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Old 05-31-2020, 05:40 PM   #1
Scooby Newbie
Member#: 421414
Join Date: May 2015

Hello NASIOC,*

* * * * * I am a seasoned Subaru specialist operating an independent pro shop (BRAP @ Snoqualmie Pass, WA) for 3 years now, and I wanted to contribute this information as I feel it may be valuable to other members trying to diagnose low/delayed boost issues. Recently an 06 Forester XT with 250,000 miles that I had built a motor for 60,000 miles ago that all of a sudden starting lagging uphill and expressing poor fuel economy and seemingly low power. This became an interesting and challenging issue to diagnose, so I thought I would itemize and elaborate on a way to track down the cause of low/delayed boost issues. I have built many EJ engines and modified/installed them in any way possible, but I had never been presented with this issue, which ended up being a partially clogged downpipe. In my research I have only come across one mention of one clogged OEM downpipe, and never in my personal experience had I encountered a clogged downpipe (though I have seen several clogged cats on n/a Subarus) until this point. I wanted to encourage other mechanics or skilled Subaru wrenches to explore that it could be a possibility in a low/delayed boost situation, so I wrote this up. It should also be noted that the catted, non-STI up-pipes common on our Forester XT's, WRX, and Legacy GT's are massive failure point that can also cause low boost and even worse disintegration causing subsequent turbo failure.
This is simply a resource to give an example of how we locate a boost leak, some may proceed differently, AND I ACCEPT NO RESPONSIBILITY SHOULD YOU DAMAGE YOUR VEHICLE ATTEMPTING THESE STEPS.
1) Datalog your A/F correction, IAM, Manifold Pressure, Fine Knock, and Wastegate Duty, focus on any irregularities.
2) Compression and or Leakdown tests.
3) Fuel pump/regulator test (T into fuel inlet, standard psi is 35 at idle, and +1psi per 1psi increase, ie 10psi boost=55psi)
3) Intake boost leak test (pressurize 10psi into the turbo inlet, block the crankcase breathers, spray soapy water)
4) Pre Turbo Exhaust leak test (pressurize tailpipe, spray soapy water at all gaskets, look for bubbles/pressure drop)
5) Wastegate function test (pressurize wastegate, observe actuator arm function, inspect turbo/wastegate vacuum T)
6) Boost Control solenoid test (apply 12v to solenoid and verify air passes through the solenoid when energized)
7) Verify mechanical boost target is being met. Start with just connecting a vacuum line from the turbo compressor nipple straight to wastegate nipple. On the 04-08 Forester XT, the mechanical spring pressure of wastegate opened at 5psi when I measured with an air regulator. This should QUICKLY (before 3000rpm) result in 5psi of manifold pressure relative to sea level on a datalog. I do not recommend disconnecting the wastegate acuator entirely, as this is where you could easily blow your engine if you aren't carefully monitoring how high your boost is going.*
8) Inspect and ditch the catted up-pipe, and swap an OEM STI or aftermarket catless pipe since you're already in there.
9) Inspect Turbocharger internally for restricted turbine or other damage that could be limiting spool.
IF YOU AREN'T MEETING MECHANICAL BOOST PRESSURE BEFORE 3000 RPM, AND ALL OF ^THESE ITEMS^ HAVE CHECKED OUT, YOU VERY LIKELY HAVE A PARTIALLY CLOGGED DOWNPIPE. I am in the process of trying to create a flow test for a down pipe, but considering the high amount of atmosphere flowing through a down pipe during boost, it has proven to be very hard to measure flow on a pipe that is only partially clogged.
Here is the account of discovering the clogged downpipe:
* * * * *After some initial datalogging, the XT was hitting a MAX of 7-8psi, and not until around 5,000 RPM. Target boost is 12psi by 3600. At 3600rpm the car was barely hitting 4psi, slowly increasing to 7 by 5000 and holding to redline. AFR corrections were acceptable in open loop, IAM ran steady at 1.0, now knock, duty cycles were below max levels, and no timing was being pulled.*
* * * * *The car was coming up it's 60k mile interval since I built the engine, so luckily we could test the compression and leakdown as I was already going to replace the spark plugs. Everything checked out well, so we ran a fuel pressure test and it held well also.
* * * * *First suspect would be a boost leak, pressurized the inlet to 10psi and it held fine, soapy water revealed no bubbles or areas of concern. Next I pressurized the tailpipe to 10psi and sprayed soapy water everywhere again to check for pre-turbo exhaust leaks and everything was good on that side as well.
* * * * *Moved onto testing the wastegate actuator, directly pressurizing the wastegate diaphragm nipple with an accurate pressure gauge. It opened just at 5psi of pressure like normal, and there was no slack in tension on the wastegate actuator arm to the exhaust gate on the turbo. Then i took it a step further to inspect the actual turbo, which revealed no excess shaft play, or difficulty for the turbine to spin freely. I also checked the action of the boost control solenoid and it operated effectively with 12v applied, opening and allowing air to pass through.*
* * * * *Next I ran a single vacuum line from the turbo compressor housing nipple straight to the wastegate diaphragm, which should have resulted in a quick and maximum boost pressure of around 5psi. It built up to 2psi at 3000 and maxed at 5psi around 5000.
* * * * *At this point I wanted to see if the turbo could even mechanically produce over than 7psi of pressure, so i disconnected the boost reference line to the wastegate from the turbo compressor housing, blocked it off, and logged a couple of WOT pulls. Still only hitting 7 PSI. At this point it was obvious there had to be a restriction on exhaust flow, as there was nothing else that could possibly restrict boost without the wastegate being active. I had a practically new OEM STI downpipe in the shop. After swapping it on, boost was immediately back to stock levels, plus 1 psi or so.*
* * * * In conclusion, it is important for our community to know that this is possible for our downpipes to clog and begin restricting boost, thanks for reading, and I hope this helps track down your boost issues.
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Old 06-03-2020, 10:48 PM   #2
Dave D.
NASIOC Supporter
Member#: 130990
Join Date: Nov 2006
Chapter/Region: NWIC
Location: Menzel Lake, WA
06 Red Ltd.Wagon
Dom 1.5xtr JJ tuned


