What If It’s Not The Gas Cap?

February 16, 2008 | By | Reply More


gas cap

Very often, an illuminated MIL and an EVAP code are due to nothing more mysterious than the driver neglecting to tighten the gas cap. That’s hardly the whole story, though.

Since 1996, OBD II has required that fuel storage systems be checked for leaks so that no errant hydrocarbons will escape to pollute our atmosphere. Well, between fuel’s Reed Vapor Pressure and temperature fluctuations between cool fuel in the tank and heated return line fuel, this is no walk in the park. Add the difficulty of running the monitor to test for fuel tank leakage and you know by now how difficult it is to diagnose one of these codes. The call two weeks later informing you that the light is back on doesn’t help either. Here is a look at the different systems BMW uses to monitor tank leakage and some testing methods that may boost your confidence in your diagnosis.

In The Beginning

BMW was ready to check for leaks in its Evaporative Emissions System (EVAP) in 1996 with program HC II. Federal EVAP emissions regulations were not that strict early on, so non-enhanced systems were allowed. You could consider BMW’s first system non-enhanced. These vehicles were known as Transitional Low Emissions Vehicles or TLEV. You can determine what EVAP system you have by knowing the engine management system. TLEV vehicles use MS41.1 systems.

smoke gas cap

You can smoke any year system to check for leaks. Here the gas cap seal is missing, allowing smoke to leak past. Notice that the tether is cut. A BMW Technical Service Bulletin warns about these tethers getting caught between the filler neck and the cap.

A rundown of the components starts with the fuel tank itself. Second would be the Activated Charcoal Canister, which has a shut-off valve mounted on it. This valve is sometimes referred to as the Carbon Canister Valve in diagrams. The next component is the Liquid Vapor Separator that has a Fuel Tank Pressure (FTP) sensor mounted in it. The job of this separator is to capture fuel vapors during refueling. Finally, there’s the actual Canister Purge solenoid. With the system purging normally, the shut-off valve is open allowing air into the canister. The purge solenoid is commanded open and gasoline vapors are drawn out of the canister as well as the liquid/vapor separator. Before the test, the DME control unit monitors the fuel tank pressure sensor signal voltage.

To test the purge system, both the canister purge solenoid and the shut-off valve are closed. The FTP reads the resulting increase in pressure in the system. If the pressure increase is below a specific threshold, the testing continues. If the pressure increase is too high, then the fuel temperature is probably too high also and the monitor is abandoned. If the pressure is right, the purge solenoid is opened and the resulting pressure drop is read, the purge function is deemed okay. Once the purge valve is switched off, the DME expects to see an increase in the pressure.


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Category: BMW TechDrive, Driveability + Emissions

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