The Evaporative Emission Control System or EVAP system as it is most often called, was designed to prevent gasoline vapors from escaping from the fuel tank and fuel system. When the computer detects a leak in the system it will trigger the check engine light to come on and warn you. A loose gas cap can cause the light to come on because the system detects that there is pressure loss. The system is designed to detect leaks as small a pin hole. BMW and Mini Cooper are equipped with a Leak Detection Pump (DMTL) for this purpose.
The most common fault codes for EVAP leaks are a PO442- Small leak detected, and PO455- Large leak detected. The PO455 code is most often caused by a gas cap that was left loose after refueling. The PO442 can be a much harder one to diagnose as it virtually impossible to diagnose visually. To diagnose the small leaks a smoke machine is used to find the leaks by feeding a mineral oil based smoke into the system under light pressure. The smoke may also have ultraviolet dye added to make it easier to spot under a UV light. These codes are most typically a result of intake system vacuum leaks, with the most common being cracked or otherwise leaking intake boots. The intake boots are an especially common problem on the 2nd gen Mini Cooper S model with turbos. On late model BMW’s that have a crankcase ventilation valve (most 6 and 8 cylinder models from the mid 1990’s to present day, equatable to a PCV valve) the common problem is the rubber check valve diaphragm ruptures, creating an internal vacuum leak.
Fixing small EVAP leaks can be a big problem even for professional technicians, and if you get a code for a small leak you will most likely have to take it to your mechanic for diagnosis as special equipment is required. The smoke machine diagnostic tool costs between $1000-$1500 depending on the manufacturer and requires some training to use properly. With EVAP leaks it is recommended that you always take your BMW or Mini Cooper to a certified repair facility as these can be nearly impossible to diagnose at home. Please watch our short 2 minute video as we demonstrate a smoke test on a 2007 BMW X3.
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