Knock Knock

[andreus009]

Ok...done a little searching but haven't found what I'm looking for. Let me describe the problem first.

 

I can drive around town all day long and have no problems. However, if I go out on the highway and get the engine fully heated up then the problem will come into play. If I'm going along at constant speed, let off of the gas, and then go back on the gas then the engine will sometimes start to miss. Sometimes it will recover on its own eventually, or I'll have to down shift, rev up and then up shift again. During the engine missing the Check Engine light comes on. I just pulled the codes today and it comes up 33 and 34. The 33 is the O2 sensor which is probably being tripped because of raw gas going through while it is missing. The 34 is the infamous Knock Sensor. So does anyone know what happens when the knock sensor goes bad? Will it cause the engine to miss or lose lots of power???

 

From what I've read so far the knock sensor signal is used by the ECU to control the ignition timing advance. Basically as I understand it, by using this signal the ECU advances the timing just to the point of knocking and then backs off a bit. So I could see how if this signal was bad, then it might affect the performance.

[Precise1]

I have not had the issue myself so I don't know it intimately, but yes, what you describe the knock sensor doing is correct.

 

MaritimeMan replaced his knock sensor and put it on the bell housing instead of digging it out of the valley. Seemed to work just fine...

There have been a few others with code 34 but I can't quote from them. I do recall the knock sensor code being a symptom, not the cause in some cases.

 

The O2 might be part of the issue or a symptom also. For example, my wife's R50 had an intermittently failing injector (when it was hot ) that would cause it to stumble, and throw codes. Codes for the injector and the O2 sensor after the cat on that side. Replaced the injector and the O2 code went away...

 

Check your O2 sensor resistance, hot and cold; that is how I verified the injector problem. The info is pinned in the Garage IIRC (or WD21 section?) or is certainly in the FSM.

 

B

[MY1PATH]

From what I've read so far the knock sensor signal is used by the ECU to control the ignition timing advance. Basically as I understand it, by using this signal the ECU advances the timing just to the point of knocking and then backs off a bit. So I could see how if this signal was bad, then it might affect the performance.

 

You may have misundersood what you have read. A knock is when the fuel combusts too quickly (pre-detonation) and pushes down on the piston coming up. This litterally makes a knock sound and is very harsh on the internals and not ecu will intentionally take the engine up to that point and dial it back when it hits it. When the engine knocks it sends a shockwave thru the block which is picked up by the knock sensor. If you had one hooked up in your hand and you sammed it against something hard you could set it off.

 

On the 87-95 ecu when the knock sensor is triggered it retards the timing by 5 degrees.

• If the sensor has failed ie, triggered by the jolt of re-accelerating, other smaller vibrations or just shorting inside your timing will retard momentarily and you will feel miss or other loss of power as the fuel chases the piston on its way down instead of helping to push it down and less of it is burned before the valve opens so either way it can give your o2 a bad reading. You will get a knock code becase ecu retarded the timing and still felt the vibration. so you change (or unplug as some ppl do...)the sensor clear the codes and all is well.
• If the sensor is working properly and you get a knock you may or may not hear it knock once and will notice almost nothing else and you engine will live longer.

I think your sensor is bad because thats what usually happens. Hell some reman VG's don't even have a knock sensor and really N/A VG's are not prone to knocking.

I don't know if the knock sensor has a pigtai harness that can be unpulggedfrom the top side of the engine (I'd assume it would) but if you can find what wire it is you can unplug it and see if your problem goes away. If you get an audiable knock instead pull over ASAP! and plug it back in.

Edited October 31, 2010 by MY1PATH

[MY1PATH]

Things that cause a knock also known as pre-detonation. (things to check if unpugging or changing doesn't solve your problem)

 

• High compression. Exessive cabon deposits inside the head on on the piston can raise the compression and fuel burns quicker under higer compression so less timing is needed.
• Too Low octane, doesn't apply to our rigs since we run the lowest octane anyway. Octane is resistance to detonation and not a measurement of power.
• Wong spark timing. if your spark timing is too far advanced you fuel will try to burn too early
• wrong valve timing. If your intake shuts sooner cyl preasure will become higher. In rare cases this can cause a knock.
• Over heating. Heat! if the cyl is too hot the fuel will burn faster OR it will ignite on its own.
• wrong spark plug(mostly heat range) If your plug retains too much heat from the previous combustion your fuel will ignite upon touching it
• Debris and deposits. If debis or deposits in your cyl are allowed to burn or smolder they will ignite fuel upon contact.

[JamesRich]

I had a knock sensor go bad on my wife's 01 quest van, not a pathy but still a Nissan. The quest has the OBDII computer system so that might make a difference, but the only problem it gave me was crappy fuel mileage. The little van usually averaged 17 mpg and with the knock sensor code it was averaging 14. It ran fine as far as smooth running and the power seemed the same, a slow turd. And yes I tore all the top end of the motor off and changed it!

James