15.1.7 Systems Pressure
This is the pressure that is seen within the system between the fuel pump and the metering head. This pressure is determined by the primary pressure regulator, situated within the metering head.
When the required pressure is obtained, the plunger within the regulator lifts off its seat and excess fuel is returned to the tank.
This system due to the nature of its operation will automatically compensate for different fuel demands under different conditions. For example if the fuel requirement is low at engine idle, the plunger will lift and return a greater volume of fuel back to the tank than when the demand is higher, when a smaller amount of fuel is returned.
When the engine is switched off, the fuel pump relay looses the coil negative signals that energise it and the voltage to the pump is removed: this subsequent loss of pressure will cause the primary pressure regulator to close. This action subsequently blocks the return flow to the tank and helps the accumulator to maintain pressure in the system.
The systems pressure is determined by the tension of the spring reacting against the plunger, if a higher pressure is required, small shims can be placed behind the spring, changing it's effective length and increasing the pressure. A shim of approximately 2 mm will increase the pressure by about 10 psi
Located within the pressure regulator is the transfer valve. This component is operated by the movement of the plunger and opens as the plunger moves off it's seat. The transfer valve's function is to block the return flow of fuel from the warm-up-regulator back to the tank, also helping to maintain residual or holding pressure.
Figure 15.1.4 shows the fuel distributor, primary pressure regulator and air flow sensor from the Bosch K Jetronic system.
The following is a guide to the fuel paths marked by each blue arrow in figure 15.1.4
A=To fuel injectors
B=To warm up regulator
C=From warm up regulator
D=To cold start injector
E=From fuel filter
F=Return to fuel tank
15.1.8 The Airflow Sensor
The airflow sensor, in most cases, is located on the air filter housing and is responsible for measuring the amount of air entering the engine. The sensor housing is conical in shape, into which the airflow sensor plate is fitted. The airflow sensor plate lifts as the throttle is opened by the incoming air.
The amount of lift is proportional to the volume of air entering the engine. The shape and angle of the cone will determine this ratio.
A neutral plate position is normally level with the bottom of the cone, this is adjustable by bending a small clip / spring that acts as a stop at the bottom of the unit. The purpose of this spring is to allow the flap to move beyond its neutral position to allow excessive pressure to escape if the engine was to backfire, passing a large volume of air back into the air filter housing.
If the system did not have this facility the pressure could split or blow off the rubber air trunking. Any splits or ill fitting air hoses that allow unmonitored air into the engine require rectification.
As the airflow lifts the sensor plate this subsequencially lifts the control plunger - the higher the lift the greater the amount of fuel delivered to the injectors.
To adjust the fuel mixture a small 3 mm Allen screw is located within the airflow sensor; this alters the relationship between the sensor arm and the control plunger. Turning the screw clockwise enriches the mixture and vice-versa. It should be noted that the screw should be turned in very small increments and the Allen key should be removed before the engine speed is raised.
NOTE :- Failure to remove the Allen key, before starting the engine, can result in damage to the airflow sensing unit.
15.1.9 The Fuel Distribution Unit
This unit delivers the correct amount of fuel to the engine via the injectors referencing to the airflow sensor plate height. As the sensor plate is lifted with inducted air volume, the control plunger is lifted proportionately, exposing small slits within the fuel distributor's barrel assembly. The barrel assembly has a series (one for each cylinder) of small slits that are machined into the barrel, and it is through these openings that the fuel passes en-route to the injector.
The width of these metered slits is only 0.2 mm across and it is this dimension, together with the plunger height, that determines the fuel delivery rate to the injectors.
At low engine speed the air volume into the engine will be minimal, this will only raise the plunger a small amount giving the requisite quantity of fuel for these engine conditions. As the throttle is opened and fuel demand is higher, the plate raises, which in turn lifts the plunger and a higher volume of fuel is delivered to the engine to match the air. The lift on the plunger will be proportionate to the air volume, this will however be exaggerated during the warm-up period when additional fuel is required by reducing the pressure acting onto the top of the control plunger.
This pressure is called the control pressure (as it controls the lift of the plunger under different operating temperatures) and is determined by the warm-up-regulator.
15.1.10 The Warm-up-Regulator
This simple device is responsible for controlling the amount of fuel delivered to the engine during it's warm-up period. The pressure acting upon the top of the control plunger varies depending on the engine temperature and provides an effective method of enrichment.
The control pressure is tapped off from the primary pressure circuit in the metering head's lower chamber through a tiny restrictive hole which gives it the ability to differentiate between the two pressures. A flexible pipe then connects the control plunger gallery to the warm-up-regulator and returns back to the metering head to a connection next to the primary pressure regulator's transfer valve. This valve is in the circuit to close the fuel from the control circuit when the engine is off, avoiding the total loss of system pressure while the engine is stationary.
