Fuel Recycle Canister Solenoid

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Fuel recycle canister solenoid.

It's function is to absorb the hydrocarbons from the fuel tank vent gasses at all times to prevent 'evaporative emissions losses'

When the engine is running, the thing needs to be purged to prevent it overflowing (breakthrough). Due to control issues, the engine can only accept a certain proportion of it's fuel (HC) as purge hydrocarbons, therefore the purge valve is used to regulate the flow of air/HC out of the can and into the inlet manifold via pulse-width-modulation (PWM) of the valve.

Because the evaporative emissions test has a standard drive cycle as a 'pre-conditioning' drive, the system is designed to purge the canister as quickly and as much as possible after a cold start, to leave it clear for when it gets SHED tested (Sealed Housing for Evaporative emissions Determination - naff US acronym for heating the car up in a sealed box to measure HC emissions evaporated off)

Also, because the ECU has (initally) no clue how much fuel is in the canister, it can only purge when closed-loop lambda (air-fuel ratio) control is operational, which is usually a few tens of seconds after start (at ~20°C)

Unfortunately, the closed-loop lambda control is also used for adapting errors and tolerances of the fuel/air metering system, but this cannot be done simultaneously with purge (due to the unknown amount of purge fuel). Neither can this be done until the oil is at a reasonable temp, since fuel that's diluting the oil will/may begin to boil off and add to the total fuelling via the PCV system (positive crankcase ventilation). Fuelling adaption must be also completed as quickly and effectively as possible during the pre-conditioning cycle to maximise the emissions capability of the car during the 'pukka' emissions test.

So, the plan is this: Nearly as soon as you can run closed-loop (lambda=1) you start purging as much as the engine/control capability will allow. Once the canister purge hydrocarbon flowrate has died down to acceptable levels (or min time period, oil temp), purge is turned off to allow the fuel system errors to be adapted for a while. Purge and fuelling adaption will then operate in turn for the duration of your drive.

...so you might hear your purge ticking coming and going, if it ever was cold enough for you to still hear it.

Systems don't tend to purge at idle for EU-2 cars (or earlier) because it adds complications to idle control. EU-3 cars tend to do so now though, because the new SHED test is 24hours long (up from 2hours? IIRC) The ECU also won't purge when you're off the throttle at higher engine speeds, as all fuelling is turned off in these conditions. That's why you can 'turn the ticking on and off' with the loud pedal.