Finding and Fixing an Evaporative Emissions Leak

the fuel filler cap on a typical automobile
Leaving the Fuel Filler Cap Loose Might be Interpreted by the ECM as an EVAP Leak.

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
Check Engine Light On? Check Your Gas Cap, First!.

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 Test for EVAP Leaks

a basic engine vacuum gauge
You Can Use an Engine Vacuum-Pressure Gauge to Check for EVAP Leaks.

For each YMM, these leak problems can often be localized depending on the code. Refer to a repair manual to help you localize the EVAP leak. The only problem is, because we’re looking for vacuum leaks, it can be nearly impossible to find EVAP leaks without special equipment.

  • Vacuum Test – All EVAP systems lead to the intake manifold, so an engine vacuum gauge can be adapted to test valves and lines for integrity. First, verify engine idle vacuum is around 21 in. Hg (inches mercury). With the engine running and the purge valve solenoid electrical connector disconnected, check for vacuum in the EVAP system – there should be none. If there is engine vacuum, this means the purge valve is stuck open.
  • Hand Vacuum Pump – A hand vacuum pump can be used to check some EVAP system valves, without the need for the engine to be running. Unpowered, the purge valve should be closed, and the vent valve should be open. The vacuum gauge will show whether the valve is holding pressure or not, and you can manually power valves to check their operation and sealing.
  • Smoke Test – The idea behind the smoke test is simple, blow smoke into the EVAP system and look for smoke escaping from a compromised valve, seal, tube, or hose.
    • Smoke testing is the best way to test the EVAP system. At the same time, it’s also either the most expensive or bravest method of doing to. Professional smoke test machines cost upwards of $600, which is out of the realm of DIY auto repair, and the author wasn’t even able to find this available as a rental.
    • On the other hand, searching YouTube and other online resources turns up several DIY smoke tester options, which involve some sort of fire and smoke. Besides dealing with flammable vapors, excessive pressure can also damage EVAP system components, leading to higher repair costs. Use a low-pressure gauge and regulator to prevent damage to the system. Use any DIY smoke tester at your own risk.
  • Bubble Test – In considering pressurizing the EVAP system safely, it is important not to overpressure the system. It may be possible to use an air mattress inflator or shop-vac outlet, which won’t inflate over 2 or 3 PSI. Simply pressurize the system and spray down EVAP system components with a soapy solution. A 50/50 solution of car wash and windshield washer fluid works well. Leaks will show up as bubbles or foam. (You can use the same solution to check for tire leaks.)

How to Repair EVAP Leaks

various o-rings and seals
Something as Simple as a Cracked O-Ring or Seal May Be the Source of an EVAP Leak.

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