Feedback Knock

I have no idea what's wrong with my car. It's stock in every sense with a stage 1 93 tune. If I'm just costing at 20mph and let off and wait till the engine rpm gets to its lowest point around 1k rpm then blip the throttle I'm getting feedback knock. I had my knock sum on there, and it was jumping up in count & my feedback knock values went to -1, -2, -3..... I also had feedback knock when driving at 60 in 5th gear, downshifted to 4th and was holding the throttle around 3k rpm to speed up and I got a -4 on the feedback knock value. What's going on? What should I check? What usually causes feedback knock on Subaru turbo's, this is my first Subaru, first turbocharged engine, first Awd car. Please someone give me an idea what to do to fix this issue.

 

Edit: also my knock sum is at crazy numbers just after a 7 min drive it said 26... and just blipping the throttle as stated above it was jumping up in like intervals of 2 or more. There isn't any audible noise, and power doesn't change at all throughout the rev range in fact it's actually stronger than before now that it's tuned. And I'm not sure if this makes a difference but it's the 5EAT transmission. If you need any more info ask and I'll provide

Edited January 5, 2022 by MarceraB16

I'm fighting with this, too, but there's a couple of things I've learned (and mine seems to be related to a vac/boost leak).

 

It seems to be pretty common for the ECU to detect knock when shifting (especially true with a 5MT, might be similar with a 5EAT - my FBKC will retard up to 4 degrees, then return to 0 after a couple of seconds), and on sharp throttle inputs. This is normal.

 

I'm assuming you've got an AP - keep an eye on your DAM and knock learning values, as well as FBKC (which it sounds like you are). If your DAM is dropping, you know you've got a severe knock event (or events) happening and need to look at it ASAP. If it's not, you might be fine.

 

There's also some stickied threads on this. Might be in the tuning section. It's a veritable treasure trove of info.

How are your long term fuel trims? close to zero?

 

 

Also, check for any loose exhaust shields.

 

 

A lean condition could trigger the knock sensor. And loose stuff connected to the engine may also cause (false) knock.

My dam hardly ever drops it's always remaining constant at 1and I'm sorry I'm not entirely sure yet how to check the long term fuel trims. How would I do that? And ok I'll check my exhaust Shields I should be getting the car into the shop beginning of next week

DAM at 1 is just an indication that so far, the ECU has not seen too much knock and is not pulling a certain amount of timing over the whole timing map.

 

 

 

The long term fuel trims should be called AF Learning A,B,C,D something like that on your AP. There should be four values, one per specific driving condition or engine load (A=idle, B=very light acceleration/cruise, C=moderate acceleration, D=getting close to WOT). These variables are in %, indicating the amount of fuel correction the ECU has learned for a given driving condition. You want these values to remain within +/-5%. Large positive values indicate that you may have a vacuum leak somewhere and the ECU noticed that you are running lean (thereby knocking). As a result it adds fuel.

 

Negative values mean you are running rich, so the ECU is removing fuel. This can happen if you have a boost leak, as metered air is not fully entering the combustion chamber. Bad MAF connection (e.g. poor ground) may also lead to negative values.

DAM at 1 is just an indication that so far, the ECU has not seen too much knock and is not pulling a certain amount of timing over the whole timing map.

 

 

 

The long term fuel trims should be called AF Learning A,B,C,D something like that on your AP. There should be four values, one per specific driving condition or engine load (A=idle, B=very light acceleration/cruise, C=moderate acceleration, D=getting close to WOT). These variables are in %, indicating the amount of fuel correction the ECU has learned for a given driving condition. You want these values to remain within +/-5%. Large positive values indicate that you may have a vacuum leak somewhere and the ECU noticed that you are running lean (thereby knocking). As a result it adds fuel.

 

Negative values mean you are running rich, so the ECU is removing fuel. This can happen if you have a boost leak, as metered air is not fully entering the combustion chamber. Bad MAF connection (e.g. poor ground) may also lead to negative values.

 

Ah! Thats how you check fuel trims with accessport! Ty! Those are the same thing like with the learning view program and the 4 ranges!

Edited January 10, 2022 by Tehnation

Those are the same thing like with the learning view program and the 4 ranges!

 

yes!