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2008 forester XT 4EAT
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2001 Forester STi 6spd with MapDCCD
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Hey there Moshe!

Looked over your logs posted here. This is pretty typical for most subaru's of your year to get some noise picked up by the sensor at low load / low RPM. Generally how i approach these types of readings will be one of two ways. Is it knock that occurs like clockwork at the same RPM and load every time, or does it happen only occasionally and randomly?

If you answered yes to the first of the two, its likely a false event and possibly something rattling at that same low RPM, like a resonate frequency, triggering the sensor to go off. The truth of the matter is that regardless of how the car is tuned, from time to time you will see some knocking here and there on pump gas fuels. Quick throttle blips, quick throttle lifts and rapid load changes between shifting can and do cause some minor pre-ignition to occur.

Seeing as your car is 12+ years old now, I tend to lean towards something more mechanical triggering this event rather than it being actual knock. Your logs have it showing right there in that 1700-2000 rpm zone too. As your car gets old, motor mounts wear out, accessory brackets get looser, and chances are if you have pulled stuff apart then there is a slight chance that stuff going back together may have put parts like AC lines and TMIC's in the line of touching one another. I have even seen a dropped 10mm nut or bolt on top of the block wreak absolute havoc with readings for the ECU thought was MASSIVE amounts of knock. The way one can tell if it is a repeatable and fake event is if you see it occurring at the same low RPM and load areas like clockwork.

Real event or not, knock being read at those low RPM zones isn't of major concern to most tuners. You aren't putting your engine at any increased risk by seeing a few events happen at low throttle like that. The ones that are worrisome are the events you see while in WOT and full load. Those are the real concerning events.

So, what can you do? First and foremost, go over the engine bay and check for anything obvious. Loose AC line brackets, loose TMIC brackets, perhaps shine a light down on the block and look (as best you can) around where the knock sensor is just under your Throttle body area for any dropped items or metallic debris that would otherwise "vibrate" whilst sitting on top of a running engine. This is often a challenge for me with tuning builds that i did not work on or see in person that have had extensive work done with motor swaps and possibly a few missed items along the way. In extreme cases, an improperly torqued knock sensor will also cause this too.

Check everything out and report back, but as i mentioned before, seeing feedback readings during low load cruising is not a terrible thing.
 
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