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Knock Sensor Replacement
 
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Pelican Technical Article:

Knock Sensor Replacement

Jared Fenton

Time:

6 hours6 hrs

Tab:

$100

Talent:

*****

Tools:

5mm hex driver, long needle-nose pliers, torque wrench

Applicable Models:

Porsche 955 Cayenne S (2003-08)
Porsche 955 Cayenne Turbo (2003-08)

Parts Required:

Knock Sensors

Hot Tip:

Use needle-nose pliers to hold the sensors in place.

Performance Gain:

Smoother running engine.

Complementary Modification:

Replace starter motor

The knock sensor is an important part of the car's ignition system. It functions by detecting instances of detonation or "knocking" within the engine and sending a signal to the vehicle's computer that in turn adjusts the ignition timing and fuel injection. The knock sensor is essentially a microphone that "listens" for certain vibrations.

So what is detonation? Detonation is when combustion of the air/fuel mixture in each cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion flame front. The air/fuel mixture is only meant to be ignited by the spark plug and also when the piston is at the right point in the stroke of the engine. Any ignition outside the normal cycle creates the pinging or knocking noise you hear. Over time, the knocking creates increased heat and stress on the pistons, eventually leading to engine damage.

It sounds a bit complicated, but the knock sensors keep all of that under control. They also give you the ability to run a lower octane fuel in a pinch at the cost of engine performance.

Typically, a knock sensor fault will trigger a check engine light and a fault code. Use a diagnostic software package or the Porsche PIWIS tester to read the codes. Unfortunately, replacing the knock sensors requires removing the intake manifold and also draining the cooling system enough to remove one of the coolant manifolds underneath.

It is also important that you disconnect the battery prior to removing the intake manifold. The starter motor sits directly behind the knock sensors, and the electrical connection is always live. Disconnecting the battery at the ground strap location in front of the driver's seat will cut the power. Please see out articles on Intake Manifold Removal and Thermostat Replacement for more information.

When installing a new sensor, take care to align and torque the fastening bolts precisely. Failure to do so could result in a sensor not working correctly. It is also important to align the sensor so that the wiring harness does not come in contact with either of the sensor housings. A stray brush across the sensor by the wire could be read as detonation. Once installed, torque the securing bolt to 23Nm (17 ft/lbs.).

Accessing the knock sensors requires you to remove the intake manifold and drain the coolant enough to remove the upper thermostat housing.
Figure 1

Accessing the knock sensors requires you to remove the intake manifold and drain the coolant enough to remove the upper thermostat housing. Removing the cover without draining will cause a huge mess. Don't forget to bleed the cooling system afterward. See our articles on Intake Manifold Removal, Thermostat Replacement and also Coolant Flush and Replacement. Once the housing cover is free, you can simply hold it with a bungee cord to keep it out of the way.

The two knock sensors are located as shown (green arrows).
Figure 2

The two knock sensors are located as shown (green arrows). The left sensor is directly below the coolant tube.

The right side sensor is easy to replace.
Figure 3

The right side sensor is easy to replace. Just loosen and remove the 5mm hex bolt (green arrow) and the sensor is removed. Pull the tab on the electrical connector (yellow arrow) and pull it off the sensor. Align the new sensor at roughly the same angle as the old one, making sure the wiring harness does not contact the sensor housing.

Shown here is the knock sensor on the left side of the engine under the coolant tube.
Figure 4

Shown here is the knock sensor on the left side of the engine under the coolant tube. This one is a bit trickier to replace. Leave the electrical connector plugged in while removing and installing the sensor. Be sure to align the sensor at roughly the same angle as before. holding it with long needle-nose pliers helps while you torque the bolt.

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Comments and Suggestions:
Kenny Comments: I just replaced the bank 1 knock sensor and the hex bit required for removal was 6 mm not 5 mm as shared here. Mine is a 2004 Cayenne Turbo.
November 8, 2016
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
Steven Comments: Hi,
A step by step on a V6 would be great.

Is there a trick to access the left hand knock sensor on the V6. The manual says hard, I would suggest impossible

Thanks
August 12, 2016
  Followup from the Pelican Staff: We don't currently have that tech article. If we get a chance to perform the procedure, we will be sure to document it.

I would grab a repair manual. It will have the procedure, special tools and torque specs.

Give our parts specialists a call at 1-888-280-7799. - Nick at Pelican Parts
 
DLF Comments: My Cayenne Turbo 2007, 957 has 4 knock sensors. I need to replace "bank 2 , sensor 2" , which one is it ?

The attached pic shows more or less where they are.
September 9, 2015
  Followup from the Pelican Staff: left side closest to firewall. I would suggest replacing all four while you have it apart. - Nick at Pelican Parts  
Matt Comments: Pretty sure you don't have to open the coolant system at all, I just did the job in two hours, just saying.
July 15, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 

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