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Old 04-21-2020, 07:33 PM   #1
someoneza
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Member#: 461463
Join Date: Jan 2017
Chapter/Region: BAIC
Location: San Francisco, CA
Vehicle:
2004 WRX wagon
Silver

Default Boost will not exceed 8 PSI on 2004 WRX; need help!

I have a 2004 WRX and my boost pressure will not go above 8 PSI, even with WOT, and was hoping someone could help me figure out why. I recently made the following upgrades:
Cobb high-flow cat downpipe
Grimmspeed catless up-pipe
Samco silicone intercooler hoses
XPT stage 2 tune (target boost of 18.5 PSI)
Stock intake and filter


After reading this forums endlessly, I have ruled out the two most likely culprits (I think):
- WG actuator/valve: I have confirmed that I cannot blow air through the WG actuator valve when car is off. I connected 12V to the valve, I heard a click, and I can blow air. I have also pulled the vacumm hose off the turbo outlet, connected compressed air to this hose, and watched the WG actuator arm move up to about 1/2". I then connected 12V to the WG valve, and the compressed air was released and the WG closed again. This all appears to be normal.
- Boost leaks: I have boost leak tested my entire intake from the intake elbow and found no leaks. I tested up to 15 PSI, and it takes about 10 seconds to drop to 10 PSI, and another 40 seconds to drop to 0 PSI. The only place I can hear air escaping is from my oil filler cap.

The maximum boost I can make is always between 7.9 and 8.9 PSI, which sounds suspiciously like wastegate spring pressure. However, I don't know what could be causing this, per my above test. I have also tried pulling the vacuum hose off the WG actuator nipple, and cautiously driving around, but boost still will not exceed 8 PSI. My IAM is at 16 (the highest value for this generation WRX).

I have attached a log of a recent 3rd gear pull. It includes AFR data from my AEM UEGO wideband. I am new at this, so hopefully there are some useful clues in the logs that I missed.


Link to logs: https://drive.google.com/file/d/1c93...ew?usp=sharing
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Old 04-23-2020, 07:20 PM   #2
bugeyerex02
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Member#: 195967
Join Date: Nov 2008
Chapter/Region: NESIC
Location: Norther Vermont
Vehicle:
2002 WRX
WR Blue

Default

Restrictor pill?
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Old 04-23-2020, 07:37 PM   #3
91 RS RA
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Have you checked for any codes?
Could it have gone into a limp mode?

Are you able to reflash the tune, maybe that will reset the issue or at least help with where the issue isn't
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Old 04-23-2020, 10:52 PM   #4
someoneza
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Silver

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Yeah, I've confirmed that my restrictor pill is still there.

No CEL light and no error codes. I have tried reflashing the tune (as well as flashing to a different tune where I was previously making 16 PSI), but still not making more than 8 PSI boost.

Any thoughts on whether this could be a problem with the BPV, causing it to open at 8 PSI?
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Old 04-24-2020, 06:58 AM   #5
Unabomber
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REPLACE your entire set of vacuum lines with OEM new. At this age, you need to also replace your factory boost control solenoid and get a new Denso MAF (you can get this off amazon). Do this and then you will KNOW your system is great and it likely will fix the issue, if not you can stand on a high mountain knowing what is new. I have an 04, these things need replacement, not check as they all can look and feel fine but actually not be.
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Old 04-24-2020, 10:06 AM   #6
Blaze13rex
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Ice Silver Metallic

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If everything is in order it is the factory boost control solenoid that has failed. JDM or USDM are the same
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Old 04-30-2020, 01:32 AM   #7
someoneza
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2004 WRX wagon
Silver

Default

Hey everyone, thanks for the help so far. I've tracked down two problems so far, based on the advice here:

1. I have a custom boost gauge that taps into the electrical output of the MAP sensor. My boost gauge was limiting the output of the MAP sensor to 3.3V, which equates to 8 PSI. I figured this out by boost leak testing the intake up to 15 PSI, and watching the boost value on the ECU and my boost gauge. Both were stuck at 8 PSI, but as soon as I unplugged my gauge, the ECU boost reading jumped up to 15 PSI. Entirely my fault there.

2. After fixing the problem with the boost gauge, I was still only getting 8 PSI while driving around. I then pulled the vacuum hose off the WG actuator nipple, and with careful throttle control, got the boost up to 14 PSI. Bam! The factory boost control solenoid was bad, as you guys said. It seemed to be working when I manually tested it with compressed air, but I guess it was sticking open when the ECU pulses it quickly.

Per Unabomber's and Blaze13rex's suggestions, I am replacing both the factory BCS and the MAF. The car is at 150k miles, so you are definitely correct that the MAF probably needs replacing too.

I have replaced most of the vacuum lines by this point (they were either split or rock-hard and not sealing properly), but I will take your advice and replace the last few too.

Thanks everyone for the help
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Old 06-03-2020, 02:58 AM   #8
someoneza
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2004 WRX wagon
Silver

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Now that I have fixed my boost problems, I just thought I'd close out this thread, in case anyone else runs into the same problem. Per above suggestions, I replaced MAF, BCS and the few remaining vacuum hoses. After doing this, my boost increased only very slightly to about 9 PSI. After thinking I was going crazy, I realized that I made a rookie mistake and accidentally threw out the restrictor pill when I replaced the last few vacuum hoses. After getting a new pill from Subaru for a whole $2, my car is now making 17-18 PSI boost. Thanks everyone for the help.

As an aside, my target boost is actually 16.8 PSI, so I am now slightly overboosting. I will try get hold of my tuner for advice (XPTTuning/Braden), but if I have no luck, I'll post a separate forum thread about that.
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