This article looks at the Denso HP2 fuel system, the way in which it operates and the common problems associated with it. This particular system was fitted to Toyotas, Nissans, Renaults, Vauxhalls/ Opels and Isuzu vehicles from around 1999 to 2007 but may vary from model to model. The HP2 system is easily identifiable because it uses two Suction Control Valves. Looking on the back of the pump you will see there are two electrically operated solenoids – one red and one green.
This is because the system is not like any other. The job of the SCV is to control the flow of fuel oil to the pumping chamber of the high pressure pump, as a means of controlling the pressure in the common rail. The pumping chamber in other systems (including Bosch, Siemens and Denso HP3), consists of a cam, piston and powerful spring. The spring provides the inlet stroke and the cam provides the outlet stroke; this means that the engine has to provide a large amount of power to operate the pump. The Denso HP2 system uses a single internal eccentric cam and two pumping chambers.
The two pumping chamber’s pistons are set at 90° to each other so that they are on opposite strokes but are operated by the same cam. In the image above, Chamber 2 is on the intake stroke; fuel from the internal feed pump enters the chamber via the check valve when the SCV is open and forces the pistons apart. The amount of fuel entering the chamber is controlled by how long the SCV is open during the intake stroke. A short opening time will only partially fill the chamber, by moving the pistons out a reduced amount so that the rollers do not touch the cam. This means the cam has to turn until the rollers touch the cam and start the outlet stroke.
The amount of fuel delivered to the common rail is a function of the angle through which the rollers are in contact with the cam – shaded red in the image below. Both the contact angle and the common rail pressure are parameters on your scan tool; the relationship between engine speed, contact angle, injection quantity and common rail pressure are monitored for plausibility.
Pump Timing
Unlike the SCVs in other systems (where the opening of the valve is controlled in degrees by using Pulse Width Modulation), the Denso HP2 system uses a timed open period. The result of this is less effort is required to operate the pump.
As a result of this method of control, the opening of the SCV has to be timed to allow the flow of fuel into the chamber when the internal cam is at its maximum stroke. This means that the pump drive pulley has to be timed to the engine. This must be kept in mind when replacing the cam belt.
What goes wrong?
The SCV fails to respond to the control signal correctly; this can be caused by various reasons but essentially, they stick. There has been some speculation about the reduction of the lubricating properties in modern diesel fuel due to the reduction of sulphur. Sulphur is not a lubricant but it can combine with the nickel content in many metal alloys to form an alloy that can increase lubrication. Also the process used to reduce the sulphur content in fuel, reduces the fuel's lubricating properties.
Over the years of production small modifications have been made to the design of the SCVs to overcome the problem, but there are still plenty of older vehicles out there that have the potential to fail.
Symptoms of SCV failure
The most common symptoms are a lack of power or rough idling; both these faults may be intermittent. DTCs P1229 and P0093 relate to excessive pressure in the system caused by the SCV sticking open longer than it should and will cause the engine to go into failsafe mode where the engine power is restricted. DTC P0627 relates to the SCV circuit, this could be caused by an open or short circuit in the SCV field coil or in the wiring.
Checks
If DTC P0627 is recorded the SCVs should be checked for resistance and insulation. The wiring from the Engine ECU to the valves should also be checked for continuity; for this you will need a wiring diagram. A quicker test can be carried out using a good oscilloscope.
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