Re: Purge Control Valve Operation - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Re: Purge Control Valve Operation
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Posted by Wayne - not Canton [Email] (more from Wayne - not Canton) on Wed, 7 Jan 2009 15:38:55 Share Post by Email
In Reply to: Purge Control Valve Operation, Wayne - Canton, Tue, 6 Jan 2009 19:53:32
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As you probably know, the evap canister is to trap fuel vapors from the tank rather than venting it to the air. The trapped vapors are then allowed to flow into the engine via the intake once it is warm enough to burn it off.

The flow from the canister to the intake is controlled by the purge valve - ie. to purge the canister.

I'm not sure exactly when it occurs, perhaps someone else will have the info, but there is an initial test of the system. The ecu sends a signal to the canister vent valve to close to seal the system. The purge valve is allowed to open, allowing the vacuum from the intake to cause a vacuum in the canister/tank system. The pressure sensor needs to see the vacuum occur, or you will get a code for "small leak," "large leak," etc. I'm not sure if all models have this tank integrity check. The closing of the vent valve only occurs during this test cycle.

Once your car is warm enough (I believe the ecu looks for 140 degree coolant temp), and sees negative pressure of the intake, it starts the purge operation. The purge valve starts operating on a "duty cycle" of opening and closing. I believe the pulse width signal is 8hz.

To make sure the engine doesn't get overwhelmed by the vapors, I believe the valve purges for 4 1/2 minutes, waits for 1/2 minute, purges for 4 1/2 minutes, waits for 1/2 minute, etc.

The normal state, with engine off, is closed.

This is my understanding of how it works...but I'm not a professional mechanic, and I don't stay at Holiday Inns.

posted by 98.229.227...

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