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Knock Sensor


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3 replies to this topic

#1 billrigsby

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Posted 19 December 2003 - 08:00 PM

Where would you find the knock sensor on a 99 2.2L
Impreza, are they hard to get at, and do they go bad.
Meaning is there a known problem with them?



Bill

#2 Legacy777

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Posted 19 December 2003 - 08:21 PM

here's the knock sensor
http://www.main.expe...rs/DCP_2481.JPG

You can get at it relatively easily

#3 alias20035

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Posted 20 December 2003 - 03:09 AM

Are you having problems?

Why are you inquiring about the knock sensor?

I will post a bunch of information anyways since I have nothing better to do right now.

First of all, the knock sensor is basically a microphone attached to the engine block that listens for knock/ping as well as the explosions related to normal combustion. It produces an electrical signal waveform that the ECU converts to digital format and analyzes the amplitude and timing of the spikes for patterns which are indicative of either knock/ping or normal combustion.

There were rumours circulating that the knock sensor can be overly sensitive and detect knock when there is in fact none. If knock is detected the ECU will increase fuel injector pulses (to get a richer fuel/air mixture) and retard ignition timing to reduce knock, this is referred to as anti-knock mode. Lean mixtures and overly advanced ignition timing are the main causes of knock along with fuel that has too low of an octane rating and either an overheating engine or extremely hot weather conditions.

A few years ago a lot of people were installing garden hose rubber washers between the knock sensor and the engine block to reduce the knock sensor's sensitivity and reduce the ECU's liklihood of entering the anti-knock mode which has the effect of reducing both engine output and fuel economy. However, a few of these people went on to have broken piston wrists due to the constant knocking sound that you can not hear. The ECU was running the ignition timing too advanced and could not detect it, and a constant low amplitude knock was the result, which is very bad for the engine.

So I would NOT recommend doing anything to the knock sensor unless the ECU detects that it is defective, in which case it will need to be replaced.

The ECU will listen to the knock sensor at engine start up and calibrate itself. If the calibration is not within a predefined range the ECU will store a problem code and the "Check Engine" light will come on. The ECU has a prediction table that indicates how "loud" the engine should be based upon the temperature reading from the thermal sensor. If the knock sensor reading does not relate to the ECU's internal table, the ECU will report a problem and switch to limp-home mode, which includes using a richer than normal fuel mixture and almost no ignition timing advance.

If you are just loosing power and are concerned that the anti-knock system is at fault please evaluate the following items before considering the knock sensor, as knock sensors are not a common point of failure (no knock sensor failures among any of my friends' 35 odd Subaru's in the last 5 years to the best of my knowledge).

* fuel octane, most Subaru's run 87 Octane, but the turbo models and the SVX's H6-3.3 require 91, and 91 is highly recommended on the H6-3.0 (may even be listed as required depending on model year). As cars age and carbon builds up you will find that fuel with a higher than recommended octane rating is required to avoid knock, especially in hot weather.

My 01 Outback hates 87 octane in summer, I usually need to use 89, and I have very little or no carbon build-up, my 93 Legacy also had the same issue. I have also found that both of my Legacy's and my current Outback really hate Shell fuel, I don't know whether it is a Shell Canada fuel formulation thing, but I really notice that the car is sluggish and poor fuel economy with Shell fuel in the tank regardless of the fuel grade. I am living at 5,000 ft, so altitude is a factor in my case. I can get away with lower octane ratings at altitude, but due to low humidity the air temperatures can often exceed 140F on the road, and it is this hot air that is then sucked into the engine.

* thermal sensor, a common failure, and often with no "Check Engine" light. Since engine noise is higher at colder temperatures, this sensor will adjust the ECU's knock sensor sensitivity. Also the fuel air ratio is set by readings from the thermal sensor, if it is not working correctly you may very well end up with lean burn which is one of the main causes of knock

* cylinder carbon buildup, seems to be a fairly common problem in Subaru's that can usually be solved by a good fuel injector/carbon deposit remover. I run fuel injector/carbon cleaner through the gas tank every 5000 km or so, and as a result I had no serious carbon build up on my 93 Legacy when it was destroyed at 466,000km.

