School me more on EVAP system

[IZwack]

I kind of understand how the EVAP works - fuel vapors collect in the canister and the purge valve (controlled by the ECU), vents these gases into the intake.

With a purge valve thats normally in the closed position, I assume the valve opens when you first crank the engine, is that correct? Also, it must open again repeatedly at some point in order to replace the volume of gasoline (that was used up by the engine) with air -- when does this happen? Is it just a timed thing like every 5 minutes the valve opens?

 

[dogfriend]

I don't believe that it opens as soon as you crank the engine - my recollection is that the purge valve opens during warm cruise conditions - once the engine is warm and under regular load. It also may open as part of the OBD2 test that is run each cycle to determine if the system is vapor tight (no leaks above a threshold value).

If you have a scan tool that can read PIDs, you should be able to monitor it to see when it opens.

Also, I believe that it is not open or closed, but it is controlled using a PWM signal that has a duty cycle (PID value is %)

 

[IZwack]

Thanks much dogfriend!

So if the vehicle is cruising down the highway, the valve is open at some percentage in order to displace the gasoline volume thats being used up?

I cant really scan mine cauz I'm going to be using MegaSquirt as the EFI controller. But I'm thinking maybe I should operate EVAP manually - have a ball valve in the tube going to the intake manifold and just open it a little maybe a few seconds after the engine starts up.

 

[dogfriend]

My EEC-IV book (Ford Fuel Inj & Electronic Engine Control) says :

Canister Purge (CANP) is open during Warm Cruise (closed loop), Warm Driveaway (open loop w/ quick switch to closed loop), Part Throttle Acceleration (closed loop) and Deceleration (open loop, lean, closed loop, some fuel cut)

CANP is closed during Engine Crank, Cold Start/Warm Up, Cold Driveaway, Full Throttle Acceleration and Warm Idle.

I think the idea is for the Purge Valve to be open when you have relatively high vacuum to draw the vapor into the intake and conditions when the extra fuel vapor won't condense or throw off the fuel/air ratio very much.