Not Just Another "Knock Sensor" Question

[211]

At least according to the search results, this is not just another knock sensor question.

 

I have a CEL P0325 Knock Sensor error that I cleared the other day but two days later it came back.

My question is, what causes the Knock Sensor to go into fault to begin with? Is it that the sensor detects a knock and sends an error or does the sensor go bad and cause the engine to knock (by not allowing the ECU to correct for the knock because the sensor is bad)?

 

Reason I ask is, I cleared the code once but then it came back two days later. Now, with the hood open and engine running I hear a slight knock or "tap". but it seems to be coming from the front of the engine and sounds like a loose fan shroud or something. Does not sound like its coming from inside the engine. For all I know the knock has been there since day one and I'm just hyper-sensitive now that there's something wrong.

 

BTW, it's not the injectors, I can hear them firing separate from the noise I described.

 

So again...

a) Bad sensor: causes knock?

or

Something bad is happening to engine: sensor is working and sending fault.

 

If B (god I hope not) what might it be?

 

Engine runs fine still even w/ light on and has been rock solid since day one.

 

2000 OBW

[OB99W]

The definition for code P0325 is "Knock Sensor Circuit Malfunction". It's set when the ECU checks the knock sensor circuit and determines there's an electrical problem with the wiring, connection, or the sensor itself. It doesn't mean that knocking was detected.

 

Engine knock of the type that the sensor is designed to detect doesn't tend to occur at idle, so what you're hearing is likely totally unrelated.

 

Assuming the sensor is connected correctly and its mounting bolt is properly tightened, it's likely that the trouble code is being triggered by a defective knock sensor (they typically develop cracks).

[211]

The definition for code P0325 is "Knock Sensor Circuit Malfunction". It's set when the ECU checks the knock sensor circuit and determines there's an electrical problem with the wiring, connection, or the sensor itself. It doesn't mean that knocking was detected.

 

Engine knock of the type that the sensor is designed to detect doesn't tend to occur at idle, so what you're hearing is likely totally unrelated.

 

Assuming the sensor is connected correctly and its mounting bolt is properly tightened, it's likely that the trouble code is being triggered by a defective knock sensor (they typically develop cracks).

 

Kay, so order a new knock sensor I and I should be good to go...

[FSRBIKER]

If I had listened to mine I would have known my head gaskets were starting to fail.

[211]

If I had listened to mine I would have known my head gaskets were starting to fail.

 

WHAT!!!!

[idosubaru]

yep get a new knock sensor. the plastic cracks and separates from the metal. i've heard of people filling those cracks before and it gets rid of the code, take that or leave it, i'm sure others hate that i even type that.

 

pay attention to torque and the angle it's installed, directions are on this site here.

[211]

When the light came on the first time someone suggested I fill the tank with premium gas. Of course I cleared the code and it didn't come back right away, but the second time it came on I put a tank of premium in it. Two days with the light on and running premium fuel and the light just went out on it's own totay . We'll see if that's just coincidence, a fix, or an intermittent failure in the knock sensor (from being cracked).

[Steves72]

I did not see the model or year of the car but for the moment will assume that the car has an OBDII computer. That generation computer will reset the code if it does not encounter the same error within a given period of time. My 2000 OBW light has been coming on and going off for over a month on that same P0325 code. Wait and see if the light comes back on once more. If it does come back plan on replacing the part. I will do mine in September when its not quite so hot outside.

[screwbaru2]

When the light came on the first time someone suggested I fill the tank with premium gas. Of course I cleared the code and it didn't come back right away, but the second time it came on I put a tank of premium in it. Two days with the light on and running premium fuel and the light just went out on it's own totay . We'll see if that's just coincidence, a fix, or an intermittent failure in the knock sensor (from being cracked).

 

Somebody who doesn't know what they're talking about is giving you bad advise. Premium gas will prevent your motor from knocking (pinging). The knock sensor does the same thing by changing the ignition timing. When you get a code for it, it means the sensor or it's wiring has a problem. No amount of any gas will change that. It's not a drivability issue but if your engine starts to pinging and the sensor does not work then the engine will continue to knock (ping). I don't understand the head gasket comment made earlier.

[FSRBIKER]

My CEL started coming on for the knock sensor but the car never ran any different(even tried Super) so i just kept clearing it. I then added one of the resistor type fixes and it stopped coming on. Not soon after this I overheated so i did the normal flush the radiator, change the thermostat type things. Overheated again, bled the sytem to get rid of any air...then the head gasket blew. From what I and several other people think the knock sensor was hearing the exhaust bubbling into the coolant. Just to justify that I kept the same knock sensor when i installed my new engine, I haven't had the CEL knock sensor code come on ever since so the sensor is/was working correctly.