* ignition problem such as bad plugs or coilpack causing a misfire, the knock sensor will detect a misfire (it will not hear the explosion when it is supposed to occur) and will take action similar to what happens if knock is detected, which includes a reduction in engine power.

* fuel supply problem, weak fuel pump (rare), clogged fuel filter, bad fuel pressure regulator, EGR problem or defective fuel injector(s). If too little fuel enters the cylinder, you end up with lean burn and usually knock.

The older Impreza's Mass Air Flow (MAF) meters are sometimes problematic, I would evaluate it carefully. How new is your primary oxygen sensor? If your MAF is misreading the air intake flow, and the oxygen sensor is old and worn, your fuel air ratio may be off and the ECU will not know causing lean burn and knock. I am not sure if your 99 EJ22 Impreza has a MAF or the better MAP sensor, some 99's have MAP while others have MAF, seems to be related manufacturing date. If you have the right fender mounted air filter box and a wiring harness on the air intake pipe just after the air filter you have the older and sometimes problematic MAF sensor. If you have a sensor wiring harness just to the left of the throttle body on the large air box attached to the throttle body, you have the less troublesome MAP system.

Since the problem with knock can be the result of many items, and in some cases not even trigger a fault code, an ODBII diagnostic unit is usually required to evaluate the sensor readings to get a sense of which sensor(s) is causing the ECU to either detect or cause knock.

Unless there is a pressing problem, the first thing to do is use a good quality tank of gas and a good fuel injector cleaner/carbon build up remover. This will often cure a sluggish and lightly knocking Subaru, and this should be repeated on a regular basis to keep fuel injector contamination and carbon build up to a minimum.

But if the knock is bad, get the car serviced pronto!!! Constant knock is very destructive, usually causing piston wrist, piston wrist pin or connecting rod failure in Subaru engines over the long term.

#4 uniberp

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Posted 07 April 2006 - 11:45 PM

Even though tis is an old thread, you might still be around.

Okay, I understand that knock isn't always audible, but what is likely the problem is the CEL light goes on after a few miles, and the only code returned indicates the knock sensor. Does that mean the sensor is faulty, or thata the engine is knocking unacceptably?

The car doesn't seem to de-tune, and the light stays on until the car sits off for a bit, which I understand is the standard reset.

I will try the injector cleaner. Do you recommend SeaFoam?

Tanx. Mpergiel, Elmhurst, IL :)

Are you having problems?

Why are you inquiring about the knock sensor?

I will post a bunch of information anyways since I have nothing better to do right now.

First of all, the knock sensor is basically a microphone attached to the engine block that listens for knock/ping as well as the explosions related to normal combustion. It produces an electrical signal waveform that the ECU converts to digital format and analyzes the amplitude and timing of the spikes for patterns which are indicative of either knock/ping or normal combustion.

There were rumours circulating that the knock sensor can be overly sensitive and detect knock when there is in fact none. If knock is detected the ECU will increase fuel injector pulses (to get a richer fuel/air mixture) and retard ignition timing to reduce knock, this is referred to as anti-knock mode. Lean mixtures and overly advanced ignition timing are the main causes of knock along with fuel that has too low of an octane rating and either an overheating engine or extremely hot weather conditions.

A few years ago a lot of people were installing garden hose rubber washers between the knock sensor and the engine block to reduce the knock sensor's sensitivity and reduce the ECU's liklihood of entering the anti-knock mode which has the effect of reducing both engine output and fuel economy. However, a few of these people went on to have broken piston wrists due to the constant knocking sound that you can not hear. The ECU was running the ignition timing too advanced and could not detect it, and a constant low amplitude knock was the result, which is very bad for the engine.

So I would NOT recommend doing anything to the knock sensor unless the ECU detects that it is defective, in which case it will need to be replaced.

