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Old 07-05-2012, 09:09 PM   #1
5knotwinds
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Default Should I be concerned with feedback knock at high rpms? Help reading logs please!

Finally got around to data logging my 07 WRX. Not too sure what I'm all looking at. Feedback Knock is bad, correct? So is this really bad?

2007 WRX - 70,000 miles
catback exhaust
Cobb stage 1

Run 1:
https://docs.google.com/spreadsheet/...oak4wWGc#gid=0

Run 2:
https://docs.google.com/spreadsheet/...ZeFdQRHc#gid=0
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Old 07-05-2012, 10:25 PM   #2
Black94Snake
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you have a DAM of .62 ... thats bad!

Clean your maf, check for pre-turbo leaks. You have something severely wrong going on.


my reasoning: 150 g/sec maf readings at 6000 rpm with 10# boost...
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Old 07-05-2012, 11:04 PM   #3
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Crap. Not what I wanted to hear. Got the car a couple years ago (first boosted car) and just left it the way it came. Haven't had a problem with anything so haven't really looked at or done anything with it since except change the oil. Looks like I got some reading to do now!

edit: I did flash a fresh stage 1/91 map this morning and just drove to and from work (15 mins each way). This was the first time updating the AP firmware and maps since getting the car. It was about 100 degrees and humid outside during the runs. Not really much of a car guru so I hope I'm not too screwed. :-/ Gave the MAF a good cleaning tonight also.

Last edited by 5knotwinds; 07-06-2012 at 01:09 AM.
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Old 07-06-2012, 08:39 AM   #4
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Quote:
Originally Posted by 5knotwinds View Post
Finally got around to data logging my 07 WRX. Not too sure what I'm all looking at. Feedback Knock is bad, correct? So is this really bad?

2007 WRX - 70,000 miles
catback exhaust
Cobb stage 1

Run 1:
https://docs.google.com/spreadsheet/...oak4wWGc#gid=0

Run 2:
https://docs.google.com/spreadsheet/...ZeFdQRHc#gid=0
You have severe knock and given the age of the car, some more extreme positive long-term fuel trims (A/F Learning 1), and the low reported airflow, the most likely culprit is a bad mass airflow (MAF) sensor. Because of how critical this sensor is, I would not bother with attempting to clean the sensor - just replace it with a new one. After you do so, reset the ECU via the AccessPORT's troubleshooting menu. Then get another run in. Before the run, reset your log list to default list (Monitor -> Reset Log List). If you can, get a passenger to come along for the run and monitor feedback knock correction via the AccessPORT's live data (while you are logging). If they see multiple counts of correction (-2.8 or worse) in the middle of the run, then instruct them to tell you to quit the run early.

Bill
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Old 07-06-2012, 09:21 AM   #5
5knotwinds
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Thanks Bill, I'll order a new MAF (of course the local Subaru dealer doesnt have one) and see if that helps.

So a bad MAF could cause all that knock or are we just scratching the surface on this one and I should expect to be replacing more?
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Old 07-06-2012, 09:38 AM   #6
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Originally Posted by 5knotwinds View Post
Thanks Bill, I'll order a new MAF (of course the local Subaru dealer doesnt have one) and see if that helps.

So a bad MAF could cause all that knock or are we just scratching the surface on this one and I should expect to be replacing more?
A bad MAF where airflow is underreported will cause you to run lean with too much ign. timing, which can most certainly cause severe engine knock (and potential engine damage). Other possibilities that go along with extreme long-term fuel trims (and knock) include post-MAF intake tract leaks, bad front o2 and other fueling issues. But, given all the evidence here, I think the most likely culprit is the MAF sensor. No guarantee that this is absolutely the problem (or the only problem), but I think this is definitely your best bet. Given how critical that sensor is - you don't want to take any chances. Unless you have another MAF sensor on hand (say from a friend's compatible Subaru) that you can swap out as a test, you'll need to just get a new one and see what happens.

