Knock Sensor - Resistor replacement

[Scott Hughes]

I've seen several posts about people replacing their knock sensors with
560K resistors. Has this process been documented anywhere? I have a 99
Impreza Outback Sport that runs AWFULLY. I've been getting a Knock Sensor
error w/ my CEL.. and the performance problem I'm having sounds very much
like what I've seen described here with the knock sensor improperly
retarding the timing.

Is the resistor replacement just a matter of removing the sensor and
splicing in a resistor, a simple wire cutter & elec. tape solution? Is the
resistor something I could pick up at the local Radio Shack? Anything else
I would need?

Also, am I pretty much guaranteeing that I'll need to upgrade to midgrade
or premium gas going this route?

TIA

-Scott

 

[Danny Russell]

I've seen several posts about people replacing their knock sensors with
560K resistors. Has this process been documented anywhere? I have a 99
Impreza Outback Sport that runs AWFULLY. I've been getting a Knock Sensor
error w/ my CEL.. and the performance problem I'm having sounds very much
like what I've seen described here with the knock sensor improperly
retarding the timing.

Is the resistor replacement just a matter of removing the sensor and
splicing in a resistor, a simple wire cutter & elec. tape solution? Is the
resistor something I could pick up at the local Radio Shack? Anything else
I would need?

The resistor just substitutes the sensor to keep the ECU happy. The lead normally connects to
ground through the 560k resistance of the sensor itself. Afterwards, it connects to ground through
the 560k resistor ...too easy. Just do it right tho... use solder, ground lug, heat-shrink tubing,
etc. If you do a hillbilly "twist-n-tape" job, you'll end up with CEL's.

Also, am I pretty much guaranteeing that I'll need to upgrade to midgrade
or premium gas going this route?

I've done it on both my Imprezas ('02-2.5 & '00-2.2). The 2.5 knocked just a little on 87 octane
with the sensor bypassed, but the 2.2 didn't knock at all.

The thing is, I also cranked the timing all the way forward by adding 47k resistance in series with
the IAT sesnsor and run 93 octane in both vehicles now.

Like most engines that have the timing retarded to cut NOX, they really come alive with the timing
restored to a power setting. I'm guessing about 6 ft./lbs torque increase, but also a livelier
"feel" thing as well.

You've gotta keep an ear out for knock though.

 

[Scott Hughes]

I've done it on both my Imprezas ('02-2.5 & '00-2.2). The 2.5 knocked
just a little on 87 octane with the sensor bypassed, but the 2.2
didn't knock at all.

The thing is, I also cranked the timing all the way forward by adding
47k resistance in series with the IAT sesnsor and run 93 octane in
both vehicles now.

Like most engines that have the timing retarded to cut NOX, they
really come alive with the timing restored to a power setting. I'm
guessing about 6 ft./lbs torque increase, but also a livelier "feel"
thing as well.

You've gotta keep an ear out for knock though.

Thanks for the response, I have a 99 OBS (2.2L) so hopefully I'll get
away w/ still running 87oct. It sounds like the IAT sensor mod you did
for additional hp (above spec), is this correct? I'll be more than happy
if I can just get normal power. So, hopefully, all I need to do to get
there should be the knock sensor replacement.

-Scott

 

[Danny Russell]

Thanks for the response, I have a 99 OBS (2.2L) so hopefully I'll get
away w/ still running 87oct.

It probably depends somewhat on the individual engine, and local fuel characteristics, but if you
keep the injectors, intake valves and combustion chamber clean, it'll help a lot.

It sounds like the IAT sensor mod you did
for additional hp (above spec), is this correct? I'll be more than happy
if I can just get normal power. So, hopefully, all I need to do to get
there should be the knock sensor replacement.

Apparently your ECU is entering into the limp mode because of the defective sensor (or connection),
which cuts about 10-15 degrees of timing, and REALLY makes it run like dog crap (below 4250 RPM
anyway, above that the advance returns).

There's an additional 5-6 degrees or so available via the AIT sensor (+ premium fuel)
trick.

 

[Scott Hughes]

Apparently your ECU is entering into the limp mode because of the
defective sensor (or connection), which cuts about 10-15 degrees of
timing, and REALLY makes it run like dog crap (below 4250 RPM anyway,
above that the advance returns).

Actually, mine runs like dog crap thru-out the rev range. It's a manual,
so I have the option of keeping the revs high if I want, but that doesn't
seem to aid my cause any.

 

[Guest]

Stupid question. Why don't you just not get a new knock sensor? It's 60-70$
and 15-20 minutes to install...

 

[Scott Hughes]

Stupid question. Why don't you just not get a new knock sensor? It's
60-70$ and 15-20 minutes to install...

Because the resistor job will probably only cost a buck or two. And as
much problems as I've read about the Subaru Knock Sensor, it seems like a
crap-shoot wether a new knock sensor will work properly or unnecessarily
retard my timing too. The cheap resistor should resolve the problem.
Mostly it's the buck or two part.. what do I have to lose? If I go w/
the resistor and for whatever reason I end up having to buy a new knock
sensor anyway.. what did I lose? A buck or two.

