Update on random check engine light (code: P0325)

[Duh_OZ]

Posted back in September? how the check engine light would randomly go
on and off and it continued doing it until about 3 weeks back where it
hasn't come back on. Today, I decided to finally take it for an
emissions test and no stored codes were found

I did start putting mid grade gas in it, so if that "cured it" or just
an onset of cooler weather (seemed to happen more often in hot
weather) was the reason for no more check engine light lately I have
no idea. I'll switch back to the low grade and see if it returns.
The longest it ever stayed on was about an hour and sometimes it would
come on 2 -3 times a week or just once a month (like I said,
random). Gas mileage never seemed to suffer(alas, almost all city
driving and red lights every few blocks) so I never really did check
into it further.

OZ, still driving his 97 Legacy (wagon).

 

[Brian]

It sounds like your knock sensor is detecting pre-detonation in your
cylinders and letting you know.

Your engine will adjust automatically, up to a point.

Your MIL/ CEL is seemingly random because it is not unusual to get more
knock occurrence in hot weather or various other driving conditions that you
may not immediately think about. Were you running the AC compressor, either
cooling the car or defrosting the windows, which obviously would tax the
engine a bit more? Going up a steep incline? Have you checked your book to
see if that engine requires a premium grade gasoline? If the engine
requires a premium grade, putting a lesser grade can end up costing you more
dollars per mile than putting the more expensive grade in, due to the engine
changing it's timing to compensate for knock conditions.

A car manual I read once summed it nicely: If you hear engine knock, you
may need to use a higher grade gasoline to compensate. This can occur based
on driving conditions, or even age related wear of the engine. When you
return to conditions that no longer generate engine knock, you can return to
a lower grade fuel. The car was designed for the low grade gasoline, but
obviously there are times this may not always be ideal.

 

[1 Lucky Texan]

Posted back in September? how the check engine light would randomly go
on and off and it continued doing it until about 3 weeks back where it
hasn't come back on.   Today, I decided to finally take it for an
emissions test and no stored codes were found

I did start putting mid grade gas in it, so if that "cured it" or just
an onset of cooler weather (seemed to happen more often in hot
weather) was the reason for no more check engine light lately I have
no idea.   I'll switch back to the low grade and see if it returns.
The longest it ever stayed on was about an hour and sometimes it would
come on 2 -3 times a  week or just once a month (like I said,
random).   Gas mileage never seemed to suffer(alas, almost all city
driving and red lights every few blocks) so I never really did check
into it further.

OZ, still driving his 97 Legacy (wagon).

In theory, the knock sensor is only sensitive to the specific
frequency of pre-ignition. But, I suppose bad motor mounts or other
issues under the hood COULD be detected by it. Or the sensor itself
has drifted or gone bad or it's wiring has chafed or is intermittent.

 

[Duh_OZ]

It sounds like your knock sensor is detecting pre-detonation in your
cylinders and letting you know.

Your engine will adjust automatically, up to a point.

Your MIL/ CEL is seemingly random because it is not unusual to get more
knock occurrence in hot weather or various other driving conditions that you
may not immediately think about. Were you running the AC compressor, either
cooling the car or defrosting the windows, which obviously would tax the
engine a bit more? Going up a steep incline? Have you checked your book to
see if that engine requires a premium grade gasoline? If the engine
requires a premium grade, putting a lesser grade can end up costing you more
dollars per mile than putting the more expensive grade in, due to the engine
changing it's timing to compensate for knock conditions.

A car manual I read once summed it nicely: If you hear engine knock, you
may need to use a higher grade gasoline to compensate. This can occur based
on driving conditions, or even age related wear of the engine. When you
return to conditions that no longer generate engine knock, you can return to
a lower grade fuel. The car was designed for the low grade gasoline, but
obviously there are times this may not always be ideal.

==========
There was never any common conditions when it would go on or when it
would go off. Didn't matter if the AC was on or off, rain or dry,
highway or side street speeds, etc.

I switched back to the lower grade fuel and after running a tank
through no check engine light.

PS. Never had any egine knocks either.

PSS. If it does start coming back I'll break down and replace the
sensor to see if that could be faulty.

 

[Brian]

==========
There was never any common conditions when it would go on or when it
would go off. Didn't matter if the AC was on or off, rain or dry,
highway or side street speeds, etc.

I switched back to the lower grade fuel and after running a tank
through no check engine light.

PS. Never had any egine knocks either.

PSS. If it does start coming back I'll break down and replace the
sensor to see if that could be faulty.

I had an old Subaru wagon with the turbo on it. It had the exact same
symptoms you describe - random check engine light. Switching grades cured
the problem. Should I have been running it on the low grade gasoline? No,
it called for the high grade, and it only threw a check engine code when
running regular, but only perhaps once every three months or so. I
eventually gave up trying to save money and ran the high grade, as gas/
mileage improvement gave me a cost per mile benefit over the lower grade
gasoline. I run newspapers with these cars so it's easy to generate some
nice numbers on a spreadsheet and see the difference. The CEL condition I
experienced in that car was never when running the AC compressor or heading
up hill either. Those were just a few conditions I was proffering in a
theorizing out loud sort of way. I've had engine knock in other cars under
those conditions that did not generate a CEL code, so who knows.

