Knock sensor

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farias326
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Knock sensor

Post by farias326 »

I got an interesting situation. Different from what I've researched here and other forums.

Knock sensor code pops up on every cold start. Will go away once car warms up. Will stay off if I'm under 40 MPH ish. Will stay off if I'm flooring it or driving in the city.

It turns on if I'm doing 50 mph ish at a constant speed, if I'm on the freeway, and going downhill above 50 mph. It won't go away until I drive for about 3 minutes in the city or I shut the car off then turn it back on.

This issue started only after I changed my spark plugs. It had the ngk (can't remember number) plugs that come gapped at .039 (the fine tip). I pulled them out and all of them were gapped out of spec (between .035 and .042). I replaced them thinking it needed new ones. I threw in the oe ngk that come gapped at .044 (v cut tip) and then the issue started. I cleared the code by connecting the 2 wires under the dash, but it came back. Then I tried a tighter gap at .035 and got the same code. Pulled nice when the code didn't come on. Tried a new set of ngk again at .039 and it's still doing it. I checked to make sure the rings on the plugs were seated properly, checked to make sure plug wires were not loose or broken, cleaned maf, changed knock sensor with I believe was Hitachi brand or the Japanese one (old one looked fine as well, no cracks).

The engine doesn't sound like it's knocking. Sounds really healthy. When the code comes on and I try to run it hard it just spools the turbo but I don't feel the boost kick in.

Someone said to try another sensor and maybe a new ecm/ecu.

Has anyone had this problem? Anything else I should try or replace?

Just signed up to this forum. If this has been solved or I need to move it don't hesitate to call me out.
Legacy777
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Re: Knock sensor

Post by Legacy777 »

Welcome to the BBS.

This is an interesting one. Out of curiosity have you checked the resistance/continuity of the knock sensor wiring from the connector in the engine to the ECU? If you haven't I would suggest doing that and verifying that the wire is still good. It's known to become brittle and crack/break over time. I had this issue and had to run a new wire.

These ECU's are relatively dumb compared to modern ECU's so I'm wondering if it's only looking for the knock sensor at certain engine conditions....and from what you described would likely be more "closed loop" conditions. If the knock sensor wire is bad that may be why you're seeing the CEL at those conditions.

Have you run the D-Check mode with just the green connectors? Does the knock sensor code come up at that time?
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
farias326
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Posts: 5
Joined: Wed Oct 12, 2022 3:27 am

Re: Knock sensor

Post by farias326 »

My cousin checked the resistance and continuity of the knock sensor wire. We noticed it was spliced, looked like a good splice though. It was not brittle, was in good shape.

He soldered the wire to see if it made a difference. He said it was reading better on the multimeter and should be more solid of a connection.

He also measure the voltage going into the knock sensor pin on the ecu and said the voltage was a little low, so if that solder didn't work then we could try an ecu.

We test drove it and same result. Reset the ecu and same results. Now this time it stays on no matter what.

I still drive it daily kinda listening to what the car does.

I recently noticed a change in the noise my bov makes. When I release the gas pedal after a pull it sounds more loud. I also noticed this when the code started popping up. Which now is leading me to believe there may be a boost leak or vacuum or something loose.

When I bought the car, I had to tighten up various hose clamps and screws that previous owner left loose after a rebuild.

I just installed a used ecu off of a 92 legacy ss today and had the same results. So now I'm thinking loose hose or something to do with the turbo system.

If I hit the gas and go full throttle, it pulls like an na car. If I gradually press the gas nice and easy, the boost creeps in, up until I go full throttle and it feels like I lost boost.

And yes I did the d check. Code 22 still pops up.
Legacy777
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Re: Knock sensor

Post by Legacy777 »

Thanks for the additional information. You may have multiple issues or certain things that may be symptoms of others.

The knock sensor voltage is very low so even just a little bit of issue on the wiring could be a problem. Do you know what the resistance value was? Also, by any chance did you check for continuity between the signal wire and shielding on the knock sensor? If they are touching that could cause your problem and your code. Any repairs that don't address this could be the cause. The knock sensor shielding goes back to the ECU and is grounded through the sensor grounds. This shielding needs to not be grounded or connected to anything else but the sensor ground pin at the ECU.
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
farias326
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Joined: Wed Oct 12, 2022 3:27 am

Re: Knock sensor

Post by farias326 »

Legacy777, turns out it was the knock sensor wire on the ecu side. Had another mechanic check it out. I think he re spliced it, but new how to un ground the opposite end of the ecu wire. Runs awesome now!

Thank you for the help.
Legacy777
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Re: Knock sensor

Post by Legacy777 »

Glad to hear you got it sorted out.
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
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