Community Member Credit: Frank / motorvate.ca
The knock sensor plays a very important role. It listens to the engine. When it hears engine knock it sends a signal to the ECU. (Engine Control Unit) The ECU normally will delay (or retard) the ignition advance. This stops the engine knock, and decreases engine power.
Engine knock can be caused by using lower octane gas than is recommended by the manufacturer, or by over advanced ignition timing. It can also be caused by a “hot spot” in the combustion chamber. During high engine load, some carbon that has accumulated in the combustion chamber can glow red. This can cause the air-fuel mixture to ignite prematurely. This is PRE-IGNITION, and it is the equivalent of taking a very large hammer and hitting the piston before it rises to the top. (TDC, top dead center) This sends a shock through the engine, and the knock sensor “hears” the knock, and tells the ECU.
For the past year, my ECU would have the code 34, knock sensor failure. I would reset the ECU, drive for 5 minutes and the code would return, BUT the check engine light (CEL) never came on. The car was running great, so I didn’t think it was in need of urgent repair, as I ALWAYS use Sunoco Ultra 94.
Early 4th generation Maxima owners were telling stories of amazing increases HP, as measured by the BUTT DYNO. (yes, it was SAE corrected) I wanted to see for myself.
Knock sensor location. It is not very easy to get at. I was able remove the knock sensor WITH OUT removing the ENTIRE intake manifold. My hands were cut up quite a bit, and it was painful, but I was done is 30 minutes. To remove, simply loosen and remove one bolt. Gently pull the knock sensor harness to remove it from the “V” of the engine.
Close up of the knock sensor.
Close up of the knock sensor. This is the bad one, with just over 235,000 Km on it.
Connect the new knock sensor to the knock sensor harness. This will prevent it slipping out of your bleeding hands, and fall to a spot that you can not reach. The red connector at the knock sensor only has ONE wire, however on the other side of the knock sensor harness there are 2 wires. Everything is fine, the sensor has a braided wire shield that is grounded. If the knock sensor falls to the floor, it can be easily damaged. BE VERY CAREFUL WHEN HANDLING THE KNOCK SENSOR.
Tighten the knock sensor bolt by hand, until the it is snug. I had just enough room to get a wrench on the bolt and tighten it. If you over tighten, the knock sensor will be VERY sensitive, and if you under tighten the knock sensor will not se sensitive enough. The factory spec is 15-20 foot-lbs.
The knock sensor is grounded by these two 10 mm bolts. Remove them, lightly sand the connector on the wires and the flat area on the intake manifold, then tighten the two bolts. This will remove any corrosion.
After a few days of driving, the car pulled a quite a bit harder. It now has more power in the lower RPM range (<3,000) Is this possible?
Yes. When the ECU is first powered on, it tests all sensors. When the knock sensor would fail, then it uses a “safe table” which retards the ignition advance slightly. This prevents the engine from knocking, as the “safe table” has values for low octane gas.
When the ECU started up, and saw a good knock sensor, it took time to optimize the ignition advance. The ignition timing advances very slowly waiting for some slight knock. This is how the ECU figures out the octane you are running. HOWEVER, if knock is detected, the timing is retarded VERY, VERY fast. This reduces the incredible stress of knock on your engine. This process repeats over and over.
If you have a trouble code 34, and your car is 4-5 years old, replace the knock sensor. Don’t forget to clean the ground connection of any corrosion.