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Re: EVAP /ELCD purge valve operation
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 18 Jan 2005 08:53:05 Share Post by Email
In Reply to: Re: EVAP /ELCD purge valve operation, TML [Profile/Gallery] , Mon, 17 Jan 2005 15:27:50
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It's not clear from the documentation, but I do believe that the purge valve continues to operate even at higher than 90Kpa. The purge valve runs while the system can tolerate the enrichment that purging provides, and there is no better time for enrichment than acceleration. I imagine that it would cut off during boost, simply because there would be no manifold vacuum to pull the vapors in.

The valve has 12 volts on one side, and the other side goes to the ECU. The ECU is a switch to ground. The switch to ground is for a very short time only, and is open, so I'd expect that measured across the coil, you'd see most of the time a voltage less than 12 volts, and every 125ms the voltage jump up to 12 volts, as the circuit is grounded. Inside the ECU is not just a transistor switch to ground - there is also a circuit that allows the ECU to read the voltage. This circuit would have some impedance, so it's reasonable that you'd get some current flow across the (purge valve) coil all the time, and that may be what's giving you the 7 volt reading.

I think it's time to take one giant step backwards and estabish what the problem is, if indeed there is a problem. I'm assuming that you get not Check Engine Light, that the car is running fine, and that the only issue is that the Purge Valve is clicking. I'd expect that at 8 hz, it's a rapid click, almost a rumble. If the purge valve isn't operating properly, if the driver in the ECU isn't operating properly, you'd get a CEL. If the Manifold Absolute Pressure sensor isn't working right, you wouldn't get a noisy purge valve - you'd get a CEL and the engine would run like crap.

So unless the engine isn't working right, I'd focus on the purge valve. It sounds like it's working right, just loud. Maybe a resonance - try changing the length of hose from the valve to the intake plenum. Try a new piece of hose. Try wrapping that hose with another piece of hose - take some bigger hose, slit it down it's length, and fold it over the existing hose - anything to act as a sound damper. Make sure the purge valve is attached properly. Make sure there are no leaks - the valve opening and closing could be opening the intake system to the outside air, and that would produce noise. And if none of that works, maybe you need to simply put in a different, hopefully quieter, purge valve.



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