What is the diesel common rail high-pressure pump and how does it work?

In today’s article, we will talk about a very important component of the common rail injection system, namely the high-pressure pump. Most drivers have definitely heard of the high-pressure pump, but most do not know what this pump is, how it works, what problems can have, and how to protect it.

The high-pressure pump is found in the injection system of the slightly newer generation engines that use the common rail injection system (CR).

To better understand, I will explain for the first time how the common rail injection system works. This injection system is much better than the classic one (PD-pumps) because the fuel injection is no longer dependent on engine speed and load, so it can be much easier to optimize the injection, the moment and the amount of fuel injected into the combustion chamber, for each engine operating point.

In the case of common rail systems, the injectors are connected to a common rail(as the name implies) and not directly to the pump.

This means that the fuel pressure is independent of engine speed and load, so if the electronic control unit commands the injectors to inject more diesel in the combustion chamber even if the engine has a low engine speed, they will do so because they are always kept under pressure, regardless of engine speed.

This independence from the point of operation of the engine has many advantages, the most important being high performance and lower consumption. For those who have more knowledge of mechanics, this system can also divide the fuel injection into several phases, namely pre-injection, main injection and finally post-injection.

The amount of fuel injected into the combustion chamber is controlled by the electronic control unit after it has collected more data from a multitude of sensors. But it is most influenced by the position of the accelerator pedal, depending on how fast or how sudden the driver accelerates, so the ECU will order the injection of a greater or lesser amount of fuel.

No matter who makes them, these injection systems are electronically controlled by ECUs, sensors and more solenoid valves. We need to know that this injection system is a bit more complicated thanks to the fact that it is controlled by many electrical components such as ECU, camshaft position sensor (to know exactly when fuel should be injected into the combustion chamber).

The engine speed sensor, the accelerator pedal sensor (for the ECU to know exactly how much power we need and how strongly we requested the engine thus determining the amount of diesel injected), the rail pressure sensor (it has the role of measuring pressure from rail so that the ECU can control the high-pressure pump further if additional pressure is needed) and some other temperature sensors and flowmeters.

From a “hardware” point of view, the common rail type injection system consists of the airflow meter, injection computer, high-pressure pump, common injection ramp, injectors, speed sensor, temperature sensor, diesel filter, and the accelerator pedal position sensor.

Well, once these aspects are established it is time to focus more attention on the high-pressure pump. In the common rail type injection system, diesel has the following route. It comes from the tank at very low pressure (normal, somewhere at 1-2 bar) and enters the diesel filter to be purified.

From there it comes out with slightly higher pressure (around 4-5 bar) and reaches through high-pressure hoses at the high pump. This complicates things a little because the high pump “pushes” through a special diesel pipeline in the injection rail with very high pressure, a pressure that exceeds 1400 bar can reach 2500 bar depending on the model.

The high-pressure pump, as the name suggests, ensures high fuel pressure at the ramp regardless of engine speed. This pump is driven by the crankshaft of the motor with the help of an accessory belt or depending on the model it is mounted on the belt or timing chain.

But the advantage is that this pump does not have to be adjusted in the sense that it should not be tuned as it is required in conventional injection pumps. These older generation pumps are limited to 3000 rpm. Diesel is sent with very high pressure to the rail where each injector is fueled.

In this type of injection, the injectors work more or less like “taps” that simply allow a certain amount of diesel to pass at a certain time.

The injectors are electrically controlled, which is why problems can occur over time.

What are the disadvantages of this system?

Well, for the beginning, it works with very high pressure, so the tolerances in the interior of the high pump and not only are very small.

Because very much electronic is involved, everything is controlled either by a sensor or by an electronic valve, nothing is simple, mechanically, everything is electrically assisted.

Because of the bad fuel, which does not have such good quality,  the high-pressure pump could fail. In most cases, only parts of the high-pressure pump give up

When the pump fails, it produces a metallic material that contaminates the entire supply system, this material reaching the tank. More serious is the fact that the injectors will block. Depending on the material, we can figure out what exactly failed in the high-pressure pump.

If the material is made of aluminum then most likely the bearing “began to exfoliate” from the inside. If the material is very thin, in the form of extremely fine filament then the pistons or system that pumps the fuel under pressure will wear.

How do we know that the high-pressure pump will fail?

Well most likely when it fails, first it will not be able to create the necessary pressure in the injection rail, so most likely the “check engine” will show and the power will be cut off feeling that when we want to accelerate.

The car will start to show these bad symptoms more and more often, especially when accelerating suddenly. In the end, the high-pressure pump will give up entirely and the engine will simply not start. If the high-pressure pump fails, you should know that you have more options for repair or replacement.

For example, you can buy a new high-pressure pump, with a warranty, but this will cost you a small fortune. Some call for the option to buy a second-hand one, from the car junkyard, which I do not recommend because you may have the misfortune of finding a pump that has not had an easy life.

The best option from our point of view would be to refurbish it, many specialized services can refurbish your old high-pressure pump at low prices than you can afford.

A disadvantage, if we can say so, is that when reconditioning the high-pressure pump, the workshop that deals with it also wants to recondition the injectors, clean the rail and injection pipes.

It is better to clean the tank and the route because if the pump started to lose material, then the entire system, including the tank, is contaminated with iron casting. After you install the freshly refurbished pump, it will fail fast. For this reason, it is good to carry out these operations.

So better try to use quality fuel and change the diesel filter in time to avoid getting in this very complicated and expensive situation.

How much does a new diesel high-pressure pump cost?

The prices for a new high-pressure pump can vary between 850 $ and 2000 $, depending on the make and model.

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