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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.
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