Gsmack wrote: ↑
Sun Jun 11, 2017 1:23 am
Thanks for the lesson on the fuel vapour process.
We have considered a failing distributor, just wanted to check everything else first.
We have used a timing light during the failing point and it still seems to be sparking appropriately, does this rule a failing dissy out?
Does the jetronic le3 have a limp home mode, I thought it was a bit early for that sort of cleverness.
Mine uses the Motronic 3.1 with wasted spark (so no dizzy) which I seem to remember has limp home, cannot recall though whether the Jetronic le3 does. But I seem toremember that the Jetronic has an electronic ecu under the seat so may well have. Here's a write up on the efi for the BX courtesy of Citroen DIY that may help understanding:
The two main inputs of the EFI ECUs are the ignition (the signal coming from the distributor) and the actual engine load (represented by the amount of air sucked in by the engine). The ignition pulses, having entered the ECU, go through a shaping and halving circuit that forms regular square pulses whose frequency is half of the ignition frequency. The required amount of fuel will be injected in two installments. This signal serves as the time base for the injectors and the frequency remains unchanged throughout the whole calculation.
The width of the individual square pulses are calculated (or, as we already explained, looked up in a stored table) based on engine speed (the ignition frequency) and engine load (the amount of air sucked in by the engine). For the purpose of measuring this second, different sensors can be used. Earlier systems (Bosch Jetronic) used an air flow sensor (AFS): as air flows through the sensor, it deflects a flap, which is connected to a potentiometer. In consequence, the resistance of the meter is proportional to the amount of air passing through it. Later systems (both from Bosch and Magneti Marelli) used a manifold absolute pressure (MAP) sensor instead, which functions as a simple pressure sender (just like the oil pressure sensor in the engine).
Simpler EFI systems—the Bosch Mono-Jetronic fitted to later 1380 ccm engines—do not use any engine load measurement at all. To reduce the number of components and the costs, these systems rely on the position of the throttle pedal as a subsitute input. This is less accurate than actually measuring the quantity of air entering the engine, but it is much simpler.
Using these two main input signals the ECU determines the base pulse width (tp). This square pulse signal would be enough to control the engine under ideal conditions. However, the operating circumstances of an egine are seldom so favorable, hence, the ECU must carry out additional calculations to modify the base pulse width according to some special requirements.
The first major real life factor is the temperature of the air entering the engine. The cooler the air is, the denser it becomes. To compensate for this difference, the AFS housing incorporates an air temperature sensor (ATS) as well. MAP-based systems use a standalone sensor—depending on its location it might measure the temperature of either pure air or air-fuel mixture. Based on the values obtained from this sensor, the ECU might decide to lengthen the pulse width to allow more fuel, a richer mixture to enter the egine.
Similarly, extreme operating conditions such as idling or full load operation require a richer fuel mixture. The throttle position switch or potentiometer (TS/TP) informs the ECU whether the throttle pedal is fully depressed, fully released or somewhere in a middle position.
Starting the engine in cold weather is an even more special situation. Part of the fuel becomes condensed on the cold engine parts, consequently, a richer mixture is required to start the engine. The ECU monitors both the position of the ignition key switch and the coolant temperature sensor (CTS). If the CTS indicates that the coolant fluid is hot—in other words, a warm start—, there is no need for the longer injection periods.
As soon as the ignition key returns to the normal position, the ECU starts a 30-second warm-up period. In the first second, the ECU adds about 50% of the normal amount of fuel. Until the end of this initial warm-up period, this surplus drops to around 25%. From that point, the fuel surplus is determined by the temperature of the warming engine, as dictated by the CTS. To stabilize the idle speed in a Jetronic-controlled engine still cold, the throttle is bypassed through an auxiliary air valve (AAV). This valve is fully open when the engine is still cold but as the temperature rises it starts to close. On a warm engine it blocks the bypass completely. The air going through the bypass is measured by the AFS, thus it tricks the ECU into providing more fuel. This device is heated electrically, hence, it will close after some time no matter what temperature the coolant has.
Later systems have a similar bypass around the throttle butterfly but instead of a simple mechanical valve, they use an electrically operated solenoid valve controlled by the ECU to decide the amount of air which is allowed to bypass the butterfly.
All these additional correction factors—air temperature, idle or full load, starting, warming up—sum up into an additional pulse width (tm). But we've not done yet. The operation of the injector solenoids depend heavily on the battery voltage fed to them. To compensate for lower voltages, the injection time period must be lengthened by ts.
The total pluse width (also called injector duty cycle) is calculated by summing up the three values we received, the base width, the various correction factors and the voltage correction: ti = tp + tm + ts.
If the revolution is above a specified limit (around 1,200 per minute) and the throttle is closed—this is called deceleration—, the momentum of the car is sufficient to rotate the engine through the roadwheels. To save fuel, the injection is cut off. As soon as the engine speed drops below the limit or the throttle is opened, the injection is reintroduced.
Finally, to avoid prolonged operation at revolutions exceeding the specification of the engine, the injection is cut off above a maximum engine speed (6,000-7,000 rpm, depending on the engine).
Models equiped with a catalytic converter use an oxygen sensor (also called lambda sensor) to measure the oxygen content of the exhaust gas to adjust the fuel mixture to achieve the ideal lambda ratio.
Theoretically, the mixture burned in the engine should contain air and fuel in proportion of 14.7 parts to 1 to achieve ideal combustion. The lambda ratio is simply the proportion of the actual mixture to the ideal. A lambda value of 1 means ideal mixture, values below 1 mean rich, those above 1 lean. By measuring the oxygen content in the exhaust gas the computer can decide how to adjust the mixture to keep the lambda value around 1. The main reason for this is the catalyst, whose efficiency depends heavily on the lambda ratio of the gas fed to it. If the lambda is just a fraction below 1, the CO emission rises sharply, and a little over 1 skyrockets the NOx emission. During cold start, acceleration or full throttle the computer decides on a different mixture but under normal operating conditions it sticks to 1, provided that everything else in the system works well. The sensor does not start working until it reaches the temperature of 350 °C.
Actually, this is the reason of the higher fuel consumption of cars equipped with a converter. Lambda 1 is ideal for the converter and is also ideal from a scientific point of view, however, it is certainly not ideal when speaking about fuel economy or driving dynamics.
There is a final safety circuit in the system. During a crash, when the engine has already stopped, the fuel squirting from the injection system could easily cause fire. Hence, the relay of the injectors is controlled by the ECU, allowing fuel injection only when the ignition signal is present.
All systems—with the exception of Jetronic—have self diagnostic subsystems which constantly monitor the signals from the engine sensors and, should a fault be present, log an error code in an internal memory. More serious errors light up the warning lamp on the dashboard as well to inform the driver of the failure. The error codes can be extracted by either a special diagnostic tool or using a simple algorithm. Whenever the ECU senses the failure of a minor sensor, it omits the signals coming from that sensor and substitutes a fixed default value. This value is characteristic of a hot engine, therefore, cold starting and the warm-up period may be less than satisfactory, however, in normal operating conditions the engine may actually run quite well.
Most systems can also adapt themselves to changing operating characteristics or engine wear. If some of the components have been renewed, the ECU should be re-calibrated, that is, allowed to repeat its learning process. Disconnect the ECU multi-connector for approximately 15 minutes (with the ignition off, this is extremely important!), during which time the ECU resets to the default values. Reconnect the connector and run the engine until it reaches the normal operating temperature.