Finding (& fixing) EVAP system leaks

Motor, May 2002 by Black, Ludlow

OBD II systems can flag an EVAP leak that's smaller than the eye of a needle. Effective diagnostics and the proper tools are required to locate these leaks.

I remember when cars came to our shop because they ran poorly. I diagnosed them, replaced or repaired some worn components and, most days, out they went running better. It gave me a sense of satisfaction, and even if the customer didn't like the price, he couldn't argue with the results. And when I missed the target, I knew about it right away because the customer would return with a symptom we could recognize. But we can't live in the past, right?

Today, a vehicle will often come into your shop with the MIL on and no symptoms. The vehicle will have a complex and invisible entity on board called OBD II. We were told how it would help us by standardizing everything. Well, its time for a reality check.

A 1996 Taurus IOL comes into your shop one morning. The customer complains about the MIL being on. He says, "The engine runs fine. Can you hook up your thing and turn out the light while I wait?" You hook up your scan tool and get a DTC P0443 (EVAP System). Then you open the hood and notice that everything is intact. You see that the engine has a green EVAP test port, so you figure that this vehicle has the system that checks for leaks. In a moment of weakness you clear the code and tell the customer that he needs to make sure he tightens the gas cap.

Before you go down that road, here are some questions to ask yourself:

*Do you think this car is coming back and, if so, how long before it does?

*How will you find the leak, if there is one?

*Does your scan tool have any special tests that will help you?

*Does the PCM need a flash update?

And now, in the words of a famous newsman, the rest of the story: You decided to clear the DTC. But the car came back in a couple of days with the MIL back on, and this time the customer agreed to leave it. The MIL is on, with a code P0443 stored in computer memory. When you look up the DTC, you see that it pertains to the electrical circuit for something called the VMV, and you need to do some meter diagnostics. Okay, you can handle this, but what if the EVAP system is leaking? Let's take a closer look at this system.

A canister purge solenoid has been used to control purge vapors in EVAP systems for several years. It's not very precise, nor does it provide the level of control required for enhanced EVAP diagnostics. It's used on nonenhanced EVAP systems-in other words, the systems that don't do a leak test.

On its nonenhanced systems, Ford uses a purge flow sensor to diagnose the solenoid. It sets a DTC P1443 if it fails. To provide more precise purge control for its enhanced systems, Ford used a new purge solenoid called a vapor management valve (VMV). It uses a vacuum source and is controlled by variable duty cycle. This valve can allow purge at idle. For this reason, the PCM can perform a functional test at idle by tracking idle speed corrections. The purge flow sensor is not needed.

Ford uses the VMV in its system, along with a pressure sensor and canister vent solenoid, to perform a .040inch leak check. The VMV has a green service cap that allows you to connect test equipment to check the integrity of the system and to find leaks.

When you assumed that you were working on a system that could set a DTC if the gas cap was loose, your logic was pretty good. This '96 Taurus has all the enhanced EVAP components, except for the software to make it work. It was designed to have an enhanced system and therefore should have been able to perform the leak check.

When the vehicle was about to be released, Ford discovered problems with the fuel sloshing in the tank during driving. There were also some fuel volatility issues that made pressure testing difficult. So Ford had to turn off the leak check function on all 1996 enhanced EVAP passenger cars, including the Taurus in our story. The next year Ford added a fuel level input to the PCM to prevent the monitor from running when the tank was empty or full, and turned everything back on.

DTC P0443 on this vehicle is a circuit code for the VMV. Once you clear this DTC, how will you know if it'll come back? Do you need to do the dreaded "drive cycle"? Since this is a continuously monitored circuit, the effectiveness of your repair should be evident quickly. If the circuit fails the next test, you know it's not fixed. The next question is, how do you know if it fails?

Many of these circuits require two trips with a failure to turn on the light. However, if they fail the test, they'll set a DTC. This code may not be listed in the Generic or Global OBD II "Read DTC" list. It may be stored in the OE "Memory DTC" list, depending on the scan tool you're using. This is a one-trip failure of a two-trip code. I've seen scan tool manufacturers refer to these codes as Memory, Continuous, Temporary and Pending. If you're using Global OBD II because your scan tool is out of date, you'll be missing many important features. It may be time for an update.