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Old 08-01-2013, 10:25 AM   #1
Scooby Guru
Member#: 134005
Join Date: Dec 2006
Chapter/Region: MAIC
Location: Pittsburgh, PA
2004 WRX Premium

Default Stumbling on startup

2004 Forester 2.5X, 146k miles, Ganzflow intake, timing belt and water pump replaced about 5k miles ago, new cat about 3-4k miles ago due to CEL.

Car has been running great for the last three years. We purchased it with about 136k and changed the timing belt soon after as we didn't know if it was changed. Soon after, we had the typical CEL for cat inefficient and had a new stock one replaced. Fixed the CEL and everything ran fine. Not uncommon for the mileage.

In the last two weeks, when I start the car, the RPMs will go to about 2k as normal. As fast as they rev up, they drop down to lower than idle levels.. about 500 RPMs and the car stumbles. The revs shoot back up to 1.5k or so and it's fine after that. Once, when the revs dropped to 500, the car stalled out.. but usually it can recover from this.

The car runs absolutely fine after this. Full power, no stalling, no issues. Any idea of where to look next? I've tried different gas as I was in need of a fillup. Didn't change anything. Fuel filter possibly may need changing. Spark plugs and wires have unknown mileage too.

Any help would be appreciated.
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Old 08-01-2013, 01:18 PM   #2
Scooby Newbie
Member#: 67083
Join Date: Jul 2004
Chapter/Region: NESIC
Location: Northfield, VT
1993 Impreza L HB
EJ25D swap, 5-speed FWD


Due to the fact your plugs and wires generally die out around 100K miles or 5 years I would replace plugs and wires with NGK plugs and NGKs or OEM wires but nothing else. I'd try that just for routine maintenance's sake, -to measure is to know- and that you now know you have new plugs and wires you can cross them off the list of culprits... Maybe a vacuum leak or sticking idle control valve. Never hurts to change the fuel filter. Unlike other cars it's easily located behind the battery. Buy it and bolt it in 30 minutes or less.
No CEL at all? Swing by or get an OBD2 reader and see if any codes come up. Sometimes if the issue isn't "too bad" the Ecu will log a code but not necessarily throw the CEL...
All I got. Good luck bud!
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Old 08-03-2013, 09:14 AM   #3
Scooby Newbie
Member#: 280689
Join Date: Apr 2011
Chapter/Region: SWIC
Location: Glendale, Az
2010 2.5i Impreza
Newport blue


Have you lost power to the computer recently? What your describing sound like a computer relearning to me. Get the car warmed up then pull the negative battery cable and press the brake for 5 or 6 seconds or pull the ECU fuse and do the same thing. Turn off everything you can on the car. Fire it up and let it idle for at least 10 mins then go for a normal drive. If it does not work all it cost you was time.
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