Carl makes some very good points in his reply. My theorizing was aimed
toward more ideal conditions, where only one condition could be causing your
knock indication, and the knock detection system itself is working properly.
In reality, every part of the system in question should be held with at
least a certain degree of suspicion.

 

[1 Lucky Texan]

I had an old Subaru wagon with the turbo on it.  It had the exact same
symptoms you describe - random check engine light.  Switching grades cured
the problem.  Should I have been running it on the low grade gasoline?  No,
it called for the high grade, and it only threw a check engine code when
running regular, but only perhaps once every three months or so.  I
eventually gave up trying to save money and ran the high grade, as gas/
mileage improvement gave me a cost per mile benefit over the lower grade
gasoline.  I run newspapers with these cars so it's easy to generate some
nice numbers on a spreadsheet and see the difference.  The CEL condition I
experienced in that car was never when running the AC compressor or heading
up hill either.  Those were just a few conditions I was proffering in a
theorizing out loud sort of way.  I've had engine knock in other cars under
those conditions that did not generate a CEL code, so who knows.

Carl makes some very good points in his reply.  My theorizing was aimed
toward more ideal conditions, where only one condition could be causing your
knock indication, and the knock detection system itself is working properly.
In reality, every part of the system in question should be held with at
least a certain degree of suspicion.

It seems this vehicle is fairly old ('97 right?) and that is why I
mentioned motormounts and 'other issues'.

 

[Duh_OZ]

If I didn't do 90% driving with short hops (> 7 miles) to work I would
try a mid grade vs low grade MPG comparison. Even the drive in to
work can vary as some times I take side streets when traffic gets too
heavy and I know I'll get almost every red light (about every 4
blocks). Of course the side streets have a stop sign on almost
every intersection (bless Chicago).

BTW, to all us "lucky" midwesterners enjoying the latest snow, stay
safe!

 

[Brian]

If your car wasn't engineered to take advantage of higher grade gasoline, it
may not show a dollar/ mile benefit with higher grades. Was really only
concerned with your knock sensor firing at random. The point can be argued
about other advantages to higher grades, such as additives and cleanliness,
but somebody else can do that.

 

[Duh_OZ]

Well, after my third fill-up back on the lower grade fuel, guess what
came back on

If my 97 sube wasn't so dang reliable I would trade her up for a new
model, but gosh darn it she hasn't let me down yet!

 

[1 Lucky Texan]

Well, after my third fill-up back on the lower grade fuel, guess what
came back on

If my 97 sube wasn't so dang reliable I would trade her up for a new
model, but gosh darn it she hasn't let me down yet!

Perhaps you have extreme carbon build-up and your 'effective'
compression is higher than stock. maybe some kinda 'top cylinder
treatment' and a search for a cause of 'running rich' is in order?

I dunno, just a WAG.

Carl

 

[Uncle Ben]

Posted back in September? how the check engine light would randomly go
on and off and it continued doing it until about 3 weeks back where it
hasn't come back on.   Today, I decided to finally take it for an
emissions test and no stored codes were found

I did start putting mid grade gas in it, so if that "cured it" or just
an onset of cooler weather (seemed to happen more often in hot
weather) was the reason for no more check engine light lately I have
no idea.   I'll switch back to the low grade and see if it returns.
The longest it ever stayed on was about an hour and sometimes it would
come on 2 -3 times a  week or just once a month (like I said,
random).   Gas mileage never seemed to suffer(alas, almost all city
driving and red lights every few blocks) so I never really did check
into it further.

OZ, still driving his 97 Legacy (wagon).

In all the time I've seen this thread, you haven't told us what code
your system is trying to tell you when the malfunction light comes on.
Have you checked the code?

If not, you are like a man who gets a letter and wonders what's
inside, but never opens it to see. (I've seen this behavior here
before and always wondered why.)

If you have, what was the code?

Uncle Ben
99 Legacy OB

 

[clare]

In all the time I've seen this thread, you haven't told us what code
your system is trying to tell you when the malfunction light comes on.
Have you checked the code?

If not, you are like a man who gets a letter and wonders what's
inside, but never opens it to see. (I've seen this behavior here
before and always wondered why.)

If you have, what was the code?

Uncle Ben
99 Legacy OB

325 is a detonation sensor code. Not uncommon on Soobs and fairly
common on '99s. Put in a new knock sensor and you should be good for
gold.

 

[Duh_OZ]

325 is a detonation sensor code. Not uncommon on Soobs and fairly
common on '99s. Put in a new knock sensor and you should be good for
gold.

============
Some day I may just do that. BTW, engine light off again (as
expected). In my younger days I would start a pool on the next time
it would come on :-O

 

[clare]

============
Some day I may just do that. BTW, engine light off again (as
expected). In my younger days I would start a pool on the next time
it would come on :-O

it is an intermittent sensor - somewhat temperature dependent, ant it
fails open ofter sensing a knock