Testing Your Evap Fuel Tank Pressure Sensor: A Step-By-Step Guide

The fuel tank pressure sensor is an integral part of a car's evaporative emissions system, also known as EVAP. It is placed atop or inside the fuel tank and is responsible for monitoring both positive and negative pressure within the tank. This sensor plays a crucial role in identifying leaks in the fuel system, detecting defective gas caps, and ensuring compliance with emissions regulations. When issues arise with the fuel tank pressure sensor, the check engine light illuminates, indicating the need for professional attention. Testing and repairing this component requires specialised tools and knowledge, making it a task best left to experienced mechanics.

Understanding the EVAP system

The EVAP system, or Evaporative Emission Control System, is an essential component in modern vehicles designed to prevent fuel vapours from escaping into the atmosphere. It captures and stores vapours in a charcoal canister while the vehicle is parked or the engine is off, and then draws them into the engine to be burned when the vehicle is running. This system helps to reduce air pollution, improve fuel efficiency, and maintain compliance with emissions regulations.

The EVAP system consists of the fuel tank, a vapor storage canister, valves, hoses, and the fuel tank gas cap. Vent lines from the fuel tank direct the vapours to the canister, where they are stored until the vehicle starts. Once the engine is running and has warmed up, the vapours are released through a purge valve and burned together with the air-fuel mixture inside the engine.

The EVAP system is designed to be active and requires no maintenance. However, faults can occur, and the system may malfunction. Therefore, the vehicle's PCM (Powertrain Control Module) monitor runs self-checks to verify airflow from the canister to the engine and to detect leaks in the fuel tank, canister, fuel lines, and vents. If a leak is detected, the PCM will trigger a fault code and illuminate the check engine light.

The fuel tank pressure sensor is a crucial component of the EVAP system. It measures both positive and negative pressure in the fuel tank, allowing the PCM to detect vapour leakages and trigger a fault code if necessary. The EVAP canister, usually filled with activated charcoal, absorbs and confines the gasoline vapours until the engine starts, at which point the vapours are drawn into the engine for combustion.

In summary, the EVAP system plays a vital role in reducing vehicle emissions and improving fuel efficiency. By capturing and burning fuel vapours, it helps to prevent pollution and ensure compliance with environmental regulations. The PCM monitor and fuel tank pressure sensor work together to detect and address any leaks or malfunctions in the system, ensuring its proper functioning.

Testing for leaks

There are several methods to test for leaks in the EVAP system:

Conduct a Visual Inspection

Firstly, visually inspect all accessible parts of the EVAP system, including hoses, valves, and connections. Check for any cracks, holes, or signs of brittleness, especially in the rubber hoses that can degrade over time. Ensure that all connections are secure and that no components are visibly damaged or disconnected, particularly around areas like the fuel tank, purge valve, and charcoal canister.

Evaluate the Gas Cap

The gas cap is often the culprit in EVAP system leaks. Ensure that the cap is tightened properly and is in good condition. Look for any cracks or a deteriorated rubber seal, which can cause the cap to lose its ability to maintain a proper seal. Replacing a faulty gas cap is usually inexpensive and can resolve a persistent check engine light issue if the leak is minor.

Smoke Test

A smoke test involves injecting smoke, sometimes combined with a dye, into the EVAP system while it is sealed. The smoke will escape through any leaks, making them visible. For an even more thorough examination, some mechanics use UV smoke, which glows under a black light, to highlight very small leaks.

Use an OBD II Scanner

An OBD II scanner is a diagnostic tool that connects to the vehicle's onboard computer. It can retrieve diagnostic trouble codes (DTCs) that indicate specific issues, including those related to the EVAP system. Codes like P0442 or P0456 directly suggest small leaks. The scanner can also initiate a system test to further diagnose the severity and location of the leak.

Apply a Pressure and Vacuum Test

This method uses specialized equipment to apply pressure or vacuum to the EVAP system and then monitor it to see if it holds the pressure or vacuum without dropping. A drop in pressure or vacuum indicates a leak. This test is precise in determining the integrity of the system and can verify that a repair was successful.

