knock sensors
 

I've read a lot about how to change the knock sensors and it seems that a lot of people are removing the alternator and a/c compressor to do the job, however wouldn't it be easier to just remove the intake manifold and attack it from the top? Oh and on a side note what exactly are the purpose of the knock sensors? Thanks in advance, chris

Yes - it is easier to replace the knock sensors if you remove the intake. In fact it is painless once the intake is off.

But - removing the intake isn't quick the first time you do it. Plan on having alot of spare 1/8th inch vacuum hose to replace all the brittle hoses under the intake. Also - be very careful with the plastic fittings that the hoses plug into. These will also be brittle and you'll have to get crafty with some superglue if you snap one off.

Removing the intake is a good idea on a higher mileage svx anyway because you are probably losing alot of vacuum with all the cracked and brittle hoses under there.

Also it would be a great time to change out the PCV valve when the intake is off. And while it's off get some valvoline synthetic carb cleaner and get the intake nice and shiny inside and also clean the gunk out of the EGR tube. :)


Almost forgot - the knock sensors detect pre-ignition (basically gas explosions before their wanted time in relation to piston travel) And retard spark timing to prevent pre-ignition (which could lead to engine death in extreme cases)

alternator yes
AC comp no
Intake manifold big NO
knock sensors listen for knocking the ecu will retard timing if needed.

interesting we have two competing recommendations, alltrac, why no for removing the manifold?

Heh heh - probably because it's a pain in the ass the first time you do it. But after that it is a breeze.

Seriously - it is a great PM measure to remove it on a 100k mile plus car just to replace the cracked vacuum hoses and clean out the plugged EGR line.

These threads should help if you decide to remove the intake
http://www.subaru-svx.net/forum/show...ghlight=vacuum
http://www.subaru-svx.net/forum/show...intake+removal

thanks matt, I'll probably be removing the intake since, like you said, it will be advantageous to address the vacuum lines.

Hijack alert...

On the subject of knock, how can you tell if your car is actually dialling in any anti-knock? (so to speak)

Matt

Removing the intake manifold is definitely more work than removing the PS pump and the alternator. Its more costly too because several gaskets and hoses will need replacing. You'll also probably break at least one of the little solenoids attached to the underside of the intake manifold. Its also time consuming. If you plan to keep the car a while, pulling the intake manifold makes more sense because all the hoses will need to be replaced at some point anyhow.

Unfortunately there is no easy way to positively check that the system is working. Worth trying, if you can set up a timing light, difficult with the plug on coil SVX arrangement, is to run the engine with a little throttle while observing the spark advance. Tap a block with a hammer, close to a sensor and the timing should retard slightly.

The usual knock sensor incorporates a metal diaphragm which absorbs shock waves from the engine block and exerts pressure against the crystals inside a piezoelectric disc. As a result, this generates a small A/C voltage signal, proportional to the frequency of the knocks. The sensor used in the SVX has a relatively high impedance of around 500,000 ohms.

Therefore in effect, the sensor is a crude crystal microphone. If you want to have some fun, you could hook one up to the input of an amplifier, run the engine and listen for some music!!

And it would sound swwweeeeeettttttttt.........

Hey Trevor, how's the weather at home? My folks have just come back to the King Country from the Mount and say it's like going back into the ice age..

Not cold in Auckland. Heavy rain overnight and flooding in many places. Hauraki Plains badly hit.

Therefore in effect, the sensor is a crude crystal microphone. If you want to have some fun, you could hook one up to the input of an amplifier, run the engine and listen for some music!![/QUOTE]

are you being serious? how exactly? just wire it to a amp or would other things need to be done first?

are you being serious? how exactly? just wire it to a amp or would other things need to be done first?[/QUOTE]

Remove existing connections to the sensor, connect a shielded audio cable and to a ground very close by the sensor, to prtevent AC hum pick up. Connect a 250-500,000 ohms potentiometer (or I meg. if you want maximum base response!), across the other end of the shield and the conductor. Connect a capacitor of a microfarad or so, to the potentiometer wiper and the other end to the imput of an amplifier, with the amp ground to the shield. Make sure the wiper is turned to the ground end for minimum volume and the amp volume likewise. Increase both level controls and you will hear the music. Those knowing exactly what is involved, could dispense with the potentiometer and capacitor, which I have suggested for isolation and as an extra safety and level control.

Is that even english?

Discombobulate the flux capacitor and reverse engineer the LMNOP barometer to produce reverse phase shift inocculation.

See, I can do that too.

i trip to your local radio shack will help you decode it... i understood it!