Diagnosing BMW DMTL / LDP EVAP System

Editor's note: This article was originally published Aug. 27, 2013. Some of the information may no longer be relevant, so please use it at your discretion.

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.

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.

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.

The Engine Control Module (ECM) is in charge of both the operation and testing of the EVAP system. It checks the system’s ability to purge HC vapors that are stored in the charcoal canister, and checks the system for leaks as small as 0.010 inch up to 0.040 inch. These tests fall under the “non-continuous” monitors, meaning that the system is tested only once during a given drive cycle. Codes set are “two trip” codes, meaning that faults discovered must be confirmed before the ECM commands the Malfunction Indicator Lamp on. In addition, many EVAP monitors have very specific enabling criteria that can make it difficult to run a monitor manually. This includes factors like how long the car has been idle (cold soak time), the amount of fuel in the tank (usually needs to be between ¼ and ¾ full), and others that you should be aware of when troubleshooting EVAP system problems. Because Mode $06 has a listing of the specific tests used by the ECM, it can be a helpful tool in your troubleshooting of intermittent EVAP faults and for verification of the final repair.

Here are a few other EVAP system basics to remember when you begin your testing:

• The Canister Purge Valve is normally closed.
• The Canister Vent Valve is normally open and needs to be closed for leak testing. Global OBD II Mode 8 will perform this function on some models, and you may be able to use your scan tool’s bi-directional features for those that don’t.
• The service port Schrader valve has reverse threads (on systems that use service ports).
• Fuel soaked charcoal canisters equal a bad liquid fuel separator. There are different names for this component, but it still has to do the same job, allowing only fuel vapors to enter the charcoal canister. If the canister becomes fuel soaked you will need to blow out the lines in a well-ventilated area with an inert gas (not air) besides replacing the liquid fuel separator and charcoal canister.
• Line and hoses are the number one areas that need to be checked for leaks.
• Vent and purge solenoids need to be energized to the open/closed or closed/open positions at least ten times. We have experienced that just simply testing the solenoids by open and closing them a couple of times may prevent you from finding a defective valve that is sticking mid way. Try using your scan tools bi- directional control or use a Power Probe to exercise the valve to uncover one that sticks.

The three EVAP systems

EVAP systems can be distinguished by how the ECM performs the leak test. It is interesting to note that many systems use some form of vacuum decay method for self-tests while most of us tend to use some form of pressure test for our troubleshooting. As with other systems, EVAP leaks might be missed relying on pressure testing alone.

The first one we’ll overview are systems that use engine vacuum as the source vacuum for testing system integrity. GM, Ford and many imports use measurements from sensors giving feedback concerning the vapor management or purge control valve, solenoid-operated canister vent, and fuel tank pressure sensor. They self-test in the following order:

1. Purge valve is closed and the canister vent valve is open. No engine vacuum is reaching the canister and with the canister vent open system pressure equals atmospheric pressure.
2. Purge valve is opened and the canister vent is closed. This should increase EVAP system pressure by 6 to 8 inches of water.
3. Purge valve and canister vent remain closed while the PCM monitors how long the system retains sufficient vacuum.

Next is an overview of how the Chrysler Leak Detection Pump (LDP) EVAP system works, and how to test it. This information is very similar to the BMW system that uses a DMTL.

The purge solenoid is normally closed, grounded by the PCM. The feed side comes from a Key On Engine Off (KOEO) fused circuit. The PCM energizes the solenoid to purge fuel vapors from the canister and to lower tank pressure. The LDP’s vent valve is incorporated in the unit and is normally open. It supplies air to the charcoal canister.

The PCM checks for EVAP leaks by first de-energizing the Purge solenoid (normally closed), and then rapidly cycling the LDP solenoid and watching the LDP switch. Once pressure (7.5 in H2O) is built up in the system, the diaphragm will be seated upwards against spring pressure. The PCM knows this since it is monitoring the LDP switch. So, the PCM compares LDP switch position against LDP solenoid cycling time to determine if leakage is present.

When manually checking for leak the vent valve must be closed. Closing of the vent valve requires that the LDP solenoid be energized and that a vacuum source be applied to the LDP solenoid. This will enable the LDP diaphragm to stroke upwards, thereby allowing the vent valve spring to close the vent valve.

If the pump runs a short time before reaching pressure it is assumed that there is a blockage and only part of the system is pressurized.

Now let’s take a look at a system that uses a strategy called “natural vacuum leak detection.”

