Passat collar fault code booster Throttle connection pressure drop

Faulty vehicle information:

Model: Passat Pilot

Model year: 2007

Configuration: 1.8T automatic gear

Mileage: 180,000 km

Symptom: A Shanghai Volkswagen Passat sedan with a turbocharged engine. Users reported that the car’s engine was running unsteadily at idle speed.

Fault diagnosis: The fault of the car has been checked by another maintenance personnel, the author first communicated with the maintenance personnel, learned that the EPC light on the instrument cluster of the car is on during normal driving, and the engine is also idling unstable. At that time, the maintenance personnel did the following inspections and repairs: check that the fuel pressure at idle speed was 400 kPa; Check the intake hose and see that there is no air leakage; Replace the air mass meter. The above repairs failed to troubleshoot.

The author started the engine and found that the EPC light on the instrument cluster did not light up, and the engine idled unstable. Next, the fault code in the engine control unit was checked using a fault diagnostic, and the fault code 17705 was found to be there, which means “supercharger – throttle connection pressure drop (static)”. Recall that users have reported that when there is a failure, the EPC light is on, so why is there no fault code about the throttle in the engine control unit? The maintenance personnel who asked the maintenance of the vehicle learned that after the fault was queried, there was indeed a fault code about the throttle, and after the fault code was cleared, the throttle was basically set, and there was no fault code, but after starting the engine, the fault code 17705 appeared.

Based on the previous experience of repairing similar faults of this model, the author has a preliminary analysis that the cause of this failure may be the disengagement of the charge air hose. After lifting the vehicle, remove the engine guard, and check that the supercharger hose is connected normally and there is no disconnection or breakage. Use the troubleshooter again to enter the engine control unit and check the data when the engine is idling as follows.

Display group 001 data: engine speed is 720~840 r/min, engine coolant temperature is 93 °C, short-term fuel correction value is 25%.

Display the data of group 002: the engine speed is 720~840 r/min, the engine load is 32%, the injection pulse width is 4.6 ms, and the air flow is 8.3 g/s.

Display the data of group 003: engine speed is 720~840 r/min, air flow is 8.3 g/s, and throttle opening is 2.0%~4.3%.

Display groups 015 and 016 are the data of engine “misfire”, all cylinders have “misfire” phenomenon, and the number of “misfires” of each cylinder is more than 40 times. Due to the unstable idle speed of the engine, the data read is also unstable, and the above data is close to the data when the engine is running, and the difference is not much.

Through the short-term fuel correction in the above data, it can be seen that the engine mixture is too thin, and the cause of the “misfire” in all cylinders may also be too thin mixture. All hoses connected to the intake manifold were inspected for no air leaks. When the engine is idling, the hoses on the intake manifold are clamped respectively, and the phenomenon of unstable engine operation does not show any improvement, indicating that the cause of the mixture is not caused by air leakage in the hose connected to the intake manifold.

The author analyzed the data of the engine again and found that the data of the air flow meter did not match the data of the throttle, and the throttle opening was 2.0%~4.3% when the idle speed was unstable, while the air flow at that time was 8.3 g/s. What is the relationship between the above two data and fault code 17705? The meaning of fault code 17705 is the supercharger – throttle connection pressure drop, since the fault code mentions the pressure drop, the so-called pressure drop, refers to the boost pressure.

The control unit obtains the boost pressure value by means of a boost pressure sensor, which is installed in the upper part of the charge air cooler. If the air flow meter detects an increase in the intake air volume, and the pressure detected by the boost pressure sensor does not increase with the intake volume, the control unit considers that the pressure has dropped and stores the fault code 17705. Considering that after unplugging the plug-in of the booster pressure sensor, the control unit will be replaced by a fixed value of 140 kPa, so the author cleared the fault code, unplugged the plug-in and started the engine, and found that the engine speed was higher than the idle speed at this time, and the unstable operation phenomenon was significantly improved, and the fault code was read and found that 17705 did not appear. This indicates that the control unit does detect a low boost air pressure, what causes the charge pressure to be too low?

Recall that the maintenance personnel who previously repaired the vehicle reported that the fuel pressure of the car at idle was 400 kPa, and the oil pressure at idle was 350 kPa, and the vacuum tube on the oil pressure regulator was unplugged, and the oil pressure became 400 kPa. The previous oil pressure of the car was 400 kPa, indicating that the pressure in the intake manifold was too high and the vacuum was reduced. The cause of this failure may be a damaged throttle that allows too much charge air to enter the intake manifold. After removing the intake hose on the throttle, the author pushed the throttle by hand and found that the gap was too large, and the gap referred to here is the gap between the gears inside the throttle body. After removing the throttle, the cover with sliding contacts was removed, and a large amount of oil was found inside, and a careful inspection revealed that the internal gears were damaged. So far, the cause of the fault has been identified.

Troubleshooting: Replace the throttle valve and rematch, idle smoothly after starting the engine, and the fault is completely eliminated.

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