Audi Q7 engine knock Troubleshooting


a 2007 Audi Q7 3.6FSI annual output of cars, mileage issued a sharp sound of metal friction to 6 000 km, the user reflect the car near the engine can be heard occasionally in the process of moving, the phenomenon lasted about 1 ~ 5 min disappears; If a failure occurs turn off the engine and then start the vehicle again, sometimes the fault disappears, the fault occurred every 2 to 3 days 1 times.

by asking the user, that the car failure phenomenon occurs mainly in hot car, the engine idling, the small-load operation will occur, but there is no relationship between the vehicle speed and travel time, the instrument panel nor any fault warning lights. Since the failure phenomenon occurs 2 to 3 days 1, and was relatively new, easy to capture every fault symptom. In consultation with the user, the user agrees to leave the car for repairs. After a long test about one day, the fault finally appeared. I heard one kind of sharp metal friction is similar to the noise. By resolution, the author determines that abnormal sound occurs in parts of the upper region of the engine head, but this time the engine in normal operation, and sound volume and the frequency of abnormal sound does not change with changes in engine speed, thereby preclude wear of mechanical parts or interfere with each other possibilities. At this time there is no inspection by VAS5052 DTCs found high pressure oil rail pressure has reached the idling 12 MPa when seen in the data stream, which is clearly not normal. It may be determined after checking audible noise portion is again the high-pressure fuel rail noise caused by high hydraulic overpressure relief valve opening noise generated.

So why will produce high pressure phenomenon? This requires analysis from the control principle FSI engine fuel supply system.


The new FSI engine fuel supply system consists of a low pressure, the high pressure of two components (Fig. 1), the two portions each corresponding function is completed . The system is composed of a low-pressure demand regulated fuel pump to the supply, demand of such adjustment means may reduce the power consumed by the fuel pump, thereby saving fuel. Fuel pump supply only the amount of fuel required for the engine, the fuel quantity is obtained under the provisions of the system pressure. This process is accomplished by an engine electronic control unit and a power control device, the electronic power control means regulates the fuel pump speed by pulse width modulation. In the low pressure system, The electric fuel pump about 0.6 MPa by the fuel filter filters the fuel supply to the high-pressure oil pump and return the high pressure pump directly into the fuel tank. In high pressure systems, a single high-pressure piston pump for about 4 ~ 11 MPa (depending on load and speed) of the fuel into the fuel distribution pipe, and then the dispensing tube is assigned to four high-pressure fuel injectors. Acting pressure limiting valve protection member is under high pressure conditions, and it opens when the system relief pressure higher than 12 MPa, the outflow fuel feed pipe enters the high pressure pump pressure relief valve opens. High-pressure pump is a single piston type pump, mechanically driven by a double cam of the camshaft. A high pressure fuel supply pump is adjustable, electric fuel pump to supply high pressure pump up to a pre-pressure of 0.6 MPa, and then high-pressure pump to generate high pressure within the fuel supply rail required.

FIG. 2

Figure 3

Here again specific analysis principle of high-pressure pump . First, when the piston moves downward, a pressure of about 0.6 MPa of the fuel flowing into the pump chamber (FIG. 2) from the low pressure pump in the tank through the inlet valve. When the piston moves upward, the fuel is compressed, and when the pressure exceeds the oil rail pressure, fuel is fed to the fuel distribution pipe (FIG. 3). Fuel metering valve (controllable valve) located between the pump chamber and the fuel inlet (FIG. 4). If the fuel metering valve is opened before the end of the supply stroke, the pressure will be relieved of the pump chamber, the fuel oil to flow back into the mouth. Between the pump chamber and the fuel distribution pipe is provided with a check valve which prevents return flow of fuel during the fuel metering valve is opened. In order to adjust fuel supply, fuel metering valve from the oil pump cam point between a travel stop is closed. When the pressure in the fuel rail to achieve the desired fuel metering valve is opened, thus preventing the oil rail pressure continues to rise. For safety reasons, the fuel metering valve (MSV) N290 is a solenoid valve (FIG. 5) is opened when not energized, i.e., all the high-pressure fuel pump is supplied through the open valve is pumped back into the low-pressure line. Fields are generated by the coil is energized, the armature together with the valve needle on the valve seat is pressed into. When the oil reaches the required rail pressure, fuel metering valve power supply is cut off, the electromagnetic field dissipates, the pressure oil from the valve needle pressure chamber the pump, so the excess fuel in the pump chamber to flow back into the low pressure line.

