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Feedback knock

13K views 6 replies 3 participants last post by  RoboTuner 
#1 ·
I have a 2004 Subaru wrx with about 200000km on the stock motor. I have a full catless exhaust system and I have a COBB stage 2 tune. I always run the feedback knock gauge on my COBB and it seems everytime I drive it I get -2.11 knock. When I really pin it usually 6k rpm or more ill get -4.22. Should I be worried. The car seems to drive just fine.
 
#2 ·
Those numbers are a bit high. Look at your learning view table. It will show you fine learning knock values. If you have those in the table anywhere then your car is not properly tuned which would explain why it's knocking.

If ap does not have a learning table view on board, add a gauge for fine learning. If it changes at all when you drive, same thing, car does not like tune.

Another thing to do would be to record some logs doing pulls. Post them up and I can take a look but make sure you're logging correct parameters (rpm, engine load, afr, feedback knock, fine learning knock, manifold relative pressure).... Or the cobb version of those, I'm using RomRaider names. Do 2k-redline pulls, 3 of them, pedal to the metal I'm a straight line, no hills. Obviously don't do that if it's icy or snowing.

Good luck!
 
#3 ·
Adding to Ruso, if your DAM hasn't changed the ecu is doing its job by pulling timing where needed as the knock is found. Still I second the logs, a couple 2k-6800 pulls, a couple 3k-6800 pulls all on flat/dry ground and logging AFR (or a wideband if you're lucky enough), FKL, FBK, DAM, RPM, TPS, MAF, Load, egt (if you still have it but not horrible if you don't), IDC or Injector Duty Cycle. I have FKL/FBK on my bugeye, but I have 565cc that are shot and flow less than the 440cc from the factory D:

Also if those number go down as you accelerate (but keep throttle consistent) then the ECU is doing its job as stated before, but if those number don't change or go up...you're looking at bigger issues (mechanical). I wouldn't keep driving in boost if I were you (Mine is sitting until I get injectors and only gets started to move fluids and make me smile from the rumble on occasion).

This only happens while in boost, right?
 
#5 ·
You should definitely log and check your AFR.

I also wanted to make sure your tune is suited for a Catless exhaust... if not you are likely overboosting/boost spiking right after spool. What's the max PSI you're hitting?

What is the AFR at that PSI, I'm not an expert on this so hopefully someone else will chime in, but if your AFR gauge is staying above it's lower limit at that RPM (on my year that's 11.4) then you are running lean, but that's hard to tell since you're below your gauges limit so it's not very accurate.

Is your intake modified in any way?

I'll say it again, learn to log, it will give you a chance to get to know your car on a deeper level. I'm still learning, just scratching the surface, but I have the tools to troubleshoot problems that are invisible to others who don't know.
 
#7 ·
First your DAM being at 13, not good! With our 16bit ecus DAM should stay at 16, when it dips down towards 0 you need to stop boosting asap. Google the Cobb documentation for Accessport and their recommended gauges for daily monitoring and you will learn the basics on what to watch out for and until you've memorized the key points from that, do yourself and car a favor and stay out of boost until you know where the issue is stemming from.

If your spiking over 20psi only when you let off the throttle it sounds like your BPV is getting stuck closed...and if it has 200k without being changed I'd start there. The dealer should charger about 1-1.5hours diag to smoke test the BPV, but don't quote me on it.

Also take your factory AFR with a grain of salt, they're known to be a not-so-wideband sensor soley for emissions (why the 08 wrx/STi had so many ringland failures). I mean that sounds about right but yeah.

What fuel are you running? There are multiple "Stage 2" OTS maps from Cobb alone and using the wrong one can do major damage down the road. For example I have a stroker EJ205, full catless but stock intake so I can't really run a real Stage 2 OTS map due to airflow, but what I have done is run the stage 2 map for ACN91 (as their fuel is crap from almost anywhere else for performance) yet run 93 or higher and it seems to be an okay point for base tuning for me.

If you go to the dealer and they say it's something else or you don't have the money atm, ask them to make it run wastegate level boost (should be 10psi), that should help a bit but I still wouldn't keep drivign your car until you fix the problem.
 
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