Finding the little ones

Your EVAP foundation
The No. 1 cause of a motorist’s illuminated check engine light is a gas cap that they did not tighten after refueling. This is why some techs delete the EVAP code and tell the vehicle owner to come back for a diagnosis if the MIL comes back on. Since loose and defective gas caps are only to blame for a part of EVAP issues, when vehicles return with EVAP codes, it is necessary to properly diagnose them. When diagnosing EVAP system problems we usually just need a smoke machine that is connected to CO₂ or nitrogen (not air pressure) to move the smoke through the EVAP system. If that does not work, you may want to try the new Bullseye leak detection system from Automotive Test Solutions. Other helpful tools to have are an information system that describes the vehicle EVAP system function and components, a wiring diagram, capable scan tool, a Power Probe that can be used to activate the solenoids, and a flow-pressure gauge. In rare situations, factory scan tools might be necessary in order to test vital EVAP components. We are going to take a look at a few different ways to diagnose the EVAP system to assist you in finding small leaks.

Before we move on, it is important to understand and interpret pressure readings since information systems provide so many different measurement specifications. The chart provided (Figure 1) will help you convert the specification from the information system to the tool you’re using. Since you may be working with different units, it’s important to have the conversion chart. You can compare actual system pressure to the fuel tank pressure Parameter Identifier (PIC) to aid with your diagnosis. Just remember to add 14.7 psi to all low-pressure gauge measurements when comparing it to the scan tool (they are calibrated to atmospheric pressure already). If the flow gauge agrees with the scan tool, then we can base our diagnosis off of the scan tool PID.

Some test using vacuum
You need to have a good game plan when trying to diagnose and locate the small leaks. The first step is always to understand the system by reading up on the system components and component location, along with the strategy of the system. This step is very important, since there are so many different systems that have different operating strategies. The following information is the fundamental information on what you need to know about most EVAP systems. Make sure to always identify and read up on the EVAP system you’re testing so you know what to expect from a particular system.

Let’s start with EVAP systems (Figure 2) that use engine vacuum to check for leaks. Since this is the most common system, you need to be familiar with it to be able to perform a proper diagnosis. GM, Ford and many imports use measurements from sensors giving feedback concerning the vapor management or purge control valve, solenoid-operated canister vent and fuel tank pressure sensor. They self test in the following order:

1. Purge valve is closed and the canister vent valve is open. No engine vacuum is reaching the canister and with the canister vent open system pressure equals atmospheric pressure.
2. Purge valve is opened and the canister vent is closed. This should increase EVAP system pressure by 6 to 8 inches of water.
3. Purge valve and canister vent remain closed while the computer monitors how long the system retains sufficient vacuum.

The simple leak detection strategy for vacuum based systems has three basic stages.

1. System at Rest - Pressure Equalization

2. Weak Vacuum Test - Pull an Initial Vacuum

3. Small Leak Test - Hold Vacuum

Step One: System at Rest — Pressure Equalization
In this step, the EVAP system is at rest. In other words, the purge valve is closed and the canister vent valve is open. This is the normal state for each valve, before the PCM activates them. With the purge valve closed, no engine vacuum can reach the canister and with the canister vent open, any pressure or vacuum in the system equalizes with atmospheric pressure. When using a standard gauge to compare values to those shown on the scan tool, be sure to correct for atmospheric pressure. Most tools are already calibrated to include the 14.7 psi as their “zero” point.

Step Two: Weak Vacuum Test
Here, the canister vent is closed and the purge valve is opened by a pulsed electrical signal from the PCM. Vacuum in the EVAP system should increase to about 6-8 inches of water (check the pressure charts – that isn’t much!) if there are no large leaks. This is sometimes referred to as the Weak Vacuum Test. If the system cannot be drawn down to this level, a P0440 is stored.

Step Three: Small Leak Test — Hold Vacuum
The purge valve is closed and the canister vent remains closed. If there are no leaks, the vacuum generated inside the EVAP system should hold steady (some small amount of decay is allowed, however, and may be caused by increased vapor pressure, not a leak). If the vacuum does not hold within an acceptable range, a fault is recorded by the EVAP monitor.

The screen shots (Figure 3, sub 1-5) from the Ford IDS are examples of how the EVAP system tests and runs the vehicle EVAP Monitor. Reviewing the Ford IDS scan tool screen shots will provide you with a good understanding of the vacuum EVAP system and what the OBD II system is looking for. All manufacturers perform similar tests to set and test the EVAP Monitor. If you are not working on a Ford or don’t have the IDS scan tool, you can use a meter, labscope, or scan tool to take the same measurements.

These waveforms show a Ford Enhanced (Figure 4) EVAP system test over a 10-second time period. Note that tank pressure falls as the purge valve opens, applying engine vacuum to the system. The Ford Fuel Tank Pressure (FTP) sensor voltage is approximately 2.4 to 2.8 volts at atmospheric pressure. Fuel tank pressure sensor voltage for this system falls to about 1.6 volts as the target test pressure (vacuum) of 7 inches of water (7”/H₂O) is reached. Then, you want to watch the rate of decay and compare it to specifications.

These waveforms (Figure 5) show a GM Enhanced system as the EVAP monitor runs. Unlike the Ford system we just learned about previously, the pressure sensor voltage in this GM vehicle is only 1.3 to 1.7 volts at atmospheric pressure. Opposite the Ford pressure sensor, this GM pressure sensor’s voltage increases as pressure decreases.

The above tests confirm whether there is a leak in the system, and proper purge and canister vent solenoid operation. You will need a scan tool with bi-directional controls or a Power Probe to open and close the solenoids.

