Recently had the CEL light come on and go off after a period of driving (amount of time before it went off varies). Checked the ECU code and it threw back 22 - Knock Sensor. I am having the usual hesitation problems that go with a faulty knock sensor, but my question is....what if it is not the sensor that is faulty, and my engine is actually knocking?

I have no idea how to tell if it is or not as I have never actually heard and engine knock (apart from on a diesel engine, where this is commonplace i believe). Is it something that I will distinctly hear at idle, or do you have to have a trained ear? I've read somewhere that copper filings in the oil is a good way of telling, and to the naked eye mine has none, although it seems the water in the cooling tank does (could this just be rust from the cap??). Any help appreicated!

 

You are talking about two different kinds of knock.

The knock that the knock sensor detects is detonation, which if sustained will lead to the second type of knock which is where the engine rod bearings get damaged and you get rod knock and copper filings in the oil plus a huge repair bill.

If your knock sensor is faulty, your engine might also knock as a result since the Subaru ECU uses the knock sensor to safely adjust the timing to whatever fuel you happen to be using (within limits).

Reset the ECU and see if the knock sensor code re-appears, if it does, then replace the knock sensor.

Knock (the detonation kind) can only be heard on load and on my engine at least, I've only ever noticed it when on boost when it can be hard to hear if you've got a loud exhaust. Sounds like small stones hitting a sheet of metal.