BMW 740i xDrive Fuel Temperature Sensor Replacement Cost

With the flexibility of computerized control systems for modern vehicles, manufacturers have nearly limitless control over engine operation and a more broad ability to tune the operation of the engine's accessory systems in order to maximize the fuel that is used during combustion. One way of maximizing the efficiency of the engine is to measure the amount of fuel used per combustion event, but taking it a step further requires that the temperature of the fuel is known. The fuel temperature sensor will read the temperature of the fuel going into the engine to determine exactly how much should be used. Let's look at how this is done.

To understand the purpose and function of the fuel temperature sensor, it is essential to understand the chemical properties of oxygen and fuel, as well as the laws of thermodynamics that play on all substances, including fuel and oxygen. Fuel and oxygen can only burn efficiently in exact amounts. This is known as the stoichiometric ratio, and it is basically a measurement of how much oxygen must be used to completely burn a given amount of fuel inside the engine. However, the amount of fuel and oxygen cannot be measured only by volume for high accuracy, but they must be measured by volume while accounting for heat expansion. Since the temperature of the fuel and air is known, the computer is not just calculating the air to fuel ratio based on volume, it can now calculate the air to fuel ratio on a molecular level, though this is would never be expressed. Plainly put: the fuel temperature sensor will allow the engine management computer to use precise and accurate measurements to ensure the most efficient burn of fuel possible.

Since the fuel temperature sensor is more of a component to maximize efficiency, the computer noticing the sensor is providing erroneous data can have one of two effects. First, the engine control module (ECM) will likely prevent the fuel temperature sensor from providing input, alert the driver with the check engine light, begin using more fuel to prevent a lean running condition, and the exhaust may smell strongly of gasoline while idling. If the fuel pressure sensor is reporting incorrect temperatures, but the temperature reported is within the specified range, the engine may begin to run rich or lean. In either case, the oxygen sensors will detect the rich or lean conditions, and the ECM will begin to run in a closed loop. This means that the fuel to air ratio will not be determined by the oxygen sensors, and this will certainly impact fuel economy and engine power. The check engine light will still illuminate as well.

Since failure of the fuel temperature sensor ultimately leads to decreased fuel efficiency and overworking the catalytic converter, it is best to have this repaired as soon as possible. It is unlikely that this sensor will cause the vehicle any substantial issues, however, the cost of wasted fuel, additional pollution, and risk to the catalytic converter all warrant having this corrected sooner than later. Like any other time the check engine light comes on, this issue should not be ignored.

A fuel temperature sensor is designed to last the lifetime of the vehicle. Unfortunately, they do occasionally go bad. Still, they are not a maintenance item and do not have a regular replacement interval. Like all electronic components, they should have a lengthy service life, but failure is also unpredictable.

When diagnosing the check engine light, the technician will need to connect a diagnostic scanner to the vehicle, read the trouble codes, and use that information coupled with classroom training and experience to identify the cause of failure. In some situations, the fuel temperature sensor will not be suspected except for the understanding that ambient temperature and fuel temperature readings are too far removed from one another. Once the fuel temperature sensor is suspected, the fuel temperature sensor readings and actual fuel temperature will be compared to note any significant differences.

Since the location of the fuel temperature sensor will be different for each make and model that uses this type of engine control sensor, the replacement procedure will vary, sometimes greatly. This may involve removing the sensor from the fuel tank, fuel rail, or fuel lines, so removal and installation may be a very quick and easy task, or it may be daunting.

If the intake manifold needs to be removed in order to access the fuel temperature sensor, the intake manifold gasket(s) should be replaced before the part is reinstalled. Proper procedure for removing, cleaning, reinstalling, and tightening is mandatory for the intake manifold to prevent immediate failure.

Do not be fooled into replacing the oxygen sensors or catalytic converter. The trouble codes from failure of the fuel temperature sensor will certainly read that the oxygen sensors have noticed a fault in the air to fuel ratio, but that does not mean they have failed.

Diagnosing the fuel temperature sensor can be very difficult. It is best done by a professional with the appropriate diagnostic scan tools, and the cost of these tools is likely to outweigh the benefit of purchasing them for this repair. If the sensor can be diagnosed, replacing the sensor can be done by the experienced DIY mechanic, but if you are unfamiliar with safety practices associated with servicing the fuel system, leave this job to a professional.