[OB99W]

...then the head gasket blew. From what I and several other people think the knock sensor was hearing the exhaust bubbling into the coolant. Just to justify that I kept the same knock sensor when i installed my new engine, I haven't had the CEL knock sensor code come on ever since so the sensor is/was working correctly.

The knock sensor "hearing the exhaust bubbling into the coolant" theoretically shouldn't be sufficient to set P0325, based on Subaru's apparent usage of that code. Anything that could be interpreted by the ECU as knock should just cause it to retard engine timing. Loss of coolant at a cylinder due to HG leak might have lead to localized elevated temperature, resulting in "real" knock that could be detected. However, that still should just have caused the ECU to retard timing, not set P0325. I wonder if heat distorted the piezoelectric element and changed its characteristics enough to cause the code.

 

OBDII P0xxx code definitions are supposed to be generic, but that doesn't mean a manufacturer won't "bend" things a bit. Perhaps Subaru isn't revealing the entire circumstances under which P0325 is set. For example, does the ECU "think" the knock sensor or wiring is defective if it already has retarded the timing the most it can, but is still getting indication from the sensor of what appears to be knock?

 

Generic knock sensor codes (for a single sensor) are:

P0325 Knock Sensor 1 Circuit Malfunction (Bank 1 or Single Sensor)

P0326 Knock Sensor 1 Circuit Range/Performance (Bank 1 or Single Sensor)

P0327 Knock Sensor 1 Circuit Low Input (Bank 1 or Single Sensor)

P0328 Knock Sensor 1 Circuit High Input (Bank 1 or Single Sensor)

 

It might be that conditions that more appropriately should set P0326 were programmed in the ECU for P0325.

 

Conjecture aside, since the knock sensors have a history of failing, I'll continue to look there before assuming other causes of P0325 ("when you hear hoofbeats, think horses, not zebras").

[screwbaru2]

My CEL started coming on for the knock sensor but the car never ran any different(even tried Super) so i just kept clearing it. I then added one of the resistor type fixes and it stopped coming on. Not soon after this I overheated so i did the normal flush the radiator, change the thermostat type things. Overheated again, bled the sytem to get rid of any air...then the head gasket blew. From what I and several other people think the knock sensor was hearing the exhaust bubbling into the coolant. Just to justify that I kept the same knock sensor when i installed my new engine, I haven't had the CEL knock sensor code come on ever since so the sensor is/was working correctly.

 

That's an interesting senerio. I guess if you have a EJ25 w/ a knock sensor code it won't hurt to look in the overflow tank for bubbles!

[OB99W]

That's an interesting senerio. I guess if you have a EJ25 w/ a knock sensor code it won't hurt to look in the overflow tank for bubbles!

I guess it wouldn't hurt to look...

 

... but knock sensors are designed to "hear" the "ringing" of the block caused by the sharp rap of knock. I'm not sure that exhaust gas bubbles in the coolant would have a similar effect.

[screwbaru2]

I guess it wouldn't hurt to look...

 

... but knock sensors are designed to "hear" the "ringing" of the block caused by the sharp rap of knock. I'm not sure that exhaust gas bubbles in the coolant would have a similar effect.

 

I know but I also think like you that "Loss of coolant at a cylinder due to HG leak might have lead to localized elevated temperature, resulting in "real" knock that could be detected. However, that still should just have caused the ECU to retard timing, not set P0325. I wonder if heat distorted the piezoelectric element and changed its characteristics enough to cause the code."

 

Which is why I say it wouldn't hurt to check for bubbles w/ a ej25.

[McDave]

Or it could have simply been a poor connection at the knock sensor connector and the r&r made the connection good again.

[OB99W]

I know but I also think like you that "Loss of coolant at a cylinder due to HG leak might have lead to localized elevated temperature, resulting in "real" knock that could be detected. However, that still should just have caused the ECU to retard timing, not set P0325. I wonder if heat distorted the piezoelectric element and changed its characteristics enough to cause the code."

 

Which is why I say it wouldn't hurt to check for bubbles w/ a ej25.

How could I argue with that logic?

 

(Sorry, screwbaru2, I thought you were agreeing with the "exhaust bubbling" theory. )

[screwbaru2]

No problem I'm often misunderstood, just ask my wife:grin: .

OB99W go you have any thoughts on my question of applying dielectirc heat disappating grease to the knock sensor mating surface?

