[Repair Case] Audi A6 stalls when warming up and has difficulty restarting

Model: Audi A6, equipped with 2.4L engine.

Driving Mileage:31068 miles.

 Issue: According to the driver, the engine is prone to stall failure during the driving process after warming up, and it is difficult to restart the vehicle after stalling.

 Troubleshooting:

There is no failure code when using the Chinese V.A.G1551 diagnostic tool to self-diagnose the engine control system. When starting the engine, it runs smoothly at idle speed in idling conditions. There was no visible aberrant data when it was examined. The problem is considered to be in the engine fuel delivery system, based on the fault characteristic mentioned by the driver, that the car is easy to stall when warming up and does not start after stalling. The performance of the fuel pump under warming-up circumstances is one of the most common causes of this failure. Another explanation is that under warming-up circumstances, the mixture concentration is incorrect. On the basis of this hypothesis, it was decided to verify the engine's fuel injection quantity first. The air flow sensor signal and the engine speed sensor signal determine the engine's basic fuel injection volume, therefore check the air flow data and fuel injection time. The air flow data in idle conditions is 3.5g/s, and the fuel injection time is 2.3ms, both of which are within the standard value range. The air flow sensor and four injectors were changed to entirely eliminate the cause of the failure, yet the failure behavior persisted. Check the fuel pressure; it should be normal, showing that the problem isn't related to the gasoline.

 The coolant temperature sensor data is within the normal range, according to the engine functioning data. Check the oxygen sensor signal and see if the adjustment value of the oxygen sensor has been changing between 0 and 25% (typical range -25 to 25%), indicating that the engine mixture is excessively rich and the oxygen sensor has been adjusted to the lean direction. It has been found that the malfunction is caused by a too rich engine mixture. To see if the problem with the oxygen sensor was causing the mixture to be out of balance, the oxygen sensor was changed, and the test was repeated, but the failure phenomena persisted.

 The fuel evaporation system can change the concentration of the mixed gas, according to the engine electronic control system's operating principle, therefore unplug the canister solenoid valve's harness plug. Feel the intake end of the canister solenoid valve with hands while the engine is idling. The suction force shows that the solenoid valve is always open, allowing gasoline vapor from the canister to enter the engine's intake duct, resulting in an overly rich mixture. Replace the canister solenoid valve and check the oxygen sensor adjustment value. It's perfectly natural. The fault symptoms vanished completely after the test run, and the maintenance work was completed.

 Summary: Observing the functioning data of the oxygen sensor is an excellent technique to estimate the concentration of the mixed gas, as shown in this example's maintenance job. The control concept of the fuel evaporation control system is also vital to understand: while the engine control unit controls the canister solenoid valve to enrich the mixture, it also controls the injector to modify the mixture concentration.

 The engine control unit opens the canister solenoid valve on a regular basis in normal circumstances. The internal air route is generally open when the canister solenoid valve leaks. There is more fuel vapor in the fuel tank when the car is heated. A lot of fuel vapor is sucked into the engine's intake, and the mixture is always at the right place. When the car is hot, the mixture in the enriched state is excessively thick, and the engine stalls.