Fourth generation common rail diesel diagnosis – suction control valve

DENSO is a world leader in diesel technology and in addition to being the first original equipment (OE) manufacturer of ceramic glow plugs, it pioneered the common rail system (CRS) in 1995. This expertise continues to allow the company to help vehicle manufacturers around the world to create increasingly responsive, efficient and reliable vehicles
Diesel 494x135


One of the key characteristics of the CRS, which has played a large part in delivering the efficiency gains associated with it, is the fact that it operates with the fuel under pressure. As the technology has evolved and engine performance improved, so has the pressure of the fuel in the system increased, from 120 megapascals (MPa) or 1,200 bar at the introduction of the first generation system, to 250 MPa for a current fourth generation system. To demonstrate the dramatic impact that this generational development has yielded, comparative fuel consumption is down by 50%, emissions down by 90% and engine power up by 120%, during the 18 years between a first and fourth generation CRS.

Safety first

Although these fuel pressure increases have brought with them considerable performance gains and emissions reductions, they have also increased the injury risks for those undertaking CRS service and repair. Naturally, wearing appropriate protective clothing and observing the correct procedures is important when working on any part of a vehicle, but additional precautions need to be followed when the CRS is involved.

While the engine is running and a high pressure test is performed, for example, technicians must never touch any of the high pressure pipes, connections or components with their fingers because leakage of hot, and highly pressurised fuel can cause serious injury. Penetration of the skin, even in the smallest of cases, can have devastating consequences, including serious infection and even result in amputations due to the development of gangrene.

CRS diesel diagnosis

DENSO Aftermarket provides technicians with many options when it comes to training and support, with useful video instruction through YouTube an important part. A great example of CRS diesel diagnosis can be viewed here and demonstrates both good customer management and efficient, best practice system diagnosis on a high mileage Toyota Corolla, suffering from rough idle and producing black smoke when starting from cold.

Other training resources include DENSO webinars and, in a CRS focus, presented by Senior Engineer/Product Specialist, Ries Schimmel, attention is drawn to diagnosing faults involving three key components, the fuel pump, injectors and the rail.

Fuel pump - suction control valve (SCV)

There are two types of HP3 diesel fuel pump SCVs: A) Normally open, and B) Normally closed. So, should the SCV become disconnected, the result will be maximum fuel delivery with a type A system because the valve is fully open, whereas with a type B system, there would be no fuel pressure as the valve is closed.

The performance of both types of SCV can be assessed to confirm whether there is an underlying fault by using a multimeter to check the duration the actuating voltage solenoid is activated and the average current across the current solenoid.

If the SCV has failed and the fuel pump needs to be replaced, it will need to be correctly calibrated after its installation, which will ensure there is the smallest possible variation due to its production tolerance or age. If this important step is not undertaken, there is the risk that the pressure in the rail could be too low, so the engine turns over, but doesn’t start, or is too high, which can cause the rail pressure limiter to open. Other issues include diagnostic fault code activation or the check engine lamp on the dashboard will be illuminated.

The Aftermarket

One of the major implications for the European independent aftermarket is that, although repair tools and techniques are under development for the DENSO authorised repair network, at present there is not a practical repair option for fourth generation fuel pumps or injectors.

Therefore, although fourth generation CRS service and repair can, and should be, undertaken by the independent sector, fuel pumps or injectors that have failed cannot currently be repaired, so must be replaced with new parts of matching OE quality supplied by reputable manufacturers, such as DENSO.

Further details of the DENSO Aftermarket programme are available online at: