One of the most problematic systems in onboard diagnostics (OBD II) is the evaporation control system. This system has caused more maintenance indicator light (MIL) illuminations than any other defect. The primary failure is a loose fuel filler cap. If the operator fails to properly tighten the cap, the system test will detect a leak. Many late model vehicles have a notice printed on the cap to warn drivers to properly tighten the cap or the MIL will illuminate.

In addition to loose filler caps, the evap system is efficient at identifying leaks in the system. The OBD II requirement is that the system must be able to identify leaks 0.04" in diameter or greater. Early results demonstrate that leaks of this equivalent size were common even in new vehicles. It appears that lack of monitoring of this system in the past led to poor quality systems. The inferior quality of evap systems is also evident by a high failure rate in infield testing by independent labs. The infield testing shows a significant failure rate with new vehicles and a rapid increase as the vehicle ages. The in-use deterioration is critical to clean air compliance.

The are two OBD II methods of testing evap systems for leaks. The first method is to apply vacuum to the tank from the intake manifold. This can be accomplished by opening and then closing the purge valve; then an in-tank vacuum sensor will monitor the vacuum decay. The system is calibrated against the 0.04" leak standard and any faster leakage will result in an MIL light and code set.

The second method uses a pressure pump to pressurize the evap system and a pressure sensor in the fuel tank. The same strategy of decay is used. The Environmental Protection Agency (EPA) may require this type of system on all cars because it appears to be more reliable at detecting leaks. The down side of this system is the extra wiring driver in the computer and the pump. However, the vacuum method may not detect poorly sealing hose connections or flap-type leaks in hoses by causing them to seal. The pressure method is less likely to seal a leak and in reality the system operates under pressure most of the time.

To set a code on the OBD II system, specific enable criteria must be met first. You don't want to purge or open exhaust gas recirculation (EGR) valves on a cold system. To do intrusive tests, not just monitor a signal, the engine needs to be operating under specific conditions. Under extremely high (Arizona) or low (Minnesota) ambient temperature conditions, the evap tests won't be enabled. If a customer has an evap code and the car is at your shop under these conditions, you will have to have the car inside the shop under driving conditions to enable the test. The bad news is that if you don't have a dyno, you may not be able to retest after repair. The good news is that the scanner will inform you if the test has not run.

The evap system is also tested for functionality; the purge valve opening can be confirmed by a response on the O2 sensor and the fuel tank sensor. The ability of the canister to store fuel can also be confirmed by the O2 sensor response when the purge valve is opened. IM240 testing required the testing of the evap system's ability to purge. Initial purge testing required inspection station workers to disconnect the purge hose from the canister and insert at "tee" for testing. This test had to be canceled due to the difficulty of access to many canisters and breaking canister connections. New systems have a test port.

Most inspection and maintenance (I/M) programs will use a gas tank pressure test at the fuel filler to test canister systems. To confirm that the system purged, it has been proposed that Helium be introduced into the fuel tank and tested for at the tailpipe. Helium, being an inert gas, will pass through the combustion process unchanged. The Helium is then detected in the exhaust stream confirming that purge took place. Many emission test programs pressure test the gas cap for leakage separately to confirm system integrity. This has created a new problem; to test these gas caps, the cap must be taken to a tester, which requires cutting the tether on the cap. The loss of the tether can lead to lost caps and angry customers.

OBD II is intended to replace tailpipe testing for determining emission failures. However, for now, tailpipe testing will be done on OBD II vehicles. While the OBD II codes for evap problems would be a better indicator of system failure than IM testing, IM methods will be used. Why not use the OBD II codes if they are better? There is no IM emission program in use today testing for codes. To validate OBD II's effectiveness, it should be compared to IM data while both tests exist. The problem is if you test for codes, do you fail the car based on codes and the tailpipe test? No state wants to have two ways to fail, so none are testing for codes. Some states do check for illumination of the MIL/Check Engine Light and will fail cars for that reason. Colorado data shows a high number of cars failing for check engine lights; this shows customers' reluctance to seek service even when it is indicated.

With HC (hydrocarbon) emissions near zero at the tailpipe, the evaporative emissions become the bulk of HC problems from vehicles. The issue of HC emissions from refueling becomes very significant. When you fill a tank with 20 gallons of fuel, you displace 20 gallons of fuel-saturated air into the atmosphere. Is this emission the responsibility of the gas station or the automobile, off-board or onboard vapor recovery? Some states have required gas stations to have off-board recovery for years. However, to bring this emission problem under compliance nationwide, the EPA now requires onboard recovery on all cars. Here's the rub: when an onboard recovery car comes into a station with off-board recovery, the off-board (station) just sucks air. This introduces air into the station tank that could result in a combustible mixture in the tank. This is causing grief for fire chiefs around the country. This is the same mechanism of failure proposed for the explosion of TWA flight 800. Every time you create a solution, it seems you plant the seed for the next problem. It will probably become necessary for the gas station recovery system to test for HC, and when few are present, disable the station recovery.

One final note: avoid the use of a siphon on late model vehicles. The hose may damage or get stuck in the one-way valve in the filler neck. GM has a special siphon hose for draining the tank when the fuel pump has failed. Be careful, you may be disassembling more than you want!

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