A case of high engine temperature failure in a BMW 730Li sedan

Failure phenomenon: A BMW E65 730Li sedan produced in 2003, equipped with an N52 engine. According to the user, the car was started in the morning. After the coolant temperature rose, the heater was turned on, but no hot air was blown out. After a while, a high temperature alarm appeared, and the multimedia I-drive display screen showed a prompt of “engine temperature is too high, stop running the engine”.

Inspection and analysis: Open the engine hood, you can see that the engine temperature is indeed very high, and the coolant is spraying out from the water filling port. Touch the upper and lower water pipes with your hands, and you feel that the upper water pipe is hot and the lower water pipe is very cold, as if the thermostat is not open. Connect the fault diagnosis instrument, read the engine system fault code, the system stores “2E84 electric cooling fan communication failure”, read the engine coolant temperature as 103 ℃, and read the temperature of the lower water pipe as 63 ℃, it can be seen that the temperature difference between the upper and lower water pipes is very large. According to maintenance experience, the reason for this large temperature difference between the upper and lower water pipes is that the coolant is not circulating, including radiator blockage and thermostat not opening. After clearing the fault code and testing the car, the fault did not occur again after a long time. However, on the way back to the factory, the warm air was gone again. A few minutes later, the water temperature alarm sounded. The fault code was read immediately, and it was still 2E84.

Using the actuator diagnosis function of the fault diagnosis instrument to drive the electric cooling fan, the diagnosis instrument prompts that the cooling fan works for 30 seconds. It can be seen that the cooling fan works steplessly from low speed to high speed for 30 seconds, confirming that the cooling fan has no fault, but the fault code cannot be cleared. After carefully analyzing the fault phenomenon, the cooling fan failure will not cause such a large temperature difference between the upper and lower water pipes. It should still be a problem with the coolant circulation. According to the information, the cooling system of this car is very different from that of previous models and is much more complicated. The BMW E65 730Li sedan is equipped with an engine thermal management system, which is mainly composed of electric water pump, electronic thermostat and stepless speed regulating cooling fan. It is already a cooling system driven entirely by electricity. The working speed of the electric water pump, the opening of the electronic thermostat and the speed of the cooling fan are controlled by the engine DME control unit. The electronic water pump, the electronic fan and the engine control unit are transmitted via serial data lines. The engine control unit calculates the engine operating temperature by integrating various data, and then transmits it to the electronic water pump and the electronic fan through the data line to keep the engine at the ideal temperature value.

The engine control unit has the following temperature nodes when adjusting the engine temperature: 112 ℃ – economic, 105 ℃ – standard, 95 ℃ – advanced, 80 ℃ – advanced and adjusted through the characteristic line thermostat. If the engine control unit recognizes that the operating range can be more “economical” based on the driving performance, the DME will adjust to a higher temperature (112 ℃), and the engine runs with relatively low fuel demand in this temperature range. I used the diagnostic function of the fault diagnosis instrument to drive the electronic water pump, and found that the water pump did not rotate at all. I used a test light to measure the electronic water pump fuse on the engine fuse bracket in the relay box on the right side of the engine compartment. There was power supply and the test light could light up normally. I unplugged the fuse and checked the fuse holder and found no poor contact. I replugged the fuse and drove the water pump again. This time the water pump could operate normally (the auxiliary water tank has a small water pipe connected to the electronic water pump. When the electronic water pump is running, this water pipe pumps water to the auxiliary water tank). It seems that the electronic water pump occasionally gets stuck, so it sometimes works and sometimes doesn’t. But why didn’t the diagnostic instrument find the fault code of the electronic water pump? So I tried to unplug the plug of the electronic fan and start the engine. I could see that the engine system had an additional fault code 2EFE (electric fan). It turned out to be a problem with the diagnostic instrument. After consulting with BMW authorized repair station, we learned that fault code 2E84 means “electronic water pump communication”, 2EF5 means “cooling control according to characteristic curve (electronic thermostat)”, 2E81 means “electronic water pump speed difference”, and 2E82 means “electronic water pump overcurrent shutdown”. The electronic water pump has a self-checking function, which can diagnose faults including speed difference, stagnation (inflexibility or jamming caused by foreign matter, etc.), incorrect coolant and water mixing ratio, air in the cooling system, and external starting using a 24 V battery.

Electric water pump

Troubleshooting: So the electronic water pump was replaced, and follow-up visits were made to confirm that the fault was eliminated. It should be noted that the operation of exhausting air from the cooling system of this vehicle needs to be driven by the actuator of the diagnostic instrument to make the electronic water pump work to exhaust air.

Review and summary: Since this vehicle is equipped with a more advanced thermal management system, fault diagnosis is essentially different from the overhaul of the transmission cooling system. To repair such faults, you must understand the basic working principles before you can start repairing them. Repairing luxury cars also requires the use of original instruments. If the original fault diagnosis instrument GT1 is used in the repair of this fault, then the fault should be solved relatively easily.