BMW E90 diesel: engine temperature stays low
What is the normal engine temperature of a BMW E90 diesel and why is a low temperature harmful?
The coolant temperature should be about 90 degrees. Below 80 degrees the diesel particulate filter (DPF) cannot clean itself by burning off soot, and it may clog in the longer run. A low temperature also increases consumption.
Which thermostats are replaced when the temperature of an E90 diesel stays below 80 degrees?
The car has two thermostats: the main thermostat and the EGR system's own thermostat. Often the temperature rises to about 90 degrees when both are replaced. Replace the main thermostat first, because the EGR thermostat alone does not always help. A thermostat usually does not give a fault code.
How can you check the condition of the EGR thermostat?
Remove the thermostat and blow into it. If air passes straight through, the thermostat is stuck open. Another way is to clamp the thermostats' hoses shut one at a time with hose clamps and see whether the temperature changes.
Is it normal for the temperature to drop as revs rise?
Yes. Coolant flow increases with revs and more heat is carried away, so the reading drops slightly. This is normal and does not point to faulty thermostats.