Fuel injection and the Jaguar XJ6 4.2 Series 3 [ Main ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ]



European non-emission cars use a micro-switch to detect closed throttle and a vacuum switch, acting at 3.5" Hg depression, to activate full load enrichment, thus at low speeds when full load is achieved at around the half throttle position, response and drivability are improved. There is no provision for extra fuel at idle, the airflow meter adjustment meeting this need, and the only reason for detecting closed throttle is to activate over-run cut off via a micro-switch so this switch is omitted on cars with manual transmission.

US emission cars use a standard Bosch throttle switch with cam operated contacts to detect the two extremes of throttle travel, therefore full load enrichment is only applied at full pedal travel, simultaneously disabling Lambda feedback. Enrichment is provided at closed throttle to obtain a satisfactory idle quality with richer mixture than Lambda feedback permits. These cars do not have over-run cut off.


As is common with most injected engines the throttle is not used as a means of adjustment for idle speed and the correct setting is to have a 0.002" gap around the periphery of the throttle disc. An air by-pass arrangement is provided from just above the throttle via the air distribution block mounted on top of the inlet manifold plenum chamber. A hex screw in this block adjusts the idle air supply but such adjustment should only be made in conjunction with fuel trim adjustment at the airflow meter subject to readings from an exhaust gas analyser. The distribution block also houses the over-run valve and feeds air into the manifold from the auxiliary air valve.

From a cold start all engines need an increased air supply whilst idling to overcome all the high friction losses of a cold engine. D Jetronic used a valve which was connected to a wax capsule suspended in the coolant. For L Jetronic Bosch adopted a different approach with an orifice plate moving across a passage under the control of a bimetallic strip (see diagram). The valve is mounted so that it follows the temperature of the engine structure but the rate of response is trimmed to suit the engine requirement by a heater coil around the bimetal strip, powered via the fuel pump relay.


The coolant sensor is just a straightforward thermistor connected as the lower part of a voltage divider chain. The signal voltage to ECU pin 13 ranges from 2 volts (330 Ohms) fully warm to around 10 volts (6K Ohms) at 0 degrees C. The corresponding nominal resistance values of the thermistor are shown in brackets. The ECU enriches the fueling when the engine is not fully warm, just like a choke device, according to the coolant sensor signal.


This does not really have much to do with the injection system proper other than using the same pressurised fuel supply. All that happens is that an electrically heated bimetal switch inside the thermotime switch causes the cold start injector to spray a fine mist of fuel into the manifold while the engine is cranking over. It is purely an aid for rapid starting from cold and once the engine is running it plays no further part. The thermotime switch limits the spray period to a maximum of 12 seconds, depending on temperature, anyway.


Only present on emission cars with catalyst this detects the presence or absence of oxygen in the exhaust stream and its output switches state accordingly. With no oxygen (rich fueling) it produces a signal of 0.8 volts, and when oxygen is present (weak fueling) the signal falls to 0.2 volts. In either case the ECU corrects the fueling gradually in the appropriate direction.

The sensor needs to be at 350 degrees C or higher to function properly and, not being of the electrically heated type as on later cars, the system runs without feedback until working temperature is reached following start up although there is no definite inhibition of the Lambda signal from cold.

Air valve

[ Main ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ]

AJ6 Engineering, 60, Henshall Road, Bollington, Macclesfield, Cheshire SK10 5DN, England
Email aj6engineering@ntlworld.com, Tel/Fax:- 0044 (0)1625 573556