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 Post subject: Sudden drop in pressure after 4500 rpms
PostPosted: Sun Nov 22, 2015 11:56 pm 
Fourth Gear
Joined: Thu Oct 17, 2002 1:39 am
Posts: 2091
Location: USA Roseville CA
So I'm running a RobTune 550 setup with a vf23. I can run 15psi on this setup without any issues. I finally hooked up a voltage readout to the MAF so I could see how far I could raise the boost levels before reaching the limits of the MAF.

What I'm finding is that when I raise boost over 16psi, once I get up to 4500-5000 rpms, the boost pressure drops suddenly and near instantly down to 10 psi and holds steady. Yet, if I'm lower than that, I can maintain upwards of 20 psi without any issue. I've already clamped all the hoses that could be getting loose, but I don't think this is a boost leak due to a hose at this point, especially if I can get a lot more boost going through those hoses without an issue.

Does this sound like a waste gate issue? If so I would think it would show up whenever I get above a specific boost amount.

Could a recirc/diverter valve cause something like that?

Bad turbo?

Any other thoughts? Any way I can test either of those devices?

-Ryan

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05 BMW R1200ST (excellent commuter when I don't have the pup with me)

87 Grey GL-10 Sedan (so long old buddy) - donated
97 Toyota 4Runner SR5 - sold
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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Fri Nov 27, 2015 9:09 pm 
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Are you using the stock boost control solenoid controlled by the Robtune or a MBC/stand alone EBC?

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Sat Nov 28, 2015 5:26 am 
Fourth Gear
Joined: Thu Oct 17, 2002 1:39 am
Posts: 2091
Location: USA Roseville CA
I'm running a gofastbits manual boost controller. The pressure sensors haven't been known to leak have they? Seems it can only be a hose (I've clamped and zip tied all of them), wastegate, or bypass valve as far as I'm thinking.

I'm going to pick up one of those Forge bypass valves with interchangeable springs and see if that helps at all. I'd think if it was the wastegate I'd see the problem in a more pronounced sort of way. The last wastegate I had just leaked boost and couldn't hold beyond a certain point. I actually can build boost, just loose it at a certain point in the rev range.

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91 Pearl White Sport Sedan
04 Java Black Pearl Forester XT
2014 Volvo S60 T5
05 BMW R1200ST (excellent commuter when I don't have the pup with me)

87 Grey GL-10 Sedan (so long old buddy) - donated
97 Toyota 4Runner SR5 - sold
2002 BMW r1150rt-p - sold


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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Wed Dec 09, 2015 6:54 pm 
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Location: Houston, Tx
Sorry for the late reply, but here's what I'd do to help narrow down my troubleshooting. I'd suggest running the wastegate line directly from the compressor side of the turbo to the wastegate solenoid on the turbo completely bypassing the manual boost controller.

If you still have issues then you know your boost controller and lines should be good. A leaking bypass valve likely wouldn't manifest itself like the symptoms you have, so the issue is probably sensor/ECU related or something with the wiring or hoses that feed those sensors.

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surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm


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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Fri Dec 11, 2015 5:31 am 
Fourth Gear
Joined: Thu Oct 17, 2002 1:39 am
Posts: 2091
Location: USA Roseville CA
Thanks for the tip Josh! Unfortunately I'm not sure it will help. The wastegate is probably set to 10 or so psi, I would imagine. It's a vf23, so I'm not certain what they are set at. My car runs fine on 15psi. No hick-ups or pressure drops at all. The ECU is a Robtune, and the ecu boost control was removed, so the sensors don't make much of a difference unless they are leaking in some fashion. The boost control solenoid isn't even hooked up to any of the lines right now. I'll take a look another look at it all, though.

I'm thinking about picking up one of those forge bypass valves after Christmas and see how it goes.

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91 Pearl White Sport Sedan
04 Java Black Pearl Forester XT
2014 Volvo S60 T5
05 BMW R1200ST (excellent commuter when I don't have the pup with me)

87 Grey GL-10 Sedan (so long old buddy) - donated
97 Toyota 4Runner SR5 - sold
2002 BMW r1150rt-p - sold


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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Fri Dec 11, 2015 7:57 am 
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Is it possible your 90* boot straight off the turbo collapsing under boost? WTDash and others spent a a bunch of time chasing that issue.

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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Sat Dec 19, 2015 1:27 am 
Fourth Gear
Joined: Thu Oct 17, 2002 1:39 am
Posts: 2091
Location: USA Roseville CA
Hmm, never even considered that. It is a silicone 90 degree elbow I have on the front of the turbo. I suppose I can keep that in mind when doing some more testing. It doesn't feel like it would collapse like that.

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91 Pearl White Sport Sedan
04 Java Black Pearl Forester XT
2014 Volvo S60 T5
05 BMW R1200ST (excellent commuter when I don't have the pup with me)

87 Grey GL-10 Sedan (so long old buddy) - donated
97 Toyota 4Runner SR5 - sold
2002 BMW r1150rt-p - sold


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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Sat Dec 19, 2015 6:39 am 
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Location: Des Moines, Wa
brweber352 wrote:
First off, this is my first post since finding this website 4 years ago when I bought my legacy! I've managed to find all my answers using the search engine, that's a surprise right LOL. Thanks to everyone who has posted their questions and answers, without them my car would still be stock :D .

Anyway, I saw this thread and it reminded of a problem I had a long time ago. I just posted this on Revtronix.com for someone who had similiar problems so i'm gonna cut and paste. BTW I could not find the answer to this problem when I had it, it took forever to find it.

Back when I first used a TD04 I used a 90 degree coupler to make the air intake work, this worked fine while I was running 12psi.

Later down the road I got an intercooler and a fuel cut defender and raised boost up to 16psi. When I mashed the throttle more than 50% the motor would bog and boost would fall way off. I found that the 90 degree coupler was actually sucking flat at high boost/load (this took 4ever to find BTW). I ended up cutting a 2.5'' piece of pipe at an angle so I could jam it further down the 90 coupler, short side of the angle facing the turbo and the long side facing the intake runner. After I did this I had no more problems and actually ran 17psi a couple times. The best fix is to have a metal 90 welded to the turbo. Remember that a turbo blows and SUCKS!!!

I'm not saying this is your problem but I figure it might help someone

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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Fri Dec 25, 2015 8:31 pm 
First Gear
Joined: Sun Aug 28, 2011 7:20 pm
Posts: 206
Location: Vancouver, WA/ Spokane, WA
I had the same issue with the silicon piece sucking in. I machined an insert on the lathe.


Image

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 Post subject: Re: Sudden drop in pressure after 4500 rpms
PostPosted: Sat Dec 26, 2015 4:04 am 
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Location: Des Moines, Wa
I grabbed a short (like 4 or 6") section of exhaust pipe from Auto zone, cut it even shorter and put it in the silicone 90. I brought the silicone 90* with to find the right outer diameter pipe.

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