Finding (& fixing) EVAP system leaks

Motor, May 2002 by Black, Ludlow

Diagnostic Strategy

My general strategy for EVAP DTC diagnosis is as follows: 1. Determine the exact manufacturer definition of the DTC. 2. Be sure to retrieve and record Freeze Frame or Failure Record data before erasing codes. 3. If you're faced with a leak DTC, find out if it's a "hard" code. In other words, is it broken now? 4. If it's a circuit or component DTC, perform the necessary circuit tests to complete your diagnosis. 5. Clear the code and operate the vehicle in the parameters that set the DTC. 6. Check to see if the system set a memory DTC.

Remember, the PCM tests for more than shorts and opens in the component wiring. It also performs cause-and-effect tests to determine if the component and system are creating the desired results. If the canister is not loaded with hydrocarbons, there'll be no change in the fuel trim when the purge solenoid is opened.

Sometimes the DTC can be misleading. For example, if the purge valve is stuck open (allowing vapors to flow into the intake all the time), the PCM will not see the expected change and may set a "no-flow" DTC. This is actually the opposite of what's really happening, since there is flow all the time.

The Dreaded Leak DTC

Before you invest a lot of time, you'll need a method to determine whether the system is leaking. Sure, you have a DTC and a Freeze Frame, but is the EVAP system leaking right now? If there's no leak, you can waste lots of time chasing your tail. Is there any real benefit in driving the vehicle to get it to "run the monitor" again? I don't think so. After all, it did fail at least twice already, or you wouldn't be diagnosing it.

First, check to see if the gas tank is full. If it's full or nearly full, the PCM may not run the following tests, and gas cap or filler neck leaks may be masked. Most OBD II vehicles since 1999 have On-board Refueling Vapor Recovery (ORVR), which traps the fuel vapors exiting the tank during refueling. One feature of these systems is a floating check ball at the end of the filler pipe in the gas tank. When the tank is full, the ball seals off the filler pipe and a gas cap leak may not be evident.

Many manufacturers build special tests into their vehicles that allow you to run a leak test right in your service bay. I liked the sound of this when I first heard it, but I quickly learned that there'd be obstacles to reaching this "diagnostic promised land." Chrysler allows you to run a leak-detection pump (LDP) test that seals the EVAP system and cycles the pump to pressurize the system and test for leaks. So far, the only scan tool I've seen that can run this test is the DRB III, which is the OE-specified scan tool.

Ford does not have a similar test that can be run with a hand-held scan tool. (The World Diagnostic System [WDS] isn't in the hand-held category.) GM has a Service Bay Test, but it cannot be run on a hot engine. I've seen this test in most major aftermarket scan tools.

What are the alternatives? Many resourceful technicians have developed ways of manipulating these systems to seal and test them. The pressures needed are extremely low and are measured in inches of water. Most EVAP testing is done at either pressure or vacuum measuring about 7 inches of water. Following is a comparison of inches of water to inches of mercury: