Originally Posted by Rymantas
I've read around and heard that our knock sensors pick up some strange events (ambient noise in engine bay). I have been getting a knock event that was very consistent on the logs. They were always spaced equally apart while driving at highway speed (70 mph to be exact) with cruise control on. The events would happen every 45 seconds or so. I thought maybe I had bad gas at first but as I looked at my logs i began to see a pattern. The knock events always occurred when the AC compressor would kick on. So I tested this by making sure recirc was on, AC was off, and blower was off. I drove 100 miles with no knock. So then I drove another 100 miles (distance to work and home) with the AC on and it knocked every time the AC compressor came on.
I then tried forcing it to happen by randomly turning the AC on and off. It would actually trigger a knock event while moving 2 out of 3 times i would do this.
I will be going through the engine bay and checking to make sure everything with the AC system is snugged up. Has anyone else had occurrences like this?
If it is a single count correction (-1.4) or so, I would not worry about it too much. The engine control unit (ECU) actually has a delay in which it prevents a correction to feedback knock taking place shortly after the A/C compressor comes on (a tuneable table in our Accesstuner tuning software). It may be something loose as mentioned (which is rattling as compressor comes on) or it may be that that some false knock due to A/C compressor coming on is normal at times (I haven't specifically check for a connection between A/C and feedback knock before via logging so I can't say for sure).