Finding and Fixing an Evaporative Emissions Leak

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Evaporative emissions leaks are difficult to identify, but there are a few steps you can take to find leaks and fix them yourself.

While we fill the fuel tank with liquid fuel, engines actually run on fuel vapors. This is pretty straightforward, since fuel readily evaporates. However, fuel vapors are harmful to the environment and human health. Smog, climate change, asthma, and lung disease are just a few problems related to evaporative emissions. Evaporative emissions (EVAP) systems are designed to keep volatile fuel vapors from escaping into the atmosphere.

EVAP System Basics and Self-Testing


Check Engine Light On? Check Your Gas Cap, First!.
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Tubes connect various parts of the fuel system, such as the fuel filler tube, fuel tank, and engine intake, to the charcoal canister. The charcoal canister is filled with activated charcoal, whose huge surface area readily absorbs fuel vapors. A series of valves regulates the flow of air and vapors into the system, the general idea being to route them to the engine to be burned.

The EVAP system, to work most effectively, should be completely sealed, including the fuel cap, tubes, valves, canister, and fuel tank. Depending on the model, the EVAP system can test itself for leaks using different strategies. Some systems use a vacuum/pressure sensor to detect when vacuum is in the system and how long it’s able to hold it. These require the engine to be running. Other systems use a dedicated pump to run a similar test, but usually when the vehicle is not running. The test circumstances vary, depending on YMM (year, make, and model), but usually include parameters such as fuel level, vehicle speed, engine run time, or engine temperature.

If the EVAP system detects a problem, it will illuminate the check engine light and store a diagnostic trouble code (DTC) in system memory. Pertaining to the evaporative emissions system, here are some of the most common DTCs:

  • P0440 Evaporative Emission Control System
  • P0441 Evaporative Emission Control System Incorrect Purge Flow
  • P0442 Evaporative Emission Control System Leak Detected (small leak)
  • P0455 Evaporative Emission Control System Leak Detected (gross leak)
  • P0456 Evaporative Emission Control System Leak Detected (very small leak)
  • P0457 Evaporative Emission Control System Leak Detected (fuel cap loose/off)
  • P1440 Purge Valve Stuck Open
  • P1442 Evaporative Emission Control System Leak Detected
  • P1443 Evaporative Emission Control System Control Valve Malfunction
  • P1444 Purge Flow Sensor Circuit Low Input
  • P1455 Evaporative Emission Control System Leak Detected (Gross Leak/No Flow)
  • P2421 Evaporative Emission Control System Vent Valve Stuck Open
  • P2450 Evaporative Emission Control System Switching Valve Performance/Stuck Open

How to Repair EVAP Leaks


Something as Simple as a Cracked O-Ring or Seal May Be the Source of an EVAP Leak.
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Finding EVAP system leaks is arguably the most difficult part of this project. Repairing EVAP leaks, though, can vary in complexity and expense, depending on which part of the EVAP system is leaking. Remove and replace is the usual repair procedure.

  • Valve prices can vary, depending on if they are available separately from other components. Standalone valves, such as the EVAP Purge Valve and some Canister Vent Valves, are usually $25 to $100 and may take just a few minutes to replace.
  • Some Canister Vent Valves are only available as part of the Charcoal Canister, which can range from $300 to over $500.
  • O-Ring Seals are located in many parts of the EVAP system and usually cost less than $2. Simply remove the old O-ring with a pick tool, spray clean the area with carburetor cleaner and allow to dry. Use spray silicone lubricant on the new O-ring and sealing surface, then reinstall.
  • EVAP system tubes, hoses, and clamps can vary in price and complexity, because they are often routed in difficult-to-access areas. Replacing these requires patience, but getting a good seal usually isn’t difficult. Use spray silicone lubricant to ease installation and prevent O-ring binding and rollover.
  • Gas cap prices usually range from $10 to $50, and gas cap O-ring prices usually range from $5 to $20. It only takes a few seconds to replace either one.

EVAP system testing and repair is not for the faint of heart, but it can be done. Because of the complexity of the system, it is often recommended to leave it to the professionals. When you’re done repairing the EVAP system, be sure to reset the DTCs

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