Bill
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Old 07-06-2012, 09:59 AM   #7
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Thank you very much for the replies this morning on this issue. I'm assuming it's recommended to get the OEM Subaru MAF as a replacement? Just wanted to double check because I see OEM ones for about $230 and off brand ones for around $80. Since it is a critical sensor, go OEM?
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Old 07-06-2012, 10:05 AM   #8
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Quote:
Originally Posted by 5knotwinds View Post
Thank you very much for the replies this morning on this issue. I'm assuming it's recommended to get the OEM Subaru MAF as a replacement? Just wanted to double check because I see OEM ones for about $230 and off brand ones for around $80. Since it is a critical sensor, go OEM?
Personally, I would go with OEM in this case, but that is entirely up to you.
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Old 07-06-2012, 12:24 PM   #9
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I would do a check for leaks in your turbo inlet before spending the money on a new MAF sensor. There is not enough data to justify a MAF just because the fuel trims are positive; positive fuel trims could be due to many different things. Throwing parts at a car is a horrible way to "diagnose" a problem. I'm disappointed someone from Cobb would do that.
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Old 07-06-2012, 12:38 PM   #10
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Originally Posted by 02RexWI View Post
I would do a check for leaks in your turbo inlet before spending the money on a new MAF sensor. There is not enough data to justify a MAF just because the fuel trims are positive; positive fuel trims could be due to many different things. Throwing parts at a car is a horrible way to "diagnose" a problem. I'm disappointed someone from Cobb would do that.
The suggestion is not based on just the extreme positive fuel trims. His reported airflow is low, which is typical of a bad MAF. A post-MAF leak is not going to impact reported airflow (the whole reason a post-MAF leak causes fueling issues is because air is lost or gained after it has been metered by the MAF).

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Old 07-06-2012, 12:45 PM   #11
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How would a post MAF leak not imact reported airflow? If the air isn't going through the MAF sensor (say, sucked in through a tear near the inlet of the turbo) it isn't going to be reported...
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Old 07-06-2012, 12:52 PM   #12
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How would a post MAF leak not imact reported airflow? If the air isn't going through the MAF sensor it isn't going to be reported...
Because you are losing or gaining after the air has already been metered by the MAF sensor (hence the distinction of a post-MAF leak). If a post-MAF leak were somehow metered accurately by the MAF sensor, then you would have no fueling issue at all. The fact that you go rich, for example, when you have a boost leak is because your actual airflow is less than the metered airflow by the MAF sensor. The opposite happens when you are adding to airflow due to a leak (you run lean because you have greater airflow than what the MAF metered).

Bill
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Old 07-06-2012, 12:57 PM   #13
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My mistake, I should have explained what I meant by leak. I was meaning a rip in the turbo inlet where air can get in, and not be sensed by the MAF sensor.
The reason I called it a "leak" is because they show up as leaks when you pressurize the turbo inlet to find them.

I have seen many torn turbo inlets (near the hose clamp around the compressor cover), that cause lean conditions, and positive fuel trims. This would have to be a pretty bad leak, but it's worth checking out before spending the $200+ on a new MAF sensor.
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Old 07-06-2012, 01:13 PM   #14
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Originally Posted by 02RexWI View Post
My mistake, I should have explained what I meant by leak. I was meaning a rip in the turbo inlet where air can get in, and not be sensed by the MAF sensor.
The reason I called it a "leak" is because they show up as leaks when you pressurize the turbo inlet to find them.

I have seen many torn turbo inlets (near the hose clamp around the compressor cover), that cause lean conditions, and positive fuel trims. This would have to be a pretty bad leak, but it's worth checking out before spending the $200+ on a new MAF sensor.
I'm using the word "leak" to refer to losing or gaining air - same concept applies - you are gaining unmetered air, which is why you go lean (and hence the positive fuel trims). If the reported airflow looked reasonable and he only had positive fuel trims, then I would put the post-MAF "leak" up there as more likely. But, with low reported airflow, it puts the MAF sensor up there as most likely.

Certainly, though, a pressure/smoke test of the intake is a good idea anyway. Wouldn't be impossible to have both a bad MAF and a leak. If you know someone with a turbo Subaru without issues, you can always temporarily swap their MAF sensor to your car as a test before you buy a new one.

Bill
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Old 07-09-2012, 06:02 PM   #15
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Got a new MAF sensor today and put that in. Reset the ECU with the AP. Let my car run for about 10 mins and then did 3 more test runs. The car no longer hesitates from 2500-3000 RPM (yay!) and seems to feel better in the short time I drove it.

Bill, how do these logs look now?

Run1 - New MAF:
https://docs.google.com/spreadsheet/...oU19la2c#gid=0

Run2 - New MAF:
https://docs.google.com/spreadsheet/...xb0RFcFE#gid=0

Run3 - New MAF:
https://docs.google.com/spreadsheet/...vMEIxbXc#gid=0

I watched the DAM all weekend while driving around with the bad MAF and it never moved from 0.6.. Reset the ECU today and did my few runs but not much driving after resetting it. Should this number move up to 1 eventually?

