The evolution of evaporative emissions systems has been driven by changes in emissions standards. While some vehicle manufacturers have introduced very different designs along the way, Ford has primarily used a vacuum-based design as a foundation. The exceptions are its hybrid electric vehicles, which use an evaporative leak check monitor (ELCM) similar to the Toyota key-off vacuum pump system, and 2011-03 Ford Fiestas, which use a natural vacuum leak detection (NVLD) system similar to the one used by Kia and BMW.
The most important step in any diagnostic process is understanding the system design, the specific components used and its theory of operation. This article will provide an overview of the different systems and, in the case of the vacuum-based system, the evolution of design enhancements used to comply with the .020-in. gas vapor leak standard.
Fig. 1 on page 20 shows the typical layout of a vacuum-based system, which is commonly used by many vehicle manufacturers. The powertrain control module (PCM) uses three simple but robust components to purge gas vapors and test the gas vapor system for leak integrity. The canister purge solenoid (CPS), canister vent solenoid (CVS) and fuel tank pressure sensor (FTPS) are the heart of the system.
The CPS is a duty-cycle-controlled, normally closed solenoid that separates the engine from the gas vapor system. The PCM operates the CPS to pull gas vapors from the charcoal canister, using intake manifold vacuum, while the engine is running.
The CVS is a basic on/off-open/closed solenoid that’s normally open when deenergized. The PCM uses the CVS to seal the gas vapor system for leak integrity checks.
The FTPS is a three-wire pressure transducer that’s used to measure pressure changes in the gas vapor system. The PCM relies heavily on the FTPS to validate the operation of the CPS and CVS and determine if any leaks exist in the gas vapor system. The system is designed to operate with very small pressure changes and is typically displayed in inches of water (in.-H2O).
To put this into perspective, 1.0 psi = 28.0 in.-H2O, and 1.0 in.-Hg (inches of mercury) = 14.0 in.-H2O. The typical range of a gas vapor system is generally not greater than 3.0 to 4.0 in.-H2O positive pressure and −10.0 in.-H2O negative pressure and/or vacuum. The typical gas cap is designed to protect the gas vapor system by releasing positive pressure buildup at approximately 1.5 psi, or 42.0 in.-H2O, and negative pressure buildup at −1.5 in.-Hg, or −21.0 in.-H2O. Bottom line: The FTPS is a very accurate sensor designed to read very small pressure changes.
In the initial stages of engine operation, the PCM checks all three components for common electrical issues and, depending on the year and model of the vehicle, will set a variety of fault codes—for example, P0443 for a CPS open or short condition and P0446 for a CVS open or short condition. The FTPS could have a variety of fault codes—P0452 (short circuit), P0453 (open circuit), P0454 (noisy signal) or P0451 (offset pressure calibration issue). All of these fault codes need to be corrected prior to performing any gas vapor testing.
A vacuum-based system goes through several phases of operation. The first phase is the workhorse side of the gas vapor system and is designed to purge the gas vapors that have been collected in the charcoal canister. The PCM commands the CPS open with the engine running, and gas vapors are purged using the intake manifold vacuum. The CVS is deenergized (open) during purge operation. In this phase, the FTPS should read a slight negative pressure—approximately −2.0 to −4.0 in.-H2O as the charcoal canister creates a slight restriction with the filters and charcoal in the canister.
If the FTPS shows a negative pressure greater than −8.0 in.-H2O, the PCM sees this as a possible restriction in the system and will likely set a P1450 for excessive vacuum buildup.
Keep in mind the purge or workhorse phase can happen at any time with the engine running; fuel level does not matter. Fuel level does matter when we move to the gas vapor integrity phase.
The evaporative gas vapor integrity test is performed in several phases. But before the test can begin, many enabling conditions must be met. In addition to the CPS, CVS and FTPS being fully operational, the mass airflow (MAF), intake air temperature (IAT), vehicle speed (VSS) and engine coolant temperature (ECT) sensors, plus a variety of engine management sensors, must be fault-free.
Fig. 2 on page 20 shows an example of the operating conditions for a 2008 Ford engine, which must be met for the monitor to run. The chart shows that the test is performed after a lengthy engine-off soak, with the vehicle driving down the road and fuel level within the proper range. This information is very useful when you’re attempting to duplicate the fault and/or validating the success of your repair.