I lost a motor due to a clogged cat overheating a cylinder years ago. Thanks for posting the info, wish I'd read it back then!
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Old 06-06-2020, 11:45 AM   #3
Scooby Newbie
Member#: 421414
Join Date: May 2015

Originally Posted by Dave D. View Post
I lost a motor due to a clogged cat overheating a cylinder years ago. Thanks for posting the info, wish I'd read it back then!
I'm sorry to hear that!
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Old 06-28-2020, 06:31 PM   #4
Scooby Newbie
Member#: 187315
Join Date: Aug 2008
Chapter/Region: MWSOC
Location: Chinatown
2007 WRX TR


Thanks for posting this. Iíve been trying to troubleshoot a low power low boost issue on my 2007 WRX TR, car has about 60k miles. I installed a TD05-18G with supporting fuel mods and in my current tune runs 22 PSI tapering off to 18 psi. The tuning was by me using Romraider and I aimed for a safe knock free tune with conservative timing in high load areas with afr of 11:1.

I ran this tune for a while (weeks) after I was happy with the results but then I noticed that max boost was down to 19psi one week and worsened to 12psi eventually.

I havenít done a compression test yet but my boost gauge shows strong vacuum (-20) at idle and it passed the oil cap smoke test.
Generally the car runs and feels strong with the only symptom being slow to build boost.
I also did the wastegate tests you mentioned in the car boosts the same. Much slower and sluggishly than before and only reaching 12 psi max.

Assuming the motor is healthy as is the new turbo (about 2000 miles on it) I suspected possibly a pre turbo exhaust leak or maybe a clogged downpipe, which is when i came across this post.
The downpipe is an aftermarket catted (SRS brand I think) and has about 50K miles on it.

Thanks for the information, Iím going to pull the downpipe off in the next week or so and check it out. Hopefully thatís it ...Iíll try and report back whatever I find.
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