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Losing boost pressure on occasion

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I have looked everywhere for people with similar symptoms and can't seem to find any. I have a 2006 legacy gt with AVO drop in filter and TMIC, sti UP, Cobb DP with cat, borla CBE an its tuned by infamous1.


I noticed awhile back that I was getting a turbo whine that I kinda just assumed was normal. After gettin the car tuned it ran fantastic. About 2-3 months ago I started loosing boost until my car had been ran for a long time (8-10psi). I made me a brilliantly engineered boos leak tester with an empty cup holder style container of dash wipes, a valve stem and transfer case rear output shaft seal for a Nissan Titan.




I plugged the exhaust an pumped the system with smoke on a evap leak smoke machine. I found one heck of an UP leak so I replaced that gasket and fixed that. Now I still lose boost but not as often or for as long. I never build boost until my thermostat is fully warm and now someone's it only hits 8-10psi until its been driven a little longer than it being warmed up and before it was almost all the time. Now when it does act up sometimes it will kick in full boost on a test pull to see of its working but it will spike 8-10 and hold until its higher rpm then shoots to 16-17psi and then will be perfectly normal until I shut the car off and let it cool down again.


I have checked for boost leaks doing the smoke thing but if I try to pressurize it more than 4psi there is a valve that makes a noise so I can't hear any leaks over it and I was told that valve is supposed to do that. I was thinking it may be my oem bcs but the fact that its electronic it would either work or not work, no in between but it's also solenoid still and had mechanical moving parts so I don't know. I'm kinda at a loss at the moment.


If anyone has seen this before or has some useful tips please help.

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I would be very happy if it was just a tune issue. But that kinda makes sense maybe, I got my car tuned before I realized I had an up pipe leak and I know it was there at the time because of the whine from the gasket acting like a reed on a wind instrument. Maybe that made it run a little richer so it got leaned out to compensate and now that its not leaking it is too lean.
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The 9% on learning D doesn't sit well with me.

Maybe you're running lean under load and knocking enough for the ecu to drop IAM and disable boost control, causing you to run on spring pressure?


except 10% is not that much and IAM isn't dropping. Unless he has 15 miles since the last reset of the ECU.

(Updated 8/22/17)

2005 Outback FMT

Running on Electrons

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At the time of the log and LV I just did it was working properly but I'm not exactly sure how much/little driving it takes to change the LV. I'm going to assume it depends on how many miles that have been driven sense the last ecu reset. I did reset it recently but its been atleast 200 miles ago.
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You have to post a log of when this is occurring. Until you do that, this is just guesswork. And contrary to what mwiener says, adding 9% fuel is not insignificant or something I would just dismiss. It's also unusual that none of your other trims are affected (they look perfectly normal.)


Get a log.

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Yeah when I did the last log it was working properly but I'm kinda afraid to drag out the RPM's when it's doing it. It's hard to have a laptop with me at all times because I use my sisters. Hopefully I'll get lucky soon.


I also find it odd it usually only does it when the temperature is could out. Usually happens below 40° but last night was 35° and it was doing fine. I'm also not too familiar with tuning and what not so does that 1.05 on the knock table mean much?


Also I don't know how to post a log.

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LTFTs are limited to +/-15% unless a tuner specifically changes it. I'm not sure what the immediate AF Correction #1 limit is, but I think 25% might actually be right for that...


Furthermore, a high LTFT in either direction (+/-) in Trim D isn't good... You've been on here long enough that you should know why it's potentially bad.


A 9% deviation is more than what a majority of people here would be comfortable with... And certainly not a 9% positive correction. A healthy LTFT tends to trend negative a tiny bit.

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I will try to get a log of it tomorrow after I get off work. I'll drive my truck and hopefully when I get to my car I can get a log in before it starts working properly.


I appreciate both your guys's help and knowledge but there is no reason to argue on the thread it's just going to make this thing more frustrating than it already is.


When I do get this log does anyone know how to post one?

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Post the log by going to "Go advanced" on the bottom where you type your message. Then look for the "manage attachments" button and click it. Upload your log file.


Also, LTFT = Long Term Fuel Trim. It's how your ECU adjusts fueling to achieve the targetted Air/Fuel Ratio (AFR). This type of learning occurs when you are in Closed Loop (CL) fueling, which means the ECU is taking readings from your front oxygen (O2) sensor to achieve a desired 14.7 AFR.


Trim A = 0 to <5.60g/s

Trim B = 5.60 to <10g/s

Trim C = 10 to <40 g/s

Trim D = anything over 40 g/s


Trim A is basically idle

Trim B is basically coasting

Trim C is light throttle/cruising

Trim D = moderate throttle/WOT


A positive number means you are adding fuel because the ECU feels you are running too lean (not enough fuel). A negative number means it thinks you're running too rich and thus it's subtracting fuel.


Somewhere at around 40-60g/s, the transition from Closed Loop to Open Loop fueling occurs. Open Loop means your ECU just blindly fuels your car based on your tune/map's Primary Open Loop Fueling table and it doesn't take readings from your O2 sensor to make adjustments.


This is where Trim D becomes important, because between 40-60g/s, you might still be in CL where some learning can occur. Once you're in OL, whatever you had previously learned in Trim D is blindly applied to your fueling.


In your case, for example, you have +9% fuel. This means your ECU is adding 9% fuel anytime you are at an airflow of >40g/s, which is anything over moderate throttle.


You might think to yourself, "Hey, that's stupid. Why would the ECU blindly apply fueling compensations without any feedback from a sensor?" and you'd be right to wonder. There are conflicting opinions here about that.


Some people feel that it is stupid, so they essentially disable Trim D. I'm not going to get into how to do that... Only that it can be done.


My personal opinion is that disabling Trim D isn't a good way to do things. Instead, adjustments should be made to the tune to minimize any fluctuations in Trim D, or ANY of the trims for that matter. That's why you'll see a lot of people here saying +/-5% in any of the trims is okay.


Larger numbers often indicate either a hardware (vacuum/intake leak, bad gasket, MAF sensor fouled, etc.) or a tune problem (MAF sensor not scaled properly, etc.) which is why your 9% caught my eye.


The reason I feel that disabling Trim D isn't a good idea is because the Trims aren't just there for fuel economy reasons (targetting 14.7 AFR) but also for safety. If you have a condition (such as vacuum/intake leak) causing a lean condition in your car (which may cause knocking = not good), your ECU can learn to compensate, up to a certain point, for each trim. If you have Trim D disabled though, your car will never compensate for a problem, and thus you will run lean under moderate to heavy throttle.


Is this a perfect system? No, but it's better than nothing.


That's my thoughts on the matter at least.


Hope this explains things somewhat... and I apologize for the super detailed response. :)

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I appreciate the detailed response, that helps to explain a lot.


I sent a log before I fixed my UP leak and he said I was getting a little bit of knock right as boost came on and up top but not much but it suprised him because my timing wasnt very aggressive at all. He also said my car was trying for all it was worth to hit target boost and I'm going to assume that was due to the UP leak that was quite large.


Does it make sense that if it was tuned with the leak that the tune would have been set to compensate for the leak and make it run not as rich because the turbo wasnt getting fed what it needed and then when I fixed it it all of a sudden had more flow so more fuel was needed and thats maybe where the +9% is coming from.

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