Fuel Tank Monitoring System

Newer vehicles often include sensors that monitor the pressure and vacuum within the fuel tank to manage vapor emissions effectively. If the system detects an inconsistency in pressure that doesn't match the operational parameters, it triggers a check engine light. This system helps in early leak detection, often before more noticeable symptoms arise.

Interpreting the check engine light

The check engine light is a car's way of alerting the driver to a potential problem. When the check engine light comes on, it is important to get the car checked out as soon as possible. While the issue may be as simple as a faulty gas cap, there could be a more serious problem with the vehicle.

The check engine light is controlled by the car's Engine Control Unit (ECU). The ECU monitors the performance of the engine and various other systems in the car. If it detects a problem, it will turn on the check engine light to alert the driver. In modern cars, the ECU will also store a trouble code, which can be read by a mechanic to help diagnose the issue.

There are many potential issues that could cause the check engine light to come on. One common issue is a problem with the fuel tank pressure sensor. This sensor is part of the evaporative emissions system (EVAP) and is responsible for detecting leaks in the fuel system. If the sensor detects a leak, or if it fails, it will trigger the check engine light.

In some cases, the check engine light may be triggered by a minor issue, such as a loose gas cap. Tightening the gas cap may resolve the issue and cause the check engine light to turn off. However, if the problem is more serious, such as a faulty fuel tank pressure sensor, the light may remain on until the issue is addressed.

It is important to note that the check engine light can also be triggered by more serious issues, such as engine misfires, faulty catalytic converters, or problems with the transmission. In some cases, the issue may be minor and the light may turn off on its own after a few days. However, it is always best to get the car checked by a mechanic to ensure that there is not a more serious problem.

Using a smoke machine

A smoke machine is a very effective tool for evap system diagnosis. It is important to note that evap systems are designed to operate and test at a very low pressure of about 12 inches of water, or about half a PSI. Testing them at pressures above 7 psi could blow off hoses or damage the pressure sensor. Therefore, it is crucial to always use a smoke machine that regulates the applied pressure to less than 7 psi.

To begin the testing process, locate the access port for the evap system under the hood of the car. The port is usually covered by a green plastic cap, which can be unscrewed. There is a valve core under the cap, but the smoke machine should be able to push smoke past it. If the vehicle does not have a test port, you can connect to the tank via the fuel filler using the appropriate adapter, such as an EVAP Service Port Adapter or a Gas Cap Adapter.

Once the smoke machine is attached, fill the system with smoke and check for leaks. If you are having difficulty locating the leak, you may need to remove the rear seat or drop down the gas tank for a closer inspection. It is recommended to test indoors, in good lighting, and without any breeze. Additionally, ensure that the smoke machine does not run out of smoke during the testing process.

After repairing the system, it is important to clear the Diagnostic Trouble Codes (DTCs) using a scan tool or code reader. Alternatively, you can clear the codes by disconnecting the negative battery terminal for about 10 seconds, but this may require resetting clocks, radio stations, and other presets. Regardless of the method used, the computer will need to run self-tests before confirming that the system is functioning properly. This process can take a few weeks of driving for some cars.

Testing the EVAP pressure sensor signal circuit

The sensor is part of the fuel pump assembly and is mounted on top of the tank or inside the tank. It is part of the evaporative emissions system (commonly referred to as "EVAP") and reads pressure in the fuel system to detect evaporative leaks, such as a loose or faulty gas cap. Fuel vapors are supposed to be contained in the EVAP system, and leaks can allow vapors to escape into the atmosphere.

If the sensor detects a leak, or if the sensor itself fails, it will illuminate the "check engine" light. Mechanics can then read a trouble code to trace the issue to a leak in the evaporative emissions system or the sensor.

It is important to note that the fuel tank pressure sensor cannot be repaired if it has failed. It must be replaced. Given the dangers of working with automotive electronics and fuel systems, it is recommended to leave this task to a professional repair shop.

Frequently asked questions