“Ford uses two types of non-enhanced evaporative emissions systems on its OBD-II cars and trucks. One system uses just a vapor management valve, the other uses a canister purge solenoid and a purge flow sensor. ”

Ford uses two types of non-enhanced evaporative emissions systems on its OBD-II cars and trucks. One system uses just a vapor management valve, the other uses a canister purge solenoid and a purge flow sensor.

The difference between the two valves is that the canister purge solenoid has just two wires and two hoses. One hose goes to the charcoal canister, the other hose goes to engine vacuum - either ported or manifold - depending on application. One of the wires gets battery voltage with the key on, and the other wire goes to the PCM to be grounded. The vapor management valve is almost the same, except that it has a third hose that also goes to engine vacuum. It has to have the electrical part of the valve energized and also has to have vacuum applied to the control port to allow vacuum to flow through the valve to the system.

The first system uses just a vapor management valve (VMV) and tests the system by monitoring the control on the idle air control valve (IACV). Basically, when the VMV is energized, it becomes an additional source for air to enter the engine. The PCM is programmed to change the command on the IACV to maintain a set engine speed, so when it energizes the VMV, it also checks to see how much it has to change the command on the IACV to maintain that engine speed. If the VMV is energized and it doesn't see the demand for a predetermined amount of correction, it assumes the VMV is not flowing enough, and will set a code for a lack of flow.

The second type of system uses a canister purge solenoid and an evaporative purge flow sensor (PFS). The PFS is a three-wire thermistor, with a battery voltage feed, a ground wire, and a signal wire going to the powertrain control module (PCM). The wire going to the PCM has 5 volts on it with the sensor disconnected, which is sent out by the PCM. The sensor then varies the resistance to ground based on the amount of flow through the sensor. As the airflow increases, the resistance increases proportionally, and the voltage increases as a result. The PFS is located between the engine and the canister purge solenoid. Depending on the vehicle, there will be either ported or manifold vacuum to the system.

When the PCM decides to test the system, it energizes the canister purge solenoid, and then monitors the voltage change on the PFS. A typical system starts out with the PFS voltage being somewhere between 0.7v and 1.7v. Once the canister purge solenoid has been energized and there is good airflow through the system, voltage should increase by approximately 1.5v.

The key is to make sure the airflow is good. One way to do this is to remove the hose from the charcoal canister so that it's open to atmosphere, and then manually ground the canister purge solenoid. The hose should then have good, strong vacuum from the engine. Obviously, if it's a ported vacuum system, it will be necessary to rev up the engine to create a vacuum on the system. If airflow through the system is good, and the voltage on the purge flow sensor does not change accordingly, verify that the power and ground feeds are OK. Then make sure there is 5v on the signal wire with the sensor disconnected to verify that the wire is not shorted to ground. If everything checks out OK, but the sensor voltage does not increase properly with increasing airflow, then you have a bad purge flow sensor. If the airflow is not good, check for a bad canister purge solenoid or blocked/collapsed hose.

The most common code that will set with these systems is P0443, which has to do with the circuit of the canister purge solenoid or vapor management valve. This code is caused by an electrical problem inside the solenoid, the wiring to it, or a bad PCM. The other common code is a P1443. This is due to the PCM not seeing enough flow through the purge flow sensor when the canister purge solenoid is energized or the IACV not needing to change enough when the VMV is energized.

Hopefully this overview of these systems will help you in the future with Ford cars and trucks that roll into your shop.

Note: For information on Ford Enhanced Evaporative Systems, see Bob Mordorski's TechTips column in the February 2002 issue of AutoInc. magazine.

Bob Mordorski is an IDENTIFIX Ford specialist. He is ASE master and L1 certified with nine years of diagnostic and repair experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2002 IDENTIFIX. All Rights Reserved.

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