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Simplified EVAP diagnosis

Time:2014-05-22 Click:1213
As I’ve spent a lot of time teaching EVAP diagnosis course, I thought I’d give some time in this column to briefly discussing EVAP diagnostics. Evaporative emissions diagnosis has plagued the automotive industry ever since OBDII systems came around in the late nineties. Technicians don’t have to be confused or frustrated when it comes to EVAP diagnosis though. Often times just remembering DTC diagnostic basics will help in diagnosing challenging EVAP DTC’s.

Understanding the EVAP system, and knowing how to use your EVAP diagnosis tools will help considerably in being able to quickly, and successfully resolve EVAP DTC challenges. Understanding the limitations of your tools is just as important as knowing what the tools are good at doing. One tool that many technicians have at their disposal for use in finding EVAP system leaks is a smoke machine. Smoke machines have made things easier when it comes to finding system leaks; however they are far from a silver bullet when it comes to diagnosis. Knowledge of the system, an understanding of the EVAP monitor, and knowing your scan tool capabilities will provide the best opportunity to successfully resolve evaporative emission challenges.

Evaporative emissions systems are designed to capture and store gasoline vapor emissions during normal vehicle operation as well as during refueling operations. The system utilizes a sealed fuel tank, an on-board refueling vapor recovery valve (ORVR), a charcoal canister, a purge valve (aka purge solenoid), and for diagnostic purposes, a canister vent valve. The system is designed to capture vapor emissions pulled from the fuel tank, store them until needed, and then burn them as part of the regular combustion process. Computer control is responsible for when, and how much, fuel vapor is burned during combustion.

Because evaporative emissions pose such an air pollution threat, the federal government requires evaporative emission systems to be monitored by the on-board diagnostic system for proper operation as well as for leaks. System leaks as small as 0.020” must be identified by the on-board diagnostic system. A leak this small can sometimes be a challenge for technicians to find. Utilizing the vehicle’s diagnostic system, as well as the technician’s skill set, and specialty tools will lead to a successful resolution.

The first step in diagnosing an EVAP DTC is to retrieve the DTC, record it along with freeze frame data, and then do a quick visual inspection. The visual inspection should be just that, visual. Don’t touch any lines, attempt to tighten the gas cap, or disturb any components. If a component’s position is changed during the inspection it may result in the problem being eliminated (at least temporarily) without the technician knowing whether or not they actually caused the condition to be eliminated. If a problem is noted during the visual inspection, a full diagnosis should be made as described below, before the problem is taken care of.

After a quick visual inspection, the technician should identify the specifics of the DTC. Note what the enabling criteria for the specific DTC is, along with the code set criteria. Understanding how the EVAP monitor is run by the diagnostic system is crucial to properly diagnosing the system. The DTC was set because a certain threshold was reached too soon, or a threshold was not reached at all. Understand what the diagnostic system thought it saw when it set the code. Look specifically for pressure thresholds and time limits.

If it is possible to force the EVAP monitor to run, do so while monitoring the fuel tank pressure data parameter. The EVAP monitor can be run by the vehicle system (PCM), by forcing it with the scan tool (if your tool has the required test functions), or by manually operating the required solenoids while watching the fuel tank pressure sensor data values.

In general, a technician should ensure complete system operation during EVAP diagnosis. This can be done by first observing fuel trims during purge solenoid operation. Make sure that fuel trim responds by dropping slightly as the fuel vapor is delivered to the engine. This indicates the purge solenoid is operating and fuel vapors are flowing.

Next, observe the fuel tank pressure sensor values. The pressure should never be equal to atmospheric pressure (14.7psi, 760 mmHg, 407”H2O) during most fuel tank operations. If on first inspection atmospheric pressure is observed at the pressure sensor, go ahead and try to tighten the fuel tank cap. As fuel in the tank heats, the tank pressure should increase slightly. If fuel tank pressure is not observed to increase, begin to seal off portions of the tank and see if you can make pressure increase. Installing a smoke machine at the filler neck may be in order at this point as well. Don’t just look for smoke, use soapy water as well to identify those really small leaks that may not emit enough smoke to see. Always test the gas cap itself if no other problems can be found. Sometimes just removing the o-ring (if equipped) and reinstalling it upside down can resolve a gas cap sealing concern.

Assuming fuel tank pressure appears to be responding normally, next turn on the purge solenoid (with the engine running of course) either manually, or with a scan tool bi-directional test. Observe fuel tank pressure readings. They should not change during purge (due to isolation of the fuel tank by the canister). If the readings begin to drop, suspect a problem with the canister vent system (plugged or restricted).

Now turn on the canister vent solenoid. This serves to seal the system completely. Fuel tank pressure readings should now drop rapidly as the purge solenoid is operated. In general, the pressure should drop around 18 mmHg over a time period of 10 to 15 seconds. If the tank pressure does not drop low enough, or rapidly enough you have a significant system leak. Use pinch off pliers to try and isolate sections of the system while retesting, or pressurize the system with a smoke machine to help identify the area of concern.

With the vent valve still closed (energized on most vehicles), now turn off the purge solenoid and observe tank pressure. If tank pressure continues to drop you have a purge solenoid that is not closely properly. Over the next 30 to 60 seconds observe tank pressure readings. They should stay the same or very slowly creep upwards. If pressure rises over 8mmHg in the first 20 seconds, you have a system leak. Use your smoke machine, or system isolation techniques to identify the source of the leak.

Once you’ve decided that the system has no leaks, open the vent valve (de-energize the solenoid) and observe fuel tank pressure readings. The fuel tank pressure should now rise rapidly indicating good airflow through the vent solenoid. If fuel tank pressure rises slowly, you have a problem with the canister vent system.

The above procedures will give you a good indication of where the problem resides that set the EVAP DTC on your problem vehicle. They are intended to be used on conventional EVAP systems, but will not work on some Chrysler LDP systems or some Key-Off EVAP systems. I’ll address these systems in future columns.

Remember, knowledge of how the monitor is testing the system is critical. Knowing what pressures the system are looking for over what time interval the system will allow a technician to test the system as the system tests itself. This is the best way of testing an EVAP system. Always consult the manufacturer’s repair information for the latest in how the monitor operates and what thresholds and time limits it is looking for.

Using a smoke machine can be a real asset, however it does pressurize the system which is not the normal way most systems diagnose themselves. If you can’t find a component problem with the smoke machine it doesn’t mean the component isn’t bad. Vacuum test any component (specifically solenoids) before assuming they are good.

The one component that is virtually impossible to truly test to conclusion is the charcoal canister. Ensure the rest of the system is operational and not leaking before condemning the canister. Don’t forget to check the vent valve operation!