Some use pressure
Since not all the EVAP systems utilize vacuum, we need to cover another popular system that is a bit different because it uses pressure. The following is an overview of how the Chrysler (Figure 6) and many other Leak Detection Pump (LDP) EVAP systems work. The procedure on how to check and test the system goes something like this. The purge solenoid is normally closed, grounded by the PCM. The feed side comes from a KOEO fused circuit. The PCM energizes the solenoid to purge fuel vapors from the canister and to lower tank pressure.

The LDP’s vent valve is incorporated in the unit and is normally open. It supplies air to the charcoal canister. The PCM checks for EVAP leaks by first energizing the purge solenoid (normally closed), followed by rapidly cycling the LDP solenoid and watching the LDP switch. The fuel at rest will increase in pressure as the fumes grow. Once pressure (7.5”/H₂O) is built up in the system, the diaphragm will be seated upwards against spring pressure. The PCM knows this since it is monitoring the LDP switch. So, the PCM compares LDP switch position against LDP solenoid cycling time to determine if leakage is present. When manually checking for leaks the vent valve must be closed! Closing of the vent valve requires that the LDP solenoid be energized and that a vacuum source be applied to the LDP solenoid. This will enable the LDP diaphragm to stroke upwards, thereby allowing the vent valve spring to close the vent valve. If the pump runs a short time before reaching pressure it is assumed that there is a blockage and only part of the system is pressurized.

Before we move on it is important to remember to activate the purge and vent solenoids multiple times. In many cases after the fifth or sixth time, the valve sticks, causing the EVAP leak problem you’ve been trying to find with smoke alone.

Leak confirmed
Once you know you have a leak, you need to locate it and eliminate it. A number of dedicated EVAP leak detection smoke machines are available. Designed specifically for this task, they combine pressure and smoke detection to locate leaks. Most EVAP leak detection machines should include the necessary EVAP port adapters that allow you to connect the machine to vehicle EVAP test port. Remember that this port has a suggested maximum pressure of 1 psi or 28 inches of a column of water that should NEVER be exceeded. The Schrader valve has a left-hand thread, and must be removed in order for the smoke to travel properly through the system. Removing the valve will allow a free flow of test gas or test smoke into the system. If the valve is left in place, the smoke breaks up, reducing its density and making it harder to see as it exits a leak.

When using the smoke machine, you will need to select the correct test mode (orifice size 0.010, 0.020 or 0.040 inch) and start to flow a non-inert gas. Make sure to always baseline the smoke machine before attaching it to the vehicle by checking for a good flow of smoke that should be exiting the hose, along with the flow ball floating at the top of the flow gauge. This very important step needs to be followed by blocking off the smoke machine output hose while observing the flow meter ball that has to be at the bottom of the flow meter confirming that the machine is not leaking. Once the machine is attached to the vehicle, either at the test port, hose, or by the gas tank adaptor, check the flow gauge for flow. If the ball is floating over the maximum leak amount for the vehicle being tested, there is a leak present. Check for smoke at all the lines, fittings, connections, filler neck, gas tank and charcoal canister. Be aware that in some cases the smoke or the dye will be impossible to detect. You may want to use your smoke machine along with a 4- or 5-gas analyzer to locate the leak while checking for CO₂ readings. Other alternatives are the use of an ultrasonic leak detector or soapy water while the system is pressurized with CO₂.

When the conventional leak detection systems don’t work in locating the small leaks, there is a new alternative from Automotive Test Solutions. The tester that ATS has is called BullsEye and uses CO₂ along with a pressure gauge, CO₂ leak finder tester and special spray foam that is used to pinpoint the leak with a confirmation by a change of color to the foam. The BullsEye Leak Detector accurately finds leaks as small as .001 of an inch. The system works by first charging the EVAP system with CO₂ and then analyzing the pressure stability on the gauge to determine if a leak is present along with its size. The electronic CO₂ leak detector works similarly to the “sniffer” you use when looking for leaks in an air conditioning system. It finds the area of the leak, while the CO₂ reactant foam is used to confirm the exact leak site. The foam is simply sprayed onto the leak site area where it will change color from pink to yellow identifying the problem. Another neat feature of the tool is the BullysEye Smart Control that allows control of any solenoid. We know that closing or opening solenoids can be accomplished a few ways. The easiest is bi-directional control from the scan tool, or you can use a Power Probe to apply power or ground as needed. If you are going to use a Power Probe you need to index the wiring to the connector. You will need to turn the key off followed by disconnecting the connector, turning the key to KOEO, and check what side of the connector has B+ and ground. Once you have completed those steps, you can then apply power and ground to the correct sides of the solenoid without burning out a diode (used in many solenoids). Now you won’t have to worry about any of that if you have the BullysEye Smart Control activator. The Smart Control automatically locates the power and control circuit (ground) and then latches the control circuit turning the solenoid on or off. This Smart Control box takes the guess work out of where to apply B+ or ground.

Take a look at this Jeep fuel tank (Figure 7) that had a 0.020 leak that was impossible to find using conventional diagnostic equipment. After using the BullsEye electronic detector that located the area of the leak, I sprayed the leak seeking foam on the area and confirmed the exact location of the leak. Take a look at the picture of the gas tank and focus your attention on the area that was leaking. The area of the leak changed the leak seeking form from pink to yellow confirming that there was indeed a leak. Before using the BullsEye detection system, I used all the conventional methods that I mention in this article only to come up empty handed while the flow meter still indicated a leak. The problem with this Jeep gas tank was that it was porous. Will the current method that you’re using to find small leaks find this leak? We need all the help we can get when it comes to diagnosing and finding the little ones.