Ford Engine Off Natural Vacuum (EONV) uses similar components to what it used on conventional engine vacuum systems. The difference is that instead of using engine vacuum, natural vacuum (or pressure) created inside the fuel system is used to test the system for leaks after the engine is shut down.

Here’s how it works:

1. When the engine is shut down and the ignition is switched off, the canister Vent remains open to allow tank and atmospheric pressures to equalize. This is no different than the other EVAP systems.

   The PCM monitors system pressure via the Fuel Tank Pressure Transducer (FTPT). If the EVAP system pressure is 1.5 inHg or more, the PCM assumes that fuel in the system is generating a lot of fuel vapor pressure and very volatile. The PCM knows that excess pressure (volatile fuel) makes the test unreliable and aborts the test. Remember, the canister vent is open. If the system can generate a positive pressure with the vent open, it’s assumed that the fuel is very volatile, creating pressure faster than the system can vent it.

2. The canister vent closes. Since the Vapor Management Valve (VMV) or purge valve is already closed, the system should be sealed. The microprocessor continues to look at feedback from the Fuel Tank Pressure sensor. If the EVAP system is leak-free, pressure should change.

   EVAP system pressure is monitored. If the system has a leak, pressure will change very little or not at all. If a large enough change in pressure is measured (either a positive pressure or vacuum), it is assumed that the system is leak-free, and the test passes. The amount of change required for a pass depends in part on ambient temperature and fuel level.

3. If a positive pressure is recorded, but it is not high enough for a pass, the first test is repeated. The canister vent opens again to vent excess pressure from the system.

BMW DMTL System

Now with the three system explanations out of the way let’s move on to the BMW DMTL/LDP system. Since BMWs don’t have many problems with EVAP other than a few gas cap and DMTL/LDP problems it not that difficult of a diagnosis or repair. It’s always a good idea to check for Technical Service Bulletins (TSBs) and check resources such as AllData, Identifix and iATN.

BMW started using the DMTL/LDP back in 1998 so it’s far from being a new EVAP system. The components in the system includes the purge valve, liquid vapor separator/rollover valve, charcoal canister, fuel tank, ORVR (On- Board Refueling Vapor Recovery), gas cap, DMTL/LDP that includes a vent valve and filter. The system utilizes the DMTL/LDP to vent the fuel through its internal vent solenoid and is also used to pressurize the system while checking for leaks. The DMTL/LDP contains a spring loaded diaphragm which moves up and down by a solenoid that controls engine vacuum. When the DMTL/LDP is in this mode, the diaphragm produces a small amount of air pressure that is used to test for leaks.

The next step is the normally open vent solenoid is closed so the pump can generate the proper pressure along with keeping the purge closed. As you can see there is nothing different from the other systems that we presented before in this article. The pump works by pulling in filtered air and then pumps the air through the charcoal canister. If the filter is clogged, the system will not be able to achieve the proper pressure and a DTC will ensue. For the system to achieve a good pressure, the DME (BMW’s term for the ECM) pulses a vacuum controlled solenoid as the pump is operating. The DME monitors the diaphragm movement via the DMTL/LDP reed valve’s feedback and compares it to stored DME vacuum solenoid frequency.

As the pump operates the frequency of the diaphragm will slow down while the reed valve signal is monitored by the DME. The diagnosis of the system is monitored by the DME when it activates the DMTL and looks for the following:

1. The DMTL is activated as air is pumped through a restrictor orifice 0.3937" = 1.0 mm (0.040 inch leak) or 0.01969" = 0.5 mm (0.020" leak) that causes the pump motor to draw a specific amount of amperage.
2. Next the solenoid valve is energized which seals the system, delivering air pressure to the complete EVAP system.

   NOTE: This is an important step. Changes in amperage readings are used for EVAP system diagnostics.

3. Large leak in the system is diagnosed when there is no amperage draw.
4. Small leak in the system is diagnosed when the amperage draw is the same as the orifice size.
5. No leak is when the amperage is higher than the specific range of the restrictor orifice size.
6. All of the above test results for amperage draw are checked in 45 to 270 seconds. Areminder once again that the BMW EVAP system is not all that different from the Chrysler LDP system. This same basic information can be used for all LDP pumps.

I hope that after reading this article, you’ll have a better understanding of EVAP systems in general and feel more confident in diagnosing and repairing all the EVAP system problems you might come across.

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