i have a 1999 Subaru Impreza Outback Sport w a EJ20 (Phase II SOHC NA). after heat soaking (driving for about 30 minutes) i get a intermittent high idling where while im driving the engine will rev to 3000 plus (without my foot on the gas) and stay there until i either come to a full stop (this doesnt always make the idle settle down) or the current fix i blip the gas real quick which causes the engine to go back to its regular idle (around 700). i have tirelessly done everything possible to fix this including cleaning the throttle body w crc throttle body cleaner, replacing the TPS(Check engine light came on for this). cleaning the maf sensor (oem), cleaning the iacv (metal barrel with the orange o ring OEM Unit). ive also disconnected the cruise control cable inspected/lubed my throttle cable and linkage, i also tried tapping the iacv with a screwdriver while it was idling high to see if the metal barrel was stuck. the only thing that fixed the high idle besides pressing the gas pedal was disconnecting the maf (part number 22794AA010)

I ordered a new maf sensor from https://partsouq.com/en/search/all?q=22794AA010 which is the only place i could find it where it wasnt aftermarket as this on says subaru man and is from Japan. hoping its OEM and fixes my problem. i also while i was at it ordered a new IACV to just rule that out of the equation as well. i know that my likely next step (probably should have been my first step) is checking for vacuum leaks since this vehicle is old and has a ej20 swap in it. the heat soak and the blipping of the gas fixing the issue are hugh clues. im sick of asking ai for answers. i need some enthusiasts help. sorry mod if this is not the right place for this as this is my first time posting and im new to the forum. thanks for reading.

Vacuum leaks is the first place I would have gone as it sounds like it's drawing unregulated air (ahead of the MAF upstream) causing it run run lean. The fuel system then takes it rich and you are then dealing with higher revs - you don't want to experience that on a two-stroke as it leads to a runaway engine (not a good outcome).

I would run a smoke test on it and see where the cracks and splits are in the hoses, gaskets, etc very cheap and easy diagnosis and 100% accurate

On 5/5/2026 at 3:06 AM, SOC Admin said:

Vacuum leaks is the first place I would have gone as it sounds like it's drawing unregulated air (ahead of the MAF upstream) causing it run run lean. The fuel system then takes it rich and you are then dealing with higher revs - you don't want to experience that on a two-stroke as it leads to a runaway engine (not a good outcome).

I would run a smoke test on it and see where the cracks and splits are in the hoses, gaskets, etc very cheap and easy diagnosis and 100% accurate

Im going to get a smoke machine to find the issue once and for all. its been a headache for sure.

i have a huge list of upgrades and parts ill be sourcing for a complete refresh of all my hoses and a whole new set of lungs since shes a 1999. her names Frank for obvious reasons.

I. Top-End, Breathing & Performance

This section focuses on airflow efficiency and thermal management to kill that heat-soak surge.

• Intake: Cosmo Racing Short Ram (SRI) (Includes 4-pin MAF adapter).

• Intake Add-ons: Hydrophobic Filter Sock & DEI Gold Reflective Tape wrap.

• Thermal Isolation: GrimmSpeed 3mm Phenolic Spacers (PN: 007002).

• Oil Control: IAG V3 Street Series AOS (Full PCV bypass for your JDM EJ20).

• Throttle: WRX Throttle Cable + Angry **** Delrin Bushings.

• Fuel System: 7ft Gates Barricade Hose (225 PSI) + 14 Fuel Injection Clamps.

• LOKOCAR Silicone Hose Vacuum Hose Kit Fits For Subaru Impreza GC8 GF8 STI EJ20 WRX (Replacing lines one-by-one).

II. Bottom-End, Oiling & Structural

Building a rock-solid foundation for the high-mileage JDM engine.

• Oil Pan Kit: 2006+ STI Oil Pan (PN: 11109AA151) + Dipstick & Windage Tray.

• Oil Pickup: Killer B Ultimate Pickup (PN: 042).

• Timing: Aisin TKF-001 Kit (Belt, idlers, and water pump).

• Oil Pump: Subaru OEM 10mm Pump (PN: 15010AA300).

• Mounts: Group N Engine Mounts (PN: D1010FE020).

• Sealant: ThreeBond 1217H (12-hour cure required).

III. The "Premium" Performance Exhaust

The high-flow setup we selected to maximize your JDM engine's output while maintaining the STI pan clearance.

• Full System: Inferno Fabrications Full Exhaust (304 Stainless, 2.25").

• Header Type: Equal Length Dualport Upgrade (For maximum performance/scavenging).

• Catalytic Option: CATTED (To maintain NA backpressure and ECU stability).

• Resonator: Short MagnaFlow (Balance of high flow and refined tone).

• Muffler: MagnaFlow Axle-back (Included in full kit).

IV. "While You're In There" Reliability Items

Hard-to-reach parts to swap while the manifold and pan are off.

• Top-End Seals: 2x Intake Manifold Gasket sets (PN: 14035AA383) & Valve Cover Gaskets/Tube Seals (PN: 10966AA030).

• Sensors: New Knock Sensor (PN: 22060AA140).

• Cooling: Coolant Crossover O-Rings (PN: 806933080).

• Rear Engine: Upgraded Stainless Steel Oil Separator Plate (PN: 11831AA210) & Rear Main Seal (PN: 806786040).

• Misc: Oil Cooler O-Ring (Orange, PN: 21317AA040).

If theres anything an enthusiast who would like to add any while im in there maintenance please do let me know. im gonna be chasing that leak and replacing those vacuum lines first and replacing my AOS at the same time. gonna find where the leak is coming from.