Bosch 0928400660 – The Metering Valve With Isolated Response To Suction Pressure Variations

A metering valve's performance is typically judged by its response to the ECU's command signal. But there is another signal that affects the valve's behaviour - the pressure at its inlet, which is supplied by the low-pressure lift pump. This inlet pressure is not constant; it fluctuates with the lift pump's output, with fuel temperature, and with the condition of the fuel filter. These fluctuations create a variable force on the armature, pushing it slightly open or closed and introducing a metering error that the ECU cannot foresee. The 0928400660 is engineered with a pressure-balanced inlet design that isolates the armature from these suction-side variations, ensuring that the valve's response is determined solely by the ECU's command, not by the fluctuations in the supply pressure.

The lift pump that supplies fuel to the high-pressure pump does not deliver a perfectly constant pressure. The pressure at the metering valve inlet typically ranges from 5 to 9 bar, but it can dip during cold starts or when the fuel filter is partially blocked, and it can spike when the lift pump is new or when the fuel is warm. This pressure acts directly on the armature face, creating a force that adds to or subtracts from the magnetic force generated by the solenoid. If the inlet pressure changes, the armature position at a given duty cycle changes, shifting the flow curve and altering the fuel delivery. The ECU does not have an inlet pressure sensor and cannot compensate for this variation in real time. The 0928400660 is designed with a pressure-compensated inlet port that balances the hydraulic forces on the armature, making the valve's response independent of inlet pressure variations within the 5-9 bar range. This balance is achieved through a pair of opposing pressure faces on the armature - one facing the inlet and one facing a reference chamber - that cancel each other's forces regardless of the inlet pressure.

The pressure compensation is achieved through the geometry of the armature and the inlet port. The armature has two effective pressure faces: the front face, which is exposed to the inlet pressure, and a rear face that is exposed to the same pressure through a calibrated passage. These two faces are sized so that the net hydraulic force on the armature is zero at all inlet pressures within the operating range. When the inlet pressure rises, the force on the front face increases, but the force on the rear face increases by the same amount, and the two forces cancel. The armature moves only when the magnetic force changes, not when the inlet pressure changes. This design relies on the precise sizing of the two pressure faces - a difference of even 0.1 mm in diameter would create a net force that varies with inlet pressure. The 0928400660 is manufactured with the two faces ground to a tolerance of ±0.02 mm, ensuring that the pressure compensation is accurate.

The pressure compensation relies on the calibrated passage that connects the inlet pressure to the rear face of the armature. If this passage becomes blocked - by varnish deposits or by debris - the rear face no longer experiences the same pressure as the front face, and the compensation is lost. The first symptom is a fuel trim that varies with engine speed - the lift pump pressure changes with speed, and the valve's flow changes with it. The engine may also show a slight hesitation on tip-in, as the lift pump pressure spikes and the valve's flow momentarily increases. The genuine 0928400660 uses a passage that is large enough to resist blockage, and the inlet screen prevents large particles from reaching the passage. The passage is also positioned so that any deposit accumulation is cleared by the fuel flow during normal operation.

Installing the 0660 is similar to other CP3 valves - clean the mounting surface, replace the O-ring and backup ring, torque the retaining nut to 24 Nm ± 2 Nm. However, there is a specific step: check the condition of the fuel filter and the lift pump pressure before installation. The pressure compensation is only effective if the inlet pressure is within the 5-9 bar range. If the lift pump pressure is below 5 bar, the compensation may be ineffective, and the valve's performance will appear degraded even though the valve itself is functioning correctly. The genuine 0928400660 includes a pressure port that can be used to verify the inlet pressure during installation. If the inlet pressure is outside the specification, address the lift pump or filter before completing the installation.

Q1: How can I tell if my valve has lost its pressure compensation without removing it?
→ Monitor the fuel trim at idle and then again at 2,500 rpm with no load. If the trim changes by more than 3 % between these two conditions, the pressure compensation may be degraded - the lift pump pressure changes with speed, and the valve's flow should not. A well-functioning 0660 will show less than 1 % change.

Q2: Can I install the 0928400660 on a pump that originally used the 0664?
→ Yes, in most CP3.3 applications. The 0660 and 0664 are interchangeable in many vehicles, but the 0660 has improved pressure compensation. Always verify the Bosch number on your existing valve and confirm compatibility with the engine application.

Q3: Why does my Duramax show a rough idle only when the fuel tank is below a quarter full?
→ This is often a sign of lift pump pressure variation. At low fuel levels, the lift pump may draw air or experience a pressure drop. The 0660's pressure compensation is designed to handle this, but if the lift pump is marginal, the inlet pressure may drop below 5 bar, at which point the compensation is no longer effective. Check the lift pump pressure and the fuel pickup tube.

Q4: What is the most common cause of the pressure compensation passage becoming blocked?
→ Varnish deposits from low-quality diesel or from infrequent oil changes (which can lead to fuel contamination through the injectors). The 0660's passage is designed to resist blockage, but regular fuel filter changes (every 20,000 km) are the most effective prevention.

Q5: I replaced the valve but still have a fuel trim that varies with engine speed - is the valve defective?
→ Before suspecting the valve, check the lift pump pressure at idle and at 2,500 rpm. If the pressure varies by more than 1 bar between these two conditions, the lift pump may be the cause. If the pressure is stable, then the valve may be the issue.

Q6: Does the 0928400660 require a specific torque for the retaining nut to maintain pressure compensation?
→ Yes. The pressure compensation relies on the correct valve body deformation. If the nut is over-torqued, the body can distort and change the pressure face geometry. The correct torque is 24 Nm ± 2 Nm. Do not exceed 26 Nm, as this can affect the compensation and the armature clearance.

 

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