The ECU will listen to the knock sensor at engine start up and calibrate itself. If the calibration is not within a predefined range the ECU will store a problem code and the "Check Engine" light will come on. The ECU has a prediction table that indicates how "loud" the engine should be based upon the temperature reading from the thermal sensor. If the knock sensor reading does not relate to the ECU's internal table, the ECU will report a problem and switch to limp-home mode, which includes using a richer than normal fuel mixture and almost no ignition timing advance.

If you are just loosing power and are concerned that the anti-knock system is at fault please evaluate the following items before considering the knock sensor, as knock sensors are not a common point of failure (no knock sensor failures among any of my friends' 35 odd Subaru's in the last 5 years to the best of my knowledge).

* fuel octane, most Subaru's run 87 Octane, but the turbo models and the SVX's H6-3.3 require 91, and 91 is highly recommended on the H6-3.0 (may even be listed as required depending on model year). As cars age and carbon builds up you will find that fuel with a higher than recommended octane rating is required to avoid knock, especially in hot weather.

My 01 Outback hates 87 octane in summer, I usually need to use 89, and I have very little or no carbon build-up, my 93 Legacy also had the same issue. I have also found that both of my Legacy's and my current Outback really hate Shell fuel, I don't know whether it is a Shell Canada fuel formulation thing, but I really notice that the car is sluggish and poor fuel economy with Shell fuel in the tank regardless of the fuel grade. I am living at 5,000 ft, so altitude is a factor in my case. I can get away with lower octane ratings at altitude, but due to low humidity the air temperatures can often exceed 140F on the road, and it is this hot air that is then sucked into the engine.

* thermal sensor, a common failure, and often with no "Check Engine" light. Since engine noise is higher at colder temperatures, this sensor will adjust the ECU's knock sensor sensitivity. Also the fuel air ratio is set by readings from the thermal sensor, if it is not working correctly you may very well end up with lean burn which is one of the main causes of knock

* cylinder carbon buildup, seems to be a fairly common problem in Subaru's that can usually be solved by a good fuel injector/carbon deposit remover. I run fuel injector/carbon cleaner through the gas tank every 5000 km or so, and as a result I had no serious carbon build up on my 93 Legacy when it was destroyed at 466,000km.

* ignition problem such as bad plugs or coilpack causing a misfire, the knock sensor will detect a misfire (it will not hear the explosion when it is supposed to occur) and will take action similar to what happens if knock is detected, which includes a reduction in engine power.

* fuel supply problem, weak fuel pump (rare), clogged fuel filter, bad fuel pressure regulator, EGR problem or defective fuel injector(s). If too little fuel enters the cylinder, you end up with lean burn and usually knock.

The older Impreza's Mass Air Flow (MAF) meters are sometimes problematic, I would evaluate it carefully. How new is your primary oxygen sensor? If your MAF is misreading the air intake flow, and the oxygen sensor is old and worn, your fuel air ratio may be off and the ECU will not know causing lean burn and knock. I am not sure if your 99 EJ22 Impreza has a MAF or the better MAP sensor, some 99's have MAP while others have MAF, seems to be related manufacturing date. If you have the right fender mounted air filter box and a wiring harness on the air intake pipe just after the air filter you have the older and sometimes problematic MAF sensor. If you have a sensor wiring harness just to the left of the throttle body on the large air box attached to the throttle body, you have the less troublesome MAP system.

Since the problem with knock can be the result of many items, and in some cases not even trigger a fault code, an ODBII diagnostic unit is usually required to evaluate the sensor readings to get a sense of which sensor(s) is causing the ECU to either detect or cause knock.

Unless there is a pressing problem, the first thing to do is use a good quality tank of gas and a good fuel injector cleaner/carbon build up remover. This will often cure a sluggish and lightly knocking Subaru, and this should be repeated on a regular basis to keep fuel injector contamination and carbon build up to a minimum.

But if the knock is bad, get the car serviced pronto!!! Constant knock is very destructive, usually causing piston wrist, piston wrist pin or connecting rod failure in Subaru engines over the long term.






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