Thursday, November 23, 2017 Detailed Auto Topics
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Evaporative emissions or EVAP systems, vent fuel tanks and help contain fumes produced by fuel. Self-testing is mandatory. Chrysler uses pressure, rather than a vacuum to tests their systems and this can create problems unique to them.

Many check engine lights are due to malfunctions detected during self-testing of the EVAP system. The previous Detailed Topic discussed the vacuum testing method. Rather than vacuum, Chrysler employs a leak detection pump, to pressurize the fuel tank. They also employ this type system on many European and some Asian vehicles.

The heart of the Chrysler system is the leak detection pump, or LDP. We do not need a pressure sensor and vent-solenoid in the fuel tank. The leak detection pump serves both functions, and creates the pressure, for testing. Most check engine lights with this setup, will be a leak or a failure of the LDP.

Conditions that the vehicle needs before testing

Before tests begin, the Chrysler system must meet several conditions, as with the vacuum style. For instance, intake air temperature must be near that of engine coolant. This means the engine has not been running and we have not heated the fuel.

The tank should be between 1/4 and 3/4 full to run the EVAP tests

Other conditions include, vehicle speed below 35 MPH and fuel level between 15% and 85%. We need A minimum fuel level of 30% for the small leak test. Ambient temperature cannot be below 40 degrees Fahrenheit or above 86 degrees. Barometric pressure must be 22 inches of mercury of more. If the vehicle has not met these conditions, the tests will abort until they are.

Vehicles at high altitudes or in extreme temperatures may not complete the tests. If codes are cleared or we disconnect the battery, readiness tests will show to be incomplete. This means the vehicle may fail State emissions testing although the system is operating.

How pressure testing works

Typical evaporative emissions leak detection pump

After the vehicle meets the preconditions, the power control module or PCM, starts the tests. First is the large leak test. The PCM opens a solenoid and the engine vacuum pulls the LDP diaphragm up. Filtered air is drawn in through the intake valve. The PCM now closes the solenoid and a spring moves the diaphragm down. Downward motion forces air out of the exhaust valve.

PCM control of the solenoid continues, creating pressure. When the pressure reaches one-quarter pound per square inch, the LDP spring lacks force to move the diaphragm down. The pressure holds it in the up position, closing the vent valve and triggering a reed-switch. An open switch notifies the PCM that testing may begin.

Phase one, measures the time needed to pressurize the system. If pressure builds too quickly, the computer assumes a pinched hose or blocked canister. This will cause a code P1486 to set, and a check engine light.

Not being able to reach a pressurized state assumes a large leak, and the check engine light comes on.

Once the reed-switch opens, showing a pressurized system, they measure time until the pressure falls. When the diaphragm moves down, the switch closes showing pressure has dropped. The PCM determines the exact time that the tests should require. Generally, the large-leak test needs 1.2 seconds to pass. The system needs around six seconds to pass the small leak test. Pressure dropping, before the predetermined time, turns on the check engine light.

Leaks in the EVAP system 

The sealed leak-detection-pump assembly, is not repairable. Most components are in this part, so it creates most check engine lights. Other causes included leaks and blockages in the system. Cracked hoses are very common and very small leaks will set a light. A charcoal-canister may also plug or break down, damaging the LDP.

improper fuel pump installation causing a check engine light

After fuel pump replacement, the seal, at the top of the tank is another common cause of leakage. Threading the retainer to the tank, requires care, to avoid cross-threading. The fuel tank uses an O-ring as a seal, and we should always replace it with a new one.

Locating leaks is much easier, with a smoke-machine. This device uses a chemical and nitrogen gas to produce pressurized smoke. Smoke flowing out, reveals the source of leaks. Such a machine can save many hours of diagnostic time and needless component replacement.

Lightly pressurizing the system and brushing suspected areas with leak detection soap, may help folks without such equipment. The Chrysler EVAP system is not complicated. With understanding and a little time, resolving most codes is straight forward.





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