98 legacy outback nock sensor issues

[wayneb]

Hi All;

I have 98 outback, 230k miles,head gaskets replaced at 170K(thanks to info hear did myself) that keeps giving knock sensor CEL.

 

I replaced the sensor and helped for a while but CEL comes on intermittently. I suspect the engine is carbonized since if my wife runs hightest, the CEL stays off and engine runs better and mileage goes up.

 

If engine is carbonized, does Subaru have recommended procedure to decarbonize engine?

 

Also, I have other suspicion that exhaust may be restrictive since I hear whistling noise under hard acceleration with windows down. anybody know what acceptable backpressure is in exhaust? Exhaust is original.

 

Thanks in advance;

 

Wayneb

[porcupine73]

Hi Wayneb. Are you sure the code you are getting is still for the knock sensor? The knock sensor codes never mean the ECU is detecting knock. It means the ECU is detecting an issue with the knock sensor circuit itself, which would be independent of the type of fuel being used.

 

Check out this article for the official Subaru products; there are several for cleaning carbon: Subaru Chemical Products: What to Use Where http://endwrench.com/images/pdfs/06SubMayEW.pdf

 

A good seafoaming through a vacuum line atop the intake manifold could help clean up the intake and such. Some throttle body cleaner will clean up the throttle butterfly and is good on the idle air control valve as well. If she's really carboned up, the old method of sucking water slowly in a vacuum line atop the intake manifold is supposed to work quite well.

[86BRATMAN]

Remove the knock sensor and clean the contact points on it and the block. Chances are there is corrosion between the two and it is not recieving signal as it should. My mom's 98 legacy ej22 was doing the same thing, intermitant code and cleaning it up has made a difference. No code for the last month, it had been on solid for about a year or more.

[idosubaru]

i'd do a search here, i've read through some informative knock sensor threads. cleaning the contacts, checking the wire visually and making sure the sensor housing isn't cracked and making sure it's bolted to the proper torque is a great start for simplicity. and of course make sure the new sensor is good, not cracked or damaged somehow.

 

i recall someone not too long ago actually relocating the knock sensor somewhere else on the engine.

 

a "trick" is to wrap the knock sensor threads with a few wraps, 10-15, of pipe thread tape and then install it. i haven't heard of anyone doing this on the newer gen stuff though, i've done it on older models.