The evaporative gas vapor integrity test begins with the PCM closing the CVS, sealing the gas vapor system, then opening the CPS to pull the gas vapor system into a negative pressure. The target pressure is approximately −8.0 in.-H2O. If the target pressure is not achieved, the PCM concludes that there’s a large leak in the gas vapor system and, after several confirmations, will set a P0455 (gross leak).
If the negative pressure exceeds the target pressure, the PCM will set a P1450 for excessive negative pressure, which would likely indicate blocked vapor lines or possibly a stuck-open CPS.
In later Ford models, as the software in the PCM evolved, you might see a P0457, which is a gross leak, which has occurred after the fuel level has increased by approximately 20%, following a key-off condition. Prior to the Check Engine light illuminating, the customer would likely get a message to check the gas cap.
If the target vacuum is achieved, the PCM moves to the vacuum stabilization phase. The CPS is deenergized (closed) and the CVS is energized (closed), keeping the gas vapor system sealed. The PCM focuses on the FTPS at this point, looking to see if the pressure remains steady. If the pressure continues to go negative, the PCM concludes that the CPS may not have fully closed, and sets a P1450. At this point, the test would be aborted. The PCM is also looking for any changes in engine load and fuel slosh, either of which can affect the test results.
If vacuum stabilization is achieved, the next phase begins—the vacuum hold and decay phase. Fig. 3 on page 22 shows a simple example of the integrity process. The CPS and CVS are both closed, sealing the gas vapor system. The FTPS shows −7.0 in.-H2O and the stopwatch represents the countdown timer inside the PCM. If the FTPS remains steady during this process (approximately 30 seconds in duration), then the PCM concludes that the gas vapor system has no leaks. Keep in mind that the test time could vary among different vehicle years, models and engines.
If the FTPS starts to decay during the vacuum hold and decay phase, the PCM can conclude there’s a leak in the system. The speed and amount of decay determine the size of the leak. For example, referring to Fig. 3, if the FTPS goes from −.7 in.-H2O to .0 in.-H2O within a few seconds, this would be considered a P0455 (gross leak). If the FTPS goes from −.7 in.-H2O to −.4 in.-H2O over a 30-second period, the PCM could set P0442 (.040-in. leak).
If the vacuum hold and decay phase is complete, the PCM moves to the vacuum release phase, where the CVS is deenergized (opened) and the negative pressure/vacuum is released. The FTPS should slowly decay until there’s no pressure in the system, close to .0 in.-H2O. If the pressure does not decay, the PCM concludes that the CVS is stuck closed.
After approximately 30 to 60 seconds, the PCM moves to the vapor generation phase. The PCM energizes (closes) the CVS, resealing the gas vapor system, then looks for a rise in vapor generation, watching the FTPS. The natural slosh of fuel inside the fuel tank, the exhaust temperature and the operation of the fuel pump should create a rise in vapor pressure. If the pressure does not rise above 2.5 in.-H2O, the PCM will set a P0442 (.040-in. leak).
If you’re following this process, you’ll notice that the gas vapor system was checked for leaks with both positive and negative pressure.
What we reviewed so far relates to leaks that are .040 in. or greater. The phase-in for .020-in. small leak testing for California vehicles started in 2000. The leak-detection process is very similar to the one described above, with a few exceptions. Obviously, the quantity of leak decay is tighter, and if a .020-in. leak is detected during a cruise test, the PCM performs a confirmation test with the vehicle sitting still at idle speed. The new fault code added is P0456, for a .020-in. small leak.
In 2005, Ford began phasing in a different method to check for .020-in. leaks. The system is called an engine off natural vacuum (EONV), and primarily uses the CVS and FTPS after the vehicle is shut off to check the gas vapor system. The PCM has been updated to include a stay-alive microprocessor that’s designed to control the CVS and monitor the FTPS key-on, engine-off (KOEO) without energizing the entire PCM. (The process of checking for .040-in. or greater leaks has not changed; the large-leak test is performed with the same enabling criteria described earlier.)
The EONV test begins when the vehicle is shut off. It uses the Ideal Gas Law principle to test the gas vapor system. The Ideal Gas Law defines a relationship between pressure and temperature in a sealed container. If the temperature increases, the pressure in the container will increase; if the temperature decreases, the pressure in the container will decrease.
