Re: Purge valve & check engine light - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Re: Purge valve & check engine light
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Posted by saabdoc [Email] (more from saabdoc) on Wed, 19 Nov 2008 17:11:06 Share Post by Email
In Reply to: Re: Purge valve & check engine light, RonF, Wed, 19 Nov 2008 15:45:56
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Yes, its a common problem with EVAP systems. I shouldn't take 2 hours to diagnose.
When the EVAP system checks for a leak in the fuel system, It first closes the canister close valve, then it operates the purge valve. This creates a slight vacuum in the fuel tank that is monitored by a pressure sensor. If the purge valve is bad and doesn't open when power is supplied, then the vacuum from the engine never reaches the fuel tank where the pressure sensor is. Because the pressure sensor doesn't see a drop in pressure during the test, the engine control module assumes There is a large leak in the system.
When you run a EVAP test with Tech2, It shows on a graph what the vacuum in the tank is, and when the close valve is activated, so if the close valve is closed and it says the test is running, you will see the line graph for vacuum drop off. if it stays flat (never drops), you know the purge valve is probably not opening or you have a very large leak. You can then test the purge valve by activating it with tech2 and seeing if you can blow through it.
We usually charge 1 hour to diagnose EVAP problems. Unless its so obvious like a missing or loose gas cap.

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