The internals of the warm-up-regulator are quite simple comprising an inlet and outlet port, a stainless steel shim, a bi-metalic heated strip and a spring.
The input to the warm-up-regulator flows into a small chamber in the top of the unit, its return is through a small drilling and back to the metering head. By controlling this return flow it will cause a change in pressure acting on the top of the control plunger. With a cold engine the flow must be fairly free giving it a lower pressure. This will allow a higher lift of the plunger which in turn will enrich the mixture under these conditions. The free flow is obtained by the internal bi-metalic strip exerting a downward pressure on the spring which decreases the pressure acting upon the shim, this lower force allows the fuel to flow almost uninterrupted.
As the bi-metalic strip is heated, by either it's heater element or natural heat soak from the engine, the downward pressure acting on the spring is gradually decreased, increasing the force of the spring, which in turn increases the control pressure.
Typical cold engine control pressure will be as low as 1.0 bar increasing over approx. 10 minutes to around 3.5 bar. Some warm-up-regulators have a vacuum connection that will sense a drop in vacuum and lower the control pressure during these acceleration periods.
The voltage supply to the regulator is from the fuel pump relay, because if the ignition was on without the engine running, all enrichment would be removed as the bi-metalic strip would be heated prematurely and the driver would not benefit from the cold engine enrichment.
The two pipes that connect to the warm-up-regulator have different sized 'banjo unions' to avoid them being connected incorrectly. The control pressures quoted are as an example only and reference should be made to the technical data as these pressures can be specific to the part number located on the unit's housing.
This unit will have a resistance value of approximately 20 to 26 Ohms.
NOTE :- it is important to disconnect the electrical connection to the unit before any pressure testing on the control circuit is performed as this will prematurely heat the bi-metalic strip and cold control pressures will not be available.
Figure 15.1.5 shows a diagram of a warm up regulator.
The connections shown in figure 15.1.5, marked with blue arrows are listed below:
A=Vacuum connection (inlet manifold)
B=Return to fuel tank
C=Control pressure (from fuel distributor)
Figure 15.1.6 shows a photograph of a warm up regulator.
15.1.11 The Cold Start Injector
To aid the starting of the engine an additional injector is located into the inlet manifold, this sprays fuel into the engine at systems pressure when the engine temperature is cold and the starter motor is activated. The length of time that this additional injector sprays is determined by the engine's temperature, seen by the thermo time switch.
The thermo time switch provides the earth path for the cold start injector via a heated bi-metalic strip, this heater is activated by a voltage from the starter motor. As the strip heats, over a period of approximately 8 to 10 seconds (when cranking only), the legs on the bi-metalic strip separate and the earth path is lost.
A warm engine will perhaps only require 2 seconds before the circuit is broken and a hot engine will already show open circuit. This simple circuit is to avoid the engine being flooded when cranking and the additional enrichment only given when essential, see illustration below.
Cold start injector
Figure 15.1.7 shows the relationship between the thermo timer switch and the cold start injector.
15.1.12 The Auxiliary Air Valve
This item is a device to aid the engine when cold by opening a small port to increase the engine's idle speed. The fast idle control is achieved by the port being held open by a bi-metalic strip that when heated by it's own heater element, or via natural heat soak from the engine, the port closes. The voltage supply to the air valve is the same as the feed to the fuel pump and the warm-up-regulator. If it is found that the idle speed will not reduce and that the speed is maintained artificially high when warm, clamp the rubber pipe between the air valve and the inlet manifold. If this action causes the engine rev's to return to normal, the fault is within a sticking auxiliary air valve.
It is worth cleaning the valve, lubricating it and re-test it's operation. The internal heater element can also be checked for continuity using a multimeter.
Auxiliary air valve
Auxiliary air valve
Figure 15.1.8 shows an auxiliary air valve diagram and figure
15.1.9 an auxiliary air valve photograph.
15.1.13 The Fuel Injectors
The injectors fitted to this system will open at a predetermined pressure and will spray a fine atomised 'mist' of fuel behind the inlet valve, waiting to be drawn in on the induction stroke. The fuel is delivered into the engine in a continuous spray and is not timed or pulsed as on other systems. The opening pressure of the injector is at approximately 3.3 bar at which point fuel is injected into the manifold; when the injector pintle opens this will cause the pressure to drop, subsequently closing the injector, which causes the pressure to rise once again and this will of course open the injector. This pintle vibration is called 'chatter' and helps to atomise the fuel before it's induction.
When the engine is switched off the fuel pressure drops below 3.3 bar and the injector closes forming a fuel tight seal, helping to avoid fuel dripping into the inlet manifold.
The spray pattern should be a conical shape and when clean and working efficiently, should emit a high frequency noise: this is the sound of the pintle 'chatter'.
Figure 15.1.10 shows a cross section of a mechanical injector.
15.1.14 System overview diagram
Figure 15.1.11 shows an overview of the Bosch K-Jetronic system