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What is the function of the evap valve under the intake manifold?


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Posted this on the 986forums and no definitive response, so I'll try here.

I'm trying to chase down a vacuum leak in the evap system and I'm having trouble figuring out how the system works.

Near the carbon canister, there is a "tank vent valve" (also called a purge valve) that is controlled by the DME to purge the carbon canister of fuel vapor under certain conditions. The TVV works in conjunction with another valve near it, the "shut off valve" which opens and closes to let fresh air into the system when the TVV is open and purging the carbon canister.

So what is the purpose of another valve (I don't even know its name) under the intake manifold thats in the same circuit as the TVV?

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I assume you are referring to #1 in the drawing below:

P0446Systemdiagram.jpg

When #1 opens, it draws the vapor laden air out of the EVAP canister into the intake to be burned.

If thats the one under the intake manifold, then yes. but my understanding is that there is a tank vent valve near the canister that opens and shuts to vent the canister, Are there two?

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There are multiple valves on the system, as well as pressure and vacuum sensors. It is designed to function as a closed loop that prevents vapors from escaping from the tank by capturing them in the activated carbon in the EVAP canister where they are held until drawn into the engine to be burned, and also preventing the gas tank going under excessive vacuum. Here is a simplified explanation of how it does this: http://www.agcoauto.com/content/news/p2_articleid/294

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To me it's sole purpose is to fail and drive you nuts trying to replace everything around the front of the car fuel related until

you discover this valve exists :-)

m

:thumbup: EVAP system problems are second only to girl friend/wife issues in terms of complexity and level of general aggravation............

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To me it's sole purpose is to fail and drive you nuts trying to replace everything around the front of the car fuel related until

you discover this valve exists :-)

m

Well, I spent a couple of hours digging into the evap system at the front of tHe car. The Shut off valve, which is often diagnosed as the problem, works fine when I applied 12 volts to it, so I'm ruling that out. So I'm going to take a chance on the valve under the intake manifold. When I pinch off the line that goes to it, right near the throttle body, the vacuum leak goes away.

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That's fascinating. I remember in one of my early cars (1957 Morris) having a fuel problem with the car stopping every ten miles or so for no apparent reason. I eventually discovered, quite by accident, when filling up with fuel near my home that I could not take off the fuel filler cap. It was held on by a vacuum in the tank. The cause was a blocked vent hole in the filler cap. Things have moved on, but the old problems seemed easier to solve.

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