that the ECU must be reset to work with the new sensor. This is probably to allow it to understand the characteristics of the sensor at idle and full throttle opening and therefore not required if replacing the same sensor.


4.6 – Fuel Feed Hoses ( )

A helicopter noise from under the bonnet, coupled with a pulsing in the fuel hoses, indicates a fuel damper or hose. The fuel damper is inline with the hoses and replacement is simple, but remember to leave the car to stand overnight or release fuel pressure carefully when undoing the pipe unions.


4.7 – Fuel Filler Drain ( )

There is a small water drain positioned alongside the fuel filler. Ensure the drain is kept free from debris, as a build up of water may then find it’s way into the tank. The drain tube itself can be accessed from inside the boot after removing the side trim panel.