[FSRBIKER]

Or it could have simply been a poor connection at the knock sensor connector and the r&r made the connection good again.

 

If this is in regards to my knock sensor then that wasn't the problem, I had unplugged it and cleaned the connections when i was first having the code. Maybe i had something else going bad in that engine but it sure would be a coincidence that it started just before my HG blew.

[2X2KOB]

.... you have any thoughts on my question of applying dielectirc heat disappating grease to the knock sensor mating surface?

 

I don't understand the point of doing this. The stuff you refer to as "dielectirc heat disappating grease" is really Thermal grease (also called thermal compound, heat paste, heat transfer compound, thermal paste, or heat sink compound)

 

http://en.wikipedia.org/wiki/Thermal_grease

 

and is used to improve the transfer of heat between two surfaces. Heat transfer or measuring (or anything else related to heat) is not the function of your knock sensor, what it does is sense a specific type of vibration.

 

Why do you think using this stuff is a good idea? What's it supposed to do for you?

[screwbaru2]

I don't understand the point of doing this. The stuff you refer to as "dielectirc heat disappating grease" is really Thermal grease (also called thermal compound, heat paste, heat transfer compound, thermal paste, or heat sink compound)

 

http://en.wikipedia.org/wiki/Thermal_grease

 

and is used to improve the transfer of heat between two surfaces. Heat transfer or measuring (or anything else related to heat) is not the function of your knock sensor, what it does is sense a specific type of vibration.

 

Why do you think using this stuff is a good idea? What's it supposed to do for you?

Heat is not good for electroincs in general. I had a couple of decades of experience using that stuff regulaly on GM IM's. The heat would slowly bake'em. I used the same 350 motor for most of those 20 years and only changed the IM once. Guys carried these IM's in their gloves boxes they went so often. I never got stuck because of the IM.

So lately I have a rash of knock sensor problems on 3 different soobs. Since these sensors sit in a fairly hot place I was speculating if applying this stuff (that Standard calls dielectirc heat disappating grease not me. I just read labels I'm not smart enough to come up with a name like that) would do any good in reducing the affects if any heat may have on the knock sensor. You seem to know something of this goop what do you think? That is if you now understand the point.

[2X2KOB]

If you would like heat to flow more efficiently from your engine into your knock sensor, then this goop is the stuff to use.

 

Really, it's not going to make one bit of difference one way or the other. Your knock sensors will be about the same temperature as whatever they are bolted to. They are designed to work in this environment. I think maybe what you have is a coincidence.

[McDave]

Heat is not good for electroincs in general. I had a couple of decades of experience using that stuff regulaly on GM IM's. The heat would slowly bake'em. I used the same 350 motor for most of those 20 years and only changed the IM once. Guys carried these IM's in their gloves boxes they went so often. I never got stuck because of the IM.

The GM Ignition Modules contained 14 transistors mounted on a heat sink, the metal rectangular area on the bottom of the IM. Since the modules were mounted in the distributor rather than the engine block, the idea was to transfer the heat from the module to the cooler aluminum distributor base.

 

Unfortunately the early 1975 and '76 transistors weren't up to snuff so failures were common, even with the dielectric paste. It didn't take them long to make better modules, but many didn't know what that tube of "grease" that came with the modules was for, so they still tended to fail anyway when it wasn't used.

 

So bottom line, I agree with 2X2KOB that it won't make any difference since you wouldn't be helping a hot electronic component transfer heat away to a cooler surface.

[screwbaru2]

Thanks guys, Apparently this would do the opposite of what I would want which would be to pull heat away from the sensor. Which was why I asked the question on the USMB becuse I knew I'd get an answer. Thanks again.

[211]

Wow, I come back three days later and this thread has grenaded!

 

Anyway, I'm sorry to say that the CEL has come back again. Haven't had the code pulled yet but I assume its the same knock sensor error.

 

Just ordered one from 1stsubaruparts.com only cost $112 and a .

 

Hopefully I'm not wasting my money.

[screwbaru2]

Wow, I come back three days later and this thread has grenaded!

 

Anyway, I'm sorry to say that the CEL has come back again. Haven't had the code pulled yet but I assume its the same knock sensor error.

 

Just ordered one from 1stsubaruparts.com only cost $112 and a .

 

Hopefully I'm not wasting my money.

I'll say this, I bought a Bosch knock sensor for my 96 Imp. This car has not been driven. I went to move it and the CEL came on. It's the knock sensor again. You're probably doing the right thing getting OME.