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How to check the pressure in the common rail?

01.12.2022

There are various methods for measuring fuel pressure in a common rail system. In the following text, we will take a look at the available measurement techniques as well as the basic methods of fuel system diagnostics.

Operation of the high pressure pump

At the outset, it is worth recalling the basics of the reservoir system, as understanding these mechanisms greatly facilitates diagnosis. The idea of the system is based on a high-pressure pump delivering fuel to a reservoir (called a rail or slat), from which it goes to the injectors. This ensures that the injectors operate at a fixed pressure, which is set by the engine controller (ECU) depending on the operating conditions. This allows more precise control of the fuel combustion process.

Fuel pressure regulator

To make this possible, a fuel pressure regulator is required. The name is usually used to refer to a control valve (PRV) that bleeds fuel from the storage tank (sometimes valves are mounted on the pump) to limit its growth. Actually, the control function is performed by the controller’s algorithm, which controls the valve (sets the PWM filling factor) based on the measured fuel pressure in the injection rail.

Discharge control valves

Additional valves are also used on the pump to regulate the pump output, allowing the engine controller to adjust the output over a wide operating range. This reduces the consumption of power and the high-pressure pump itself. Normally open or normally closed valves are used – the former allow maximum fuel flow in the absence of valve control. Such a pump is capable of reaching a pressure of 700 bar during a start-up test with the valve and injectors disconnected. Knowledge of the type of valve used is essential for more accurate diagnosis.

How to detect fuel pressure problems?

Since the pressure in common rail systems is constantly changing, the key information is to compare the actual pressure in the rail with the setpoint. This is usually done by reading the operating parameters from a diagnostic tester. This method will work best if the engine starts and runs stably at least under certain conditions.

By examining the pressure, we observe the keeping up of the actual value with the setpoint in the operating range from slow speed up to full power operation. An efficient system is capable of maintaining the required pressure over the full range of operation.

Pay attention to when symptoms occur

Important are the extreme points – under idling conditions we have the minimum fuel flow – here it will be easiest to occur too high pressure due to, for example, a clogged control valve. High pressure, on the other hand, is necessary for high load, which requires both a functioning pump, control valve and injectors.

Many people too hastily link pressure problems to the condition of the high-pressure pump. It is important to remember that under typical conditions the capacity of the pump should be much higher than the actual demand. Since the pressure value is regulated by the solenoid valve, it will be the same under the given conditions with a new pump as with a partially worn pump. Only when there is a significant infirmity will the pressure generated be too low.

Start-up problems

In the case of start-up problems, measurement by on-board diagnostics is not always satisfactory. Sometimes the offered sampling rate (the number of measurements per unit of time) does not allow you to know exactly what is going on or there is a delay in data transmission. In this case, you can either measure the signal from the sensor on the injection rail with an independent meter or oscilloscope, or attach an external pressure gauge to the system (connecting it instead of one of the injectors) and measure the pressure independently.

The undoubted advantage of the pressure gauge is the ability to detect a fault in the sensor itself, which is not possible without an independent measurement. The disadvantages are the relatively slow operation and the need to physically observe the pointer during the start-up test. It can be assumed that the minimum pressure is 180 to 250 bar, depending on the generation of the system (newer systems require higher pressures).

Symptoms while driving

Problems with fuel pressure while driving are most often manifested by a lack of power after adding gas or unevenness in operation, described as engine jerking. Keep in mind that the cause of similar abnormalities can be many and not necessarily related to the pressure on the injection rail. An interesting possibility is the EDIA-PRO tester, which allows to test the engine during operation. The system uses an oscilloscope method to test the system, allowing in many cases to distinguish between a malfunction of the injection pump and the injectors themselves.

Dedicated tester

The RPT-5 device is an easy-to-use tester for independent pressure measurements in the common rail system. Thanks to its independence from on-board diagnostics, it can be used in any vehicle equipped with a tank system (including gasoline ones), but also in agricultural, construction machinery or aggregates. Connection to the sensor is very simple – using either an adapter or needle probes. The tester does not require knowledge of the connection layout. When you turn on the ignition, it will show the layout of the leads and start measuring both the sensor signal and the supply voltage from the controller.

Samochód - Pojazd silnikowy

What about in the case of a defective sensor? The tester offers the unique functionality of using a typical, working pressure sensor as a reference sensor. The additional sensor is powered from the tester. After attaching such a sensor to the system (e.g., using a blinded common rail), we gain the ability to compare the readings of both sensors at once. This will avoid unnecessary replacement of the sensor.

The device can simply display values on the display, but it can also record signal waveforms from the sensor. The included Android app allows them to be conveniently read at a higher resolution on the screen of a mobile device.

Causes of too low pressure

The most common problem found is that fuel pressure is too low. Below we will present an example of how to deal with such a case.

  1. Check the fuel supply system, starting with the tank itself. Inspect the fuel filter, pre-pump and the condition of the fuel lines. A clogged filter is a common and easy cause of problems. Check that the pre-pump works properly (in some systems it works only on startup) and applies the required pressure to the CR pump. Leaks can cause the system to air up.
  2. Check the injector overflows. Damaged injectors that generate a very large overflow can even prevent the system from generating sufficiently high pressure. Evaluation of injector overflow is simple and does not require expensive equipment.
  3. Check the rail pressure control solenoid valve. Before checking the pump itself, it is a good idea to evaluate the operation of the valve. It may be that the engine does not start because the pressure is too low, and the valve at that time lets the fuel overflow.
  4. Check the valve circuits of the pump and the fuel pressure sensor.
  5. As a last resort, we decide to check the high-pressure pump by testing it on a test table.

Causes of too high pressure

Although this type of problem is less common, excessively high blood pressure is also sometimes a problem. It is good to detect the fault in time because excessive pressure can lead to accelerated wear or damage to injection system components.

  1. The most common cause of the problem is the fuel pressure control valve. Sometimes contamination leads to pinching of the valve, which leads to an excessive increase in pressure. A jammed control valve can also manifest itself in pressure ripples during operation.
  2. Check the fuel pressure sensor. If there are problems with the sensor or its circuit, an error signaling high pressure may be reported.

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