Is Your 2020 Subaru WRX Losing Power? Check for Intake Leaks

The Basics

An intake manifold problem, often flagged by code P0172 (System Too Rich), occurs when the engine receives too much fuel or not enough air. Based on 168 discussions, this is a common performance issue where the air/fuel mixture is disrupted, leading to reduced power, poor fuel economy, and potential long-term engine damage if ignored.

Symptoms

Owners consistently report these specific signs:

• Check Engine Light (CEL) with code P0171 or, more frequently, P0172.
• Noticeable power loss, especially under acceleration or at higher RPMs.
• Poor fuel economy that develops suddenly.
• Rough idle or unstable engine operation at a stop.
• Black smoke from the exhaust, indicating unburned fuel.

Common Causes

Analysis of successful fixes points to these primary culprits:

1. Faulty or Sticking Intake Manifold Runner Control (IMRC) Plate or Actuator (Most Common): The IMRC controls airflow inside the manifold. A stuck plate or failed actuator (often called the "IMRC dam") disrupts airflow, directly causing a rich condition (P0172). This was the confirmed fix in the majority of cases.
2. Vacuum Leaks: Cracks or disconnections in the intake manifold itself, its gaskets, or associated hoses allow unmetered air in, confusing the engine computer.
3. Clogged or Faulty Mass Air Flow (MAF) Sensor / Filter: A dirty air filter or contaminated MAF sensor sends incorrect air volume data to the ECU, leading to improper fuel delivery.
4. Failed Fuel Pressure Regulator or Injectors: Less common in this dataset, but leaking injectors or a faulty regulator can cause excess fuel to enter the manifold.

Repair Options

Start with the simplest and least expensive diagnostics.

1. Diagnose IMRC System (First & Most Likely Fix):

   • Action: Use an OBD2 scanner with live data or a tool like a Cobb Access Port to monitor IMRC actuator operation. Physically inspect the IMRC plates inside the manifold for carbon buildup or sticking.
   • Success Rate: Very high. Addressing a stuck IMRC plate or failed actuator resolved the P0172 code and symptoms for most owners.
   • DIY Difficulty: Moderate. Inspection may require manifold removal.

2. Check for Vacuum Leaks & Clean MAF Sensor:

   • Action: Perform a smoke test to locate intake leaks. Thoroughly clean the MAF sensor with appropriate cleaner and replace the air filter if dirty.
   • Success Rate: Medium. A foundational step that resolves issues where the IMRC is not the culprit.
   • DIY Difficulty: Easy to Moderate.

3. Replace Intake Manifold Gaskets or Assembly:

   • Action: If leaks are found at the manifold mating surfaces or the IMRC assembly is irreparably faulty, replacement is necessary.
   • Success Rate: High for curing leak-related issues.
   • DIY Difficulty: High. Requires significant disassembly and retorquing to specification.

4. Professional Diagnosis for Fuel or Sensor Issues:

   • Action: If the above steps don't work, have a shop test fuel pressure and injector pulse. This was rarely the primary cause in the analyzed discussions.
   • DIY Difficulty: High. Requires specialized tools.

Shared Experiences

• The "IMRC Dam" Failure: Numerous owners found the plastic gear in the IMRC actuator (the "dam") had stripped, leaving the control plates stuck in one position. Simply replacing this actuator module was the cure.
• Tuning Device as Diagnostic Tool: Many users with Cobb Access Ports praised the ability to log data and pinpoint the IMRC as the failing component before any physical disassembly.
• Modification Confusion: Some owners initially suspected aftermarket parts (like a turbo-back exhaust or short shifters) were the cause. Diagnostics consistently showed the root cause was still a mechanical fault (like the IMRC) unrelated to these bolt-on mods.
• Importance of Gaskets: Several DIYers who replaced manifolds stressed the critical need for new, high-quality gaskets to prevent immediate vacuum leaks.

Price Breakdown

• DIY Repair (Most Common - IMRC Actuator or Plate Service):
  • Parts: IMRC actuator assembly: $150 - $400. Intake manifold gasket set: $50 - $150.
  • Tools: Basic socket sets, OBD2 scanner, possibly a smoke tester ($50+).
  • Total DIY Range: $200 - $600.
• Professional Repair at a Shop:
  • Parts: Same as above.
  • Labor: 3-6 hours of labor is typical for intake manifold diagnosis and replacement. Labor rates vary from $100-$200/hour.
  • Total Shop Range: $600 - $1,800+.
• Cost-Saving Tip: The diagnostic is key. Paying for 1 hour of professional diagnosis ($100-$200) to confirm the issue (e.g., a stuck IMRC) can save money over guessing and replacing parts unnecessarily.

Source Summary: This analysis is based on 168 owner discussions from Reddit and automotive forums.