Last edited by 5knotwinds; 07-09-2012 at 06:34 PM.
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Old 07-09-2012, 06:51 PM   #16
Black94Snake
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Much better. Your MAF is now seeing more than 150 g/sec. Now stop driving like a dil-hole and let the DAM rise.
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Old 07-09-2012, 08:06 PM   #17
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Edit, thank you Black94 for taking a look.

Last edited by Solhays85; 07-09-2012 at 09:00 PM.
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Old 07-09-2012, 11:16 PM   #18
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Quote:
Originally Posted by 5knotwinds View Post
Got a new MAF sensor today and put that in. Reset the ECU with the AP. Let my car run for about 10 mins and then did 3 more test runs. The car no longer hesitates from 2500-3000 RPM (yay!) and seems to feel better in the short time I drove it.

Bill, how do these logs look now?

Run1 - New MAF:
https://docs.google.com/spreadsheet/...oU19la2c#gid=0

Run2 - New MAF:
https://docs.google.com/spreadsheet/...xb0RFcFE#gid=0

Run3 - New MAF:
https://docs.google.com/spreadsheet/...vMEIxbXc#gid=0

I watched the DAM all weekend while driving around with the bad MAF and it never moved from 0.6.. Reset the ECU today and did my few runs but not much driving after resetting it. Should this number move up to 1 eventually?
The MAF sensor was clearly the issue - your reported airflow now looks good and knock is no longer an issue. It goes to show how critical the MAF sensor is to proper operation - when you have an issue with it, it can cause all kinds of problems.

The DAM will learn up to 1.0 eventually. For these maps, moderate steady boost from 2000-3000 RPM should speed things along.

Bill
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Old 07-09-2012, 11:27 PM   #19
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Thank you so much for the replies one this, everyone. I did also run a boost leak test and could tell there was a small leak somewhere that I have to find/fix yet, but things seem to be running much better now. I'll keep an eye on that DAM and make sure that moves.

Stage 2 next
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Old 07-16-2012, 01:53 PM   #20
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How long goes the DAM usually take to learn up to 1? After the new MAF last week, it learned up to 0.9 (displayed on the AP) but never hit 1. I reset it again yesterday just to try and watch it learn up but it's stiill @ 0.6. I'm probably just watching it too close now but I'm not really sure what to expect here. Thanks!
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Old 07-16-2012, 02:41 PM   #21
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Stop reseting your ecm...
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Old 07-16-2012, 02:48 PM   #22
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I understand resetting it is not helping but I'm just curious how long it normally takes. A week? A month? A tank of gas? I'll be flashing to Stage 2 later this week so unfortunately it will be reset once again soon. Just wondering what a normal time frame is. Sorry, just trying to learn more.
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Old 07-16-2012, 03:07 PM   #23
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Quote:
Originally Posted by 5knotwinds
I understand resetting it is not helping but I'm just curious how long it normally takes. A week? A month? A tank of gas? I'll be flashing to Stage 2 later this week so unfortunately it will be reset once again soon. Just wondering what a normal time frame is. Sorry, just trying to learn more.
The answer is coded into your tune. The only way to know 100% is to look at the actual tune.

Typically it should not take longer than a single driving cycle.
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Old 07-16-2012, 03:25 PM   #24
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Typically it should not take longer than a single driving cycle.
Ok, thanks for the info. Maybe keeping an eye on this isn't a bad idea afterall. I was driving around daily for a week (at least two 20 mile trips per day plus some extra driving here and there) making sure I was giving moderate boost between 2k-3k rpms like Bill suggested earlier and the DAM was still hanging at 0.9.

I'll try to be more patient and see how things go.
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Old 07-17-2012, 09:03 AM   #25
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Quote:
Originally Posted by 5knotwinds View Post
Ok, thanks for the info. Maybe keeping an eye on this isn't a bad idea afterall. I was driving around daily for a week (at least two 20 mile trips per day plus some extra driving here and there) making sure I was giving moderate boost between 2k-3k rpms like Bill suggested earlier and the DAM was still hanging at 0.9.

I'll try to be more patient and see how things go.
These maps are a little slower to raise the DAM (something we will fix in a future map update). You can give it some time (and the mentioned moderate steady boost around 2000-3000 RPM helps). The other options would be to make a change to the map with the AccessTUNER Race software. If you feel comfortable doing this, I can walk you through it. Here's how you get the free software:
http://www.cobbtuning.com/accesstune...st-s/70716.htm

Bill
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