Not everyone understands EVAP systems especially if they have to diagnose and repair one on a BMW. We understand that you might have some anticipation about working on different EVAP systems since manufacturers use so many different parts and names for their components. Be assured that in this article we will provide a solid overview of the different systems that will make your diagnostic routine easier.
The EVAP system on this BMW shares similarities with the Chrysler LDP system. This car came in with code set for a major leak.
Because there are only three types of evaporative emissions (EVAP) systems used it won’t be that difficult to understand. The three systems are vacuum, pressure and natural vacuum and are defined by the method used for leak detection. In the following paragraphs we will provide a brief description of each before we move specifically into the BMW Diagnostic Module Tank Leakage (DMTL) / Leak Detection Pump (LDP) EVAP system.
Before we move on to the system descriptions we need to address the common misnomer that an EVAP problem is “just a gas cap.” According to SAE paper No. 1999-01-1463 the gas cap is to blame only a quarter of the time for an EVAP diagnostic trouble code (DTC). So, if you are in the habit of sending motorists with an EVAP-related DTC on their way after a quick twist of the fuel cap, expect that they will come back with the same problem they originally showed up with. Better to learn how to pin-point the real cause of EVAP issues.
EVAP System Theory
Because we are limited to the length of this article we are going to cover only generic principals of EVAP systems. The following will provide EVAP theory essentials, so that you know enough to be able to make educated judgments during your diagnosis.
A capable scan tool is a must for the proper diagnosis of today’s complex vehicle systems.
The EVAP system prevents hydrocarbon (HC) vapors from the fuel system from escaping into the atmosphere. On a vehicle with its engine off, the fuel in the tank starts to evaporate. Vapors travel through the vent hoses/tubes to be stored in the charcoal canister. When the engine is started, vapors are drawn/sucked into the intake and burned during combustion.
Most systems consist of a purge solenoid, charcoal canister, pressure/vacuum gas cap, fuel tank pressure (FTP) sensor and vent solenoid or valve. Any problem with these or associated parts (fuel filler neck, gas tank, associated rubber lines, etcetera) will prevent proper operation of the EVAP system.
EVAP has grown steadily more complicated now that vehicles are mandated to have ever more efficient systems that prevent fuel vapors escaping to the atmosphere. The demands placed on the EVAP system have increased in an effort to reduce HC vapors.