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Knock Sensor Replacement
 
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Pelican Technical Article:

Knock Sensor Replacement

Nick Czerula

Time:

4 hours4 hrs

Tab:

$300

Talent:

*****

Tools:

Set of sockets (13mm), flathead screwdriver

Applicable Models:

BMW X3 Sport Utility (2004-06)

Parts Required:

Knock sensors, knock sensor fasteners, intake manifold gasket set

Hot Tip:

Work with a cool engine

Performance Gain:

Remedy knock sensor fault codes

Complementary Modification:

Replace breather hoses

Engine knock or detonation in spark-ignition internal combustion engines occurs when combustion of the air and fuel in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air explode outside the area of the normal combustion front, causing a shock wave and a collision, resulting in the knocking sound. The shock wave creates the characteristic metallic pinging sound, and cylinder pressure increases dramatically. Effects of engine knocking range from non-damaging to completely destructive.

Pre-ignition in an engine is the event wherein the air / fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark plug, such as hot spots in the combustion chamber, a spark plug that runs too hot, or carbon deposits in the combustion chamber.

BMW X3 M54 6-cylinder models utilize two sensors to monitor engine knock. The knock sensors are mounted to the engine block, beneath the intake manifold. The knock sensors failure will set a check engine light and a knock sensor fault code. When a knock sensor fails, the ignition timing on your engine may be held in a retarded position until the fault is remedied, therefore reducing engine power and the chance of detonation. The sensors are replaced in pairs and you should also replace the fasteners. In this tech article I will go over how to replace the engine knock sensors on your BMW X3 with an M54 6-cylinder engine.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Testing knock sensors: To test the sensors, you will have to access the DME electrical connector. See our tech article on DME replacing. Working at the electrical connector, you will backprobe the connector with the ignition ON. There are four wires at the connector, two for each sensor. One is a ground, the other is a reference voltage used for circuit integrity. To test the bank 1 knock sensor, backprobe Pin 29 at the DME, yellow wire. To test the bank 2 knock sensor, backprobe pin 31 at the DME, green wire. Double check the wiring for your vehicle, this is what it was for my subject vehicle. Here's the quick test, use a DVOM, connect the black lead to battery negative, then connect the positive lead to the sensor wires. One wire should read close to) volts, this is the sensor ground. The reference voltage wire should read around 2.5 volts, for a good sensor. A bad sensor will either short the reference voltage to ground, or not pull it down to 2.5, therefore giving a reading of about 5 volts.

BMW X3 M54 6-cylinder models utilize two sensors to monitor engine knock.
Figure 1

BMW X3 M54 6-cylinder models utilize two sensors to monitor engine knock. The knock sensors are mounted to the engine block, beneath the intake manifold (red arrows), intake manifold shown removed.

Disconnect negative (-) battery cable.
Figure 2

Disconnect negative (-) battery cable. See our tech article on Battery connection notes. Remove engine covers. See our tech article on removing engine covers. Remove the intake manifold. See our tech article on intake manifold removing. Follow the knock sensor wiring harness to the single junction connector. Unclip the tabs (red arrow) and disconnect the connector.

Follow the knock sensor harnesses to the coolant pipe and cut the tie straps (red arrow).
Figure 3

Follow the knock sensor harnesses to the coolant pipe and cut the tie straps (red arrow). Bank 2 sensor tie strap shown, bank 1 is similar.

Working at the knock sensor, remove the 13mm knock sensor fastener (red arrow).
Figure 4

Working at the knock sensor, remove the 13mm knock sensor fastener (red arrow). You will have to squeeze the socket in past the coolant pipe. Bank 1 shown, bank 2 is similar. Remove bank 2 fastener.

Remove the knock sensor fastener.
Figure 5

Remove the knock sensor fastener. Be sure to use a new fastener if threads are not free of corrosion (red arrow).

Then remove the knock sensors (red arrow) from your engine.
Figure 6

Then remove the knock sensors (red arrow) from your engine. Be sure the mounting surface is clean (blue arrows), then install new knock sensors finger tight. Then torque the knock sensor fasteners. Reassemble engine and check DME for fault codes.

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Page last updated: Wed 5/24/2017 03:18:49 AM