Jaguar XJ (X350). Manual — part 771

This subsequent combustion of the unburnt and partially burnt carbon monoxide (CO) and

hydrocarbon (HC) particles help to reduce the emission of these pollutants from the exhaust system.

The additional heat generated in the exhaust manifold also provides rapid heating of the exhaust

system catalytic converters. The additional oxygen which is delivered to the catalytic converters also

generates an exothermic reaction which causes the catalytic converters to reach their optimum

operating temperature and 'light off' quickly.

The catalytic converters only start to provide effective treatment of emission pollutants when they

reach an operating temperature of approximately 250°C (482°F) and need to be between

temperatures of 400°C (752°F) and 800°C (1472°F) for optimum efficiency. Consequently, the heat

produced by the AIR 'afterburning' reduces the time delay before the catalysts reach an efficient

operating temperature.

The AIR system comprises the following components:

AIR pump

AIR switching valve

AIR control valve

AIR vacuum reservoir

AIR pump relay

AIR pressure sensor (North American specification vehicles only)

AIR Pump

The AIR pump is located behind the right-hand side of the front bumper cover. The pump is fitted on

rubber mountings to help prevent noise which is generated by AIR pump operation. The AIR pump is

powered from the vehicle battery by a dedicated relay and supplies approximately 10 to 15 kg/hr (22

to 33 lb/hr) of air when the engine is at idle speed and the ambient temperature is below 20°C

(68°F).

Air is drawn into the AIR pump through vents in its front cover and is then passed through a foam

filter. The air is delivered to the exhaust manifold on each side of the engine through a combination

of plastic pipes and stainless steel tubes.

One second after the AIR pump is energised, the ECM switches on the AIR switching valve, which

opens to allow vacuum from the AIR vacuum reservoir to be applied to the vacuum operated AIR

control valve. When the vacuum is applied to the AIR control valve, it opens to allow the air from the

AIR pump through to the exhaust manifolds.

When the ECM switches off the AIR switching valve, the vacuum supply to the AIR control valve is

cut-off and the valve closes to prevent further air being injected into the exhaust manifolds. With an

approximate five second delay after as the AIR switching valve is closed, the ECM removes power

from the AIR pump relay, and this in turn stops the AIR pump from operating.

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AIR Switching Valve

The ECM switches on the AIR switching valve with a one second delay after initiating AIR pump

operation. When the AIR switching valve is open, a steady vacuum supply is allowed through to open

the vacuum operated AIR control valve. When the ECM switches off the AIR switching valve, the

valve closes and immediately shuts off the vacuum supply to the AIR control valve. The pump

continues to operate for a further five seconds for system diagnostic purposes.

When the AIR switching valve is switched off, the vacuum supply line opens to atmosphere, and this

causes the AIR switching valve to close automatically to prevent any further injection of air.

AIR Control Valve

The injected air from the AIR pump is controlled by the AIR control valve. This allows the correct

amount of air to be injected directly into the exhaust manifolds. The AIR control valve prevents

exhaust gasses from blowing back into the AIR pump.

The AIR control valve is assisted in operation by a vacuum source from the AIR vacuum reservoir

located in the right-hand side of the engine bay. This assistance allows the actuation of the AIR

control valve independently from the intake manifold vacuum levels available.

When the pressure in the exhaust system is higher than in the AIR system, the AIR control valve

closes the circuit, and this protects the AIR system from exhaust gasses blowing back into the AIR

system.

Vacuum to the AIR switching valve is provided from the intake manifold vacuum by the AIR vacuum

reservoir. A small bore vacuum hose provides the vacuum route between the AIR vacuum reservoir

and AIR switching valve. A further small bore vacuum hose is used to connect the AIR switching valve

to the AIR control valve.

AIR Vacuum Reservoir

The AIR vacuum reservoir is located on the right-hand side of the engine bay.

The AIR vacuum reservoir is included in the vacuum supply line between the intake manifold and the

AIR switching valve.

The AIR vacuum reservoir contains a one-way valve to stop vacuum leaking back towards the intake

manifold side. The AIR vacuum reservoir holds a constant vacuum so that the AIR control valve opens

as soon as the AIR switching valve is switched on.

AIR Pump Relay

The AIR pump relay is located on the AIR pump mounting bracket. The ECM is used to control the

operation of the AIR pump by the AIR pump relay.

The AIR system receives its voltage supply through the AIR pump relay. The ECM monitors the state

of the relay for correct operation as part of its system diagnostic.

AIR Pressure Sensor - North American specification vehicles only

The AIR system is monitored by measuring the system pressure by using the AIR pressure sensor at

several instances during its cycle of operation.

The AIR system pressure is measured before operation of the AIR pump. The AIR pump is then

switched on and with a one second delay, the AIR switching valve is opened. After a stabilizing

period, the system pressure is measured again, this time by taking the average of a one second

duration of readings, and normalising for variations in battery voltage and atmospheric pressure. If

the system pressure measured at this time has not risen enough with respect to the initial AIR

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