The Evaporated Emissions system is designed to prevent the vehicle fuel system from emitting unburned hydrocarbons to atmosphere. It does this by the simple expedient of subjecting the petrol tank to vacuum through a carbon vapour store, which collects the evaporated fuel and stores it. At suitable moments the PCM opens the EVAP store to the inlet manifold which draws the fuel into the engine and uses it in combustion. The owner confirms that the EVAP is working every time he drives to a service station and opens the fuel cap to fill up. The hiss of released vacuum is caused by the EVAP.
Several different systems exist in OBD2, but we shall concentrate on the version used on the Scorpio.
The EVAP system has its own Electronic Vacuum Control valve, similar to that used on the EGR system, built into the Canister Purge Valve CANP,) mounted down on the offside engine bay. This connects a pipe to the EVAP Canister, a plastic container mounted near the petrol tank. The purpose of this valve is to open the EVAP store to inlet vacuum and hence purge it of fuel, commanded by the PCM.
The following conditions must be met for the EVAP monitor to run:
An additional sensor can be used to check for the correct vacuum, but this is not installed on the Scorpio so the DTCs P1444 and P1445 may not be encountered.
The only way in which a driver may decide that the EVAP system is not working is when there is no hiss of vacuum as the fuel cap is released after the engine has run.
The EVAP system is a simplified version for the Scorpio. A Purge Flow Sensor is apparently omitted, since there is no mention of it on the Ford Catalogue or in the Circuit Diagrams. The diagram below is a representation of the the Scorpio system as far as can be determined.
A careful check of all vacuum lines beneath the vehicle, near the fuel tank and in the engine bay, particularly after work in these areas should be made. Testing the CANP should be carried out only after this.
Diagrams © Ford (Europe)
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