The screen capture in Fig. 4 below
left shows the phases of the EONV test. The red trace represents the CVS and the blue trace represents the FTPS. P0 is the fuel stabilization phase with the CVS deenergized (open), which allows the gas vapor pressure to stabilize close to atmos-
pheric pressure. The PCM monitors the FTPS, and if the value is greater than 1.5 in.-H2O, it will abort the test.
The next phase (P1) begins when the PCM energizes (closes) the CVS, sealing the gas vapor system. The PCM monitors the FTPS for changes in gas vapor system pressure. If the fuel temperature is still rising from the heat of the exhaust system components or ambient temperature, the pressure will rise. When the fuel temperature starts to cool down, the pressure will begin to decrease inside the gas vapor system. If during P1 the pressure rises but does not go above the positive pass threshold, P2 will kick in, which relieves the pressure in the gas vapor system and accelerates the process of negative pressure buildup.
Phase 3 shows the FTPS moving in the negative pressure direction as the fuel temperature decreases. Keep in mind that a 3° positive/negative temperature change will change the pressure approximately 1.0 in.-H2O positive or negative.
Phase 4 completes the EONV test. The PCM records the results and deenergizes (opens) the CVS. The total time on the chart is 30 minutes, but the PCM has a 45-minute evaluation timer. If the vacuum and/or pressure thresholds are not met, the PCM will record a P0456 (.020-in. small leak). Keep in mind the gas vapor system had to pass the .040-in. leak test prior to starting the EONV test KOEO.
As you can see, the EONV test is performed during a KOEO condition, which means the results will be more accurate than a running test.
Diagnosing gas vapor system leaks on Ford vacuum-based systems is straightforward, but you might need more than just a basic smoke machine to get the job done. It will be very useful to have an in.-H2O gauge to measure the pressure in the gas vapor system. Fig. 5 above shows a smoke machine with an in.-H2O gauge included, but if your smoke machine doesn’t have one, you’ll want to adapt one to your testing setup.
The in.-H2O gauge will help you duplicate the test Ford is performing on the vehicle. With the evap test machine and/or in.-H2O gauge connected to the gas vapor system, engine running, the CVS deenergized (open) and the CPS energized (open), this should cause the in.-H2O gauge to go negative, approximately −2.0 to −4.0 in.-H2O. If you energize (close) the CVS with the CPS open, the negative pressure should increase to −7.0 to −10.0 in.-H2O. If at this point you deenergize (close) the CPS, you’ll have a sealed system. All you need to do is watch the in.-H2O gauge. If the needle remains steady, no leaks are present.
If the vehicle has recorded a leak code, you’ll want to take the next step and begin shaking the evap system components while monitoring the in.-H2O gauge. If the gauge begins to drop, you know one of the components you moved around is creating the intermittent issues. If the needle quickly drops to atmospheric pressure, you have a large leak and need to search for it with the smoke machine.
To fully understand the Ideal Gas Law principle, use your in.-H2O gauge and seal the gas vapor system. This is best done by plugging the CVS outlet. Now all you need to do is wait for the fuel temperature in the gas vapor system to change. You need only a 3° temperature change to see the results. You could use a space heater and/or fan to accelerate the temperature change process.
Ford has two more systems on its vehicles today. The internal combustion engine on a hybrid electric vehicle may not operate during a drive cycle, or would likely not run long enough to take advantage of the EONV system. In this case, Ford uses the ELCM system, which is similar to the Toyota key-off vacuum pump system and is used during KOEO conditions.
Ford’s 2011-13 Fiestas are equipped with the NVLD II system, which also relies on the Ideal Gas Law principle. The NVLD II module has a vacuum switch that closes when the gas vapor system cools down. If the NVLD switch closes, the PCM concludes that gas vapor integrity is good. The test is also performed KOEO for increased leak-testing accuracy.
Great free resources from Ford are the OBD II Theory and Operation manuals found at https://www.motorcraftservice.com/freeresources/obd. You can read about all the evap systems described above, as well as get a full explanation of a variety of engine management systems from 1996 to the present.
The nice thing about Ford evaporative emissions systems is that they have pretty much stuck with a similar design with minor variations, so the learning curve is not too steep and diagnosis is fairly straightforward.