Figure 4

Figure 5

The high-pressure fuel system works can be seen, because the rail pressure is too high can rule out the possibility of damage to the high pressure pump, high pressure pump should adjust the control device there is a problem. In the operation of the fuel metering valve may be utilized oscilloscope fault diagnostic function, checking the observed signal waveform. So I connected fault diagnostic apparatus, a control signal using the oscilloscope function of the fuel metering valve N290 observed, and found that the signal is always at high level, which is determined to verify the previous author. There cause abnormal fuel metering valve operating two: First, the engine control unit generates an error control signal, and second fuel metering valve N290 line emergence of the phenomenon of short circuit of the positive electrode. According to the analysis of ideas, the author first line of the fuel metering valve N290 were carefully examined, but did not find problems on the positive line short circuit. It seems that the problem should be receiving an error from the engine control unit in which the control signal.

The engine control unit then why would it send the wrong signal to control the fuel metering valve? This requires how to generate a control signal to the engine control unit, and what issues will affect the output signal of understanding. In the vehicle fuel control system, an engine control signal to determine the fuel metering valve associated with the load signal, the intake air amount and the throttle signal and the like according to the state signal, it is necessary to check the signal. Observation by using a scanner fault dynamic data flow engine control system, the signal can be determined are normal. Effects Since the engine control unit controls the fuel metering valve output of the input signal are normal, then the need to check the engine control unit. Based on past experience in the maintenance, in addition to human reason, the general case where the engine control unit is less damaged, most of the problems on its power supply, grounding line.

Figure 6

As we prepared for the power supply and the grounding of the engine control unit checks, fault has disappeared. The engine data are normal, the high-pressure also returned to normal. Faced with such an embarrassing situation refused to concede defeat, but by the previous inspectionTroubleshooting has narrowed a lot, so I re-organize your thoughts, ready to continue checking. After the power supply lines and grounding of the engine control unit to check found no problems, so there is a problem author determines the engine control unit. We decided to models with trouble-free replacement of the engine control unit, allowing users to continue to observe and use. A few days later, you call the rescue call, said car battery power loss can not be started. Open back to the factory after the maintenance staff will jump start the car, then the engine appeared the original malfunction. But it found during the inspection of a new fault phenomenon, ie, the engine can not be started after flameout, similar to the phenomenon of electricity without the battery, but this time the battery is fully charged. It seems that the car’s power cord out of the question? So I decided to gradually inspection and the connection points of the wiring harness from the battery. When checking the positive electrode located in the engine compartment on the left side of the pile head jumper (FIG. 6), it is found to have significant loosen ablation (FIG. 7) at the connection with the pile head harness. The position of the main battery positive fraction is branched to each connection point, the connection point of loosening ablation, among such branch harness produced phenomenon of poor contact, the point of failure is the failure of the power supply leads to the engine control unit, whereby there has been an error of the control signal of the fuel metering valve N290.

Figure 7

After the processing of the point of failure, troubleshooting test.

Summary: Symptom car’s engine control unit after all, or power lines caused by bad, since the fault belongs to the occasional failure, check up that there are some problems, but also take some detours. By excluding car failure, remind us first of all of the relevant power line Ground detailed and thorough examination failures should check the control unit and the like, do not rush to replace the corresponding control unit.


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