-Scott

 

[Scott Hughes]

The resistor just substitutes the sensor to keep the ECU happy. The
lead normally connects to ground through the 560k resistance of the
sensor itself. Afterwards, it connects to ground through the 560k
resistor ...too easy. Just do it right tho... use solder, ground lug,
heat-shrink tubing, etc. If you do a hillbilly "twist-n-tape" job,
you'll end up with CEL's.

I'm seeing anywhere from 1/8W 560K OHM resistors to 2W 560K OHM
resistors. What wattage should I be looking for?

 

[John O]

I'm seeing anywhere from 1/8W 560K OHM resistors to 2W 560K OHM

resistors. What wattage should I be looking for?

You can't go wrong by using a larger wattage...a 2W resistor is physically
bigger but electrically identical.

-John O

 

[-rick-]

You can't go wrong by using a larger wattage...a 2W resistor is physically
bigger but electrically identical.

-John O

If there were 12V across it 12V^2/560K is only 257 micro watts.

1/8 or 1/4 watt is plenty, get one that is mechanically convenient.

-rick-

 

[Nick Lamendola]

Hello Scott,

Did you try the resistor replacement? How did you get access to the knock
sensor connector?

Thanks
Nick L

 

[Dtitan]

Please be carefull when performing knock sensor modifications. Many of the
subaru sensors and just too damn sensitive. Adding resistors and such can
completely defeat any sort of knock protection. However, adding another
washer or gasket made of a soft material (like a real thin piece of rubber)
at the base of the sensor will help lower it's sensitivity but still give
you some knock protection.
However, this does not gaurantee good results. Ask yourslef if there is any
other reason the car might be knocking first. My assumption is that you are
running 87 octane. This is understandable due to the increase in gas
prices. however, running the next higher grade in gas doesn't really affect
the pocket book much. The average consumer will spend about 13-20 dollars
more a month on gas by using the next higher grade. That's nothing compared
to the cost of a new engine because you eliminated knock protection.

It is my experience that these engines LOVE 89 octane when naturally
aspirated. and 91 or higher when turbo charged.

Please give better gas a chance before you risk something expensive.


-Dominic Acia

 

[SlickWilly]

Please be carefull when performing knock sensor modifications.
Many of the
subaru sensors and just too damn sensitive. Adding resistors
and such can
completely defeat any sort of knock protection. However,
adding another
washer or gasket made of a soft material (like a real thin
piece of rubber)
at the base of the sensor will help lower it's sensitivity but
still give
you some knock protection.
However, this does not gaurantee good results. Ask yourslef if
there is any
other reason the car might be knocking first. My assumption
is that you are
running 87 octane. This is understandable due to the increase
in gas
prices. however, running the next higher grade in gas doesn't
really affect
the pocket book much. The average consumer will spend about
13-20 dollars
more a month on gas by using the next higher grade. That's
nothing compared
to the cost of a new engine because you eliminated knock
protection.

It is my experience that these engines LOVE 89 octane when
naturally
aspirated. and 91 or higher when turbo charged.

Please give better gas a chance before you risk something
expensive.


-Dominic Acia





 >> On Mon, 11 Apr 2005 19:08:28 +0000 (UTC), Scott
Hughes
 >> <(e-mail address removed)> wrote:
 >>
  >>>I've seen several posts about people replacing
their knock sensors
  >>>with 560K resistors. Has this process been
documented anywhere? I
  >>>have a 99 Impreza Outback Sport that runs
AWFULLY. I've been getting
  >>>a Knock Sensor error w/ my CEL.. and the
performance problem I'm
  >>>having sounds very much like what I've seen
described here with the
  >>>knock sensor improperly retarding the timing.
  >>>
  >>>Is the resistor replacement just a matter of
removing the sensor and
  >>>splicing in a resistor, a simple wire cutter &
elec. tape solution?
  >>>Is the resistor something I could pick up at
the local Radio Shack?
  >>>Anything else I would need?
 >>
 >> The resistor just substitutes the sensor to keep the
ECU happy. The
 >> lead normally connects to ground through the 560k
resistance of the
 >> sensor itself. Afterwards, it connects to ground
through the 560k
 >> resistor ...too easy. Just do it right tho... use
solder, ground lug,
 >> heat-shrink tubing, etc. If you do a hillbilly
"twist-n-tape" job,
 >> you'll end up with CEL's.

I’m in the same boat as the original poster...A few weeks ago, my 98
Outback’s thermostat blew and had to be replaced. A few days after I
picked up the car from the mechanic, the CEL came on. A quick trip
back to the mechanic showed that it was a bad knock sensor. The
mechanic said the car’s driveable, so I’ve been making short trips
with it. The weird thing is that sometimes the CEL doesn’t come on,
but there’s no rhyme or reason to when it goes off. (For example, I
parked the car and started it again a few minutes later and no light.
Another time, the light was on, I turned on the A/C, and the light
went off. Having said that, it’s on the overwhelming majority of the
time.)

I’d like to change the flipping sensor just to be safe, but I can’t
find one online and the local parts store wants $165 for one. Also,
I’m not the greatest with cars, but this sounds like a simple fix. Is
there a manual that anyone can recommend? Anything online I can find
that will show me where the knock sensor is?

Thanks in advance for any help.