EVAP System Diagnosis

Evaporative (EVAP) control systems contain the fuel vapors from the fuel tank and injection system and prevent these fumes from being emitted to the atmosphere. The vapors are collected in a canister, then released into the intake system to be burned in the combustion process. Problems are usually detected when the Check Engine light or Malfunction Indicator Lamp (MIL) illuminates. Other times, engine driveability problems can occur. Problems can be diagnosed either visually or electronically with a scan tool. Let’s review those procedures.

EVAP SYSTEM BASICS

In the days before emission control regulations, the fuel system vented to the atmosphere to allow the flow of fuel. The downside to this setup was the emission of a large amount of hydrocarbons (HC), essentially gasoline fumes, directly to the atmosphere. When you opened the hood of a car, you could smell the oil and gas. That is no longer true. As the result of the installation of the EVAP control system, these fumes are collected and no longer escape into the atmosphere. The entire fuel system is closed and does not allow the venting of gasoline vapors directly to the atmosphere. Even the fuel filler cap, which incorporates a two-way relief valve, is closed to the atmosphere during normal operating conditions.

The EVAP system is relatively simple and works quite well. When gasoline evaporates in the fuel tank, the vapors pass through vent hoses to an activated carbon-filled canister (Figure 1). The canister temporarily holds the fuel vapors until the intake manifold vacuum draws the vapors into the combustion chamber. The Powertrain Control 
Module (PCM) purges the canister through the proportional purge solenoid (Figure 2). This is done at pre-determined intervals and engine conditions.

The Leak Detection Pump (LDP) is another component in the EVAP system. In later model vehicles equipped with the Next Generation Controller (NGC), the LDP was replaced by the Natural Vacuum Leak Detection (NVLD) system. Later in the decade, the system evolved to the Evaporative System Integrity Monitor (ESIM). A typical NVLD valve is shown in Figure 3.

The NVLD system is basically a pressure system for the EVAP system. By monitoring vacuum, it can detect leaks throughout the system. In addition, during cool down it prevents an excess build-up of vacuum. When the engine is off, it maintains a seal on the EVAP system to prevent the release of fuel vapors. Finally, when the vehicle is fueling and pressure builds up in the evaporative system, it pressure vents to the fresh air filter. .

EVAP SYSTEM PROBLEMS

Quite often, EVAP system problems will not affect driveability; however, the Malfunction Indicator Lamp (MIL) will illuminate. As we all know, an illuminated MIL can represent a wide range of problems. To pinpoint the potential cause of the problem, connect your DRBIII® or equivalent scan tool to access the Diagnostic Trouble Codes (DTCs). Let’s pick an example vehicle, a 2007 Dodge Caravan. For this vehicle, there are 10 DTCs, which are listed below:
PO440 – General EVAP System Failure
PO441 – EVAP Purge System Performance
PO443 – EVAP Purge Solenoid Circuit
PO452 – NVLD Pressure Switch Sense Circuit Low
PO453 – NVLD Pressure Switch Sense Circuit High
PO455 – EVAP System Large Leak
PO456 – EVAP System Small Leak
PO457 – Loose Fuel Cap
PO498 – NVLD Canister Vent Valve Solenoid Circuit Low
PO499 – NVLD Canister Vent Valve Solenoid Circuit High

Of all the potential problems listed above with the DTCs, leaks are probably the most common ones that occur. With that in mind, let’s concentrate on leak detection. And before we wheel out the Evaporative Emissions Leak Detector (EELD) to run smoke tests to find the leak, let’s do a visual inspection. There’s a good chance that such an inspection can locate the leak. If we go back and review the EVAP system basics presented in the earlier paragraphs, you can see the areas in which a leak can occur.

The first item discussed was the fuel filler cap with the 2-way relief valve. The fuel cap must be rotated until several clicks are heard. The sound indicates the cap is fully seated. Although this might seem very basic, when a customer tells you the MIL illuminated within the last day or two, check the fuel filler cap. If it is loose, that will generate a fault code PO457 and illuminate the MIL. Properly tighten the cap to correct that problem.

The components of the system were discussed next, but it was not mentioned that hoses and tubes connect these pieces together, allowing the flow of fuel vapors. Due to the harsh environment in which the vapor canister and the NVLD are located, breakdown of the hose material is common. This is particularly true at the connection points. Inspect the hoses and tubes and look for cracks and splits in the material. Remove the hoses at the connection points and examine the ends of the line. Don’t overlook the plastic lines. .

LEAK DETECTION

In older vehicles (prior to 2002), a DTC was set when a minimum leak size of .040" was detected. With the NVLD, the detectable leak size shrunk to .020". Now, a .010" leak can be sensed. To aid technicians in locating the leak, a new tool was developed, the EELD (Essential Special Tool #8404). We’ll review the procedure to locate vacuum leaks using the smoke feature.

Operating the EELD is simple. It has a shop air connection for airflow and to introduce smoke into the EVAP system. The EELD uses a special fluid to generate nontoxic smoke and has two different light sources to help you find the location of the leak. In many cases, the smoke can be seen without any aid, but the bright white light can help in the detection of a particularly small opening. An ultraviolet light is included—used with special goggles—to check for traces of a dye that is left by the smoke.

For best results, leak testing should be (1) performed indoors, away from drafts (moving air can dissipate the smoke, making it difficult to detect, especially in low light areas under the vehicle), (2) with the engine cold (warm fuel tends to hide leaks) and (3) with the fuel tank between 20 and 80 percent full.

The EELD will pressurize the EVAP system with smoke in order to detect the source of the vacuum leak. Remove the fuel cap and insert one end of the adapter hose into the fuel tank fill neck. Attach the other end of the adapter hose to the fuel cap.

Pressurize the system (this might take as long as five minutes for large volume 
fuel tanks). To use the smoke feature, remove the air hose and insert the black smoke hose. On the EELD, set the SMOKE/AIR control switch to the SMOKE setting. Press the remote SMOKE/AIR start button to begin filling the EVAP system with smoke.

To verify that the system is filled with smoke, remove the fuel cap adapter and watch for smoke escaping from the fuel tank fill neck. Now let’s look for leaks.

If a leak is present at a connection, such as the fuel tank vent connection, smoke will easily be seen. A more common sight for a vacuum leak is at one of the connections (Figure 1). A closer inspection of the sight will reveal which connection is leaking.

If you are still having difficulty locating the leak, use a UV light and goggles to look for the residual dye that was left by the escaping smoke. This will show the exact location of the leak because the dye only appears where the smoke passes through a small hole, causing it to condense.