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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 01-20-2007, 01:12 AM
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Fulltone74 Fulltone74 is offline
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knock sensor on 540i -- how robust?

Hi all:

How reliable are the knock sensors on the BMW 540i?
At 100K miles, should they be changed?

On my previous car (a nissan), the knock sensors were known to be flaky and/or over-sensitive with age, and yet show no fault codes.

How easy is it to access the knock sensor on a 540i?
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  #2  
Old 01-20-2007, 06:21 AM
franka franka is offline
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I've not heard of anyone replacing them. I'm sure they would throw a CEL if failed or failing since they are an integral part of the motor control system.

I believe they are located in the valley, one against each bank of cylinders. That means the intake manifold and the valley pan would have to come off.
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  #3  
Old 01-20-2007, 10:08 AM
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Fulltone74 Fulltone74 is offline
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Yeah, the knock sensor in my Nissan was also in the valley.

But how would the engine computer actually know that the knock sensor was bad?

When a car really is pinging, will that cause erratic o2 sensor readings? If so, then that would be how the computer could cross reference the knock sensor vs. the O2 to see if the knock sensor was really bad.

But its my understanding that knocking or pinging is the spontaneous ignition of all the fuel, so maybe the O2 sensor wouldn't indicate very rich or very lean in either direction with such a condition. So if that were true, then how would the engine computer have any way of knowing the knock sensor was bad?
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1998 BMW 540i 6-speed (SOLD)
Jet black on tan
Dinan springs, Dinan Koni dampers, Dinan front strut tower bar, M5 rear sway bar,
Dinan Stage-1 software, Dinan clutch, Dinan short throw shifter, CDV delete, Bosch Design III injectors,
Dice I-pod kit,
Hawk ceramic brake pads....

2001 BMW M5
Silverstone on Caramel
All stock.
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  #4  
Old 01-20-2007, 01:20 PM
Hotswimmer Hotswimmer is offline
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Looking at the manual for my Peake code reader, there are fault codes for bad knock sensors - right down to which one of the four is bad.
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Old 01-20-2007, 03:15 PM
franka franka is offline
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Quote:
Originally Posted by Fulltone74 View Post
Yeah, the knock sensor in my Nissan was also in the valley.

But how would the engine computer actually know that the knock sensor was bad?

When a car really is pinging, will that cause erratic o2 sensor readings? If so, then that would be how the computer could cross reference the knock sensor vs. the O2 to see if the knock sensor was really bad.

But its my understanding that knocking or pinging is the spontaneous ignition of all the fuel, so maybe the O2 sensor wouldn't indicate very rich or very lean in either direction with such a condition. So if that were true, then how would the engine computer have any way of knowing the knock sensor was bad?
Pre-ignition and detonation are different but both would cause a knock. In both cases all the fuel/air mixture is burned but not instantaneously. The oxygen sensor would not be able to detect the condition.

Many systems on the car are self diagnosing by sending a test signal to the unit. So even if your car was not knocking but your knock sensor went out the system would detect it and send a CEL.
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