Welcome to the North American Subaru Impreza Owners Club Sunday January 21, 2018
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Go Back   NASIOC > NASIOC Technical > Factory 2.5L Turbo Powertrain (EJ Series Factory 2.5L Turbo)

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads. 
Thread Tools Display Modes
Prev Previous Post   Next Post Next
Old 08-25-2010, 10:55 PM   #1
Scooby Newbie
Member#: 58709
Join Date: Apr 2004
Chapter/Region: SCIC
Location: Los Angeles
2004 WRX STi

Default Surging Idle and P0638 CEL Issue Solved

For the past few months Iíve had this bizarre surging idle problem that would manifest itself whenever I would go into boost and then slow down for a stoplight by holding the clutch in or shifting into neutral such that the engine rpms would freely drop from the higher rpm range. The rpm would freely drop from, say, 4000-5000 until they got past maybe 1500 and then it would catch itself and begin to surge up and down between maybe 2500 and 1000 rpm. And it would actually throw a Throttle related CEL code if I let it (P0638). I learned that I could prevent the code by goosing the throttle which seemed to snap the car out of the surging do-loop.

I spent all kinds of time looking for vacuum leaks; I swapped out both the ECU and the TB with used replacements, took the accelerator pedal assembly apart and cleaned the contacts, and also swapped out the front and rear O2 sensors. None of it made a lick of difference and I was pretty much at wits end and on the verge of bringing it to a trained professional.

So, with all that said, today I solved the problem. Turns out it was all due to a stuck open check valve on my catch can system (vendor is sending me a free replacement valve asap). This check valve is positioned between the intake manifold and a Tee for both the Turbo Inlet and the crankcase, and was there to prevent the intake manifold air from ever flowing into either of those places. It basically acts in similar function to the OEM PCV valve, so this failure would also be equivalent to an OEM PCV valve stuck in the open position.

Iím trying to visualize what may have been physically taking place but itís not totally obvious. Iím pretty sure that it did create some sort of weird airflow loop around the turbo under boost since much of the boost pressure was also blowing out into the intake pre-turbo. And it was also likely pressurizing the crankcase. Maybe even creating some sort of hydraulic accumulator effect such that when I shifted and the intake manifold pressure went negative, that accumulated air in the crankcase would suddenly get blown back into the intake manifoldÖ maybe that explains the surging.

I also had been smelling methanol after spraying (nozzle located in TMIC pre-TB). As it turns out, the methanol was actually getting blown all the way out through the air intake filter when I shifted and the intake suddenly/briefly pressurized. I had erroneously thought that I had developed a leak in the HSF-5 system, but now, after fixing the check valve problem, I no longer smell it.

Hopefully my troubles and final solution may help someone having similar issues. I know there are numerous posts involving throttle related CEL/Issues and Idle problems.


CNís: Stuck open check valve on catch can system causes surging idle and a P0638 CEL code.
gandm99 is offline   Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Rough idle and throwing CEL marcus712 Factory 2.5L Turbo Powertrain (EJ Series Factory 2.5L Turbo) 4 11-30-2009 02:25 PM
High Idle and P1507 cel - Need new IACV? Asterix Factory 2.0L Turbo Powertrain (EJ Series Factory 2.0L Turbo) 38 01-28-2009 12:47 AM
Rough idle issue solved via TSB sticky!!! Gregmatic Baja Forum 0 03-08-2006 12:49 AM
Erratic idle and surge shikataganai Factory 2.0L Turbo Powertrain (EJ Series Factory 2.0L Turbo) 15 02-13-2006 10:55 PM
idle and no cel cowlypso Normally Aspirated Powertrain 1 05-17-2004 02:13 PM

All times are GMT -4. The time now is 08:40 AM.

Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2018 Axivo Inc.
Copyright ©1999 - 2017, North American Subaru Impreza Owners Club, Inc.