EVAP purge valve problem

[justdave]

Have an "Evaporative Emissions Control System" purge valve, a solenoid-activated open/closed valve with only two vacuum hoses in and out of it, sitting front top left of my V-6.
Sometimes, it is found to be pulsing on/off at a rapid rate (30HZ?), which i did NOT expect to find happening, ever, with such a valve. [I have removed it from it's metal mounting bracket when checking it, to make sure the vibration was not coming from the nearby injectors.] As far as i know, these kinds of EVAP valves (on or leading-to the Charcoal fume-storage Canister, and on or near the Fuel Tank) should be either open or closed, period!
I DO know that the PCM in this 99 Bonneville SE does perform scheduled "tests" of 3 kinds, on the EVAP system/parts, to make sure it is all working correctly. (Expect that is part of Federal emissions Law that all cars are supposed to do so.) The car has NEVER generated any DTC code related to the EVAP system or parts of it.
Anyone know for-a-fact IF this purge valve is EVER supposed to cycle on/off rapidly for some valid reason??
All i can think of, is that perhaps in the event of raw gasoline "liquid" getting into the EVAP lines, and maybe all-the-way into the Canister (on hard cornering or over big bumps), the system is supposed to cycle the engine vacuum quickly to try to suck the heavier-to-handle liquid fuel into the engine and clear it out of the system??
*On this Bonne, the input to this purge valve is a vacuum line from the throttle body (getting excellent vaccum, and with no cracks in line or no leaks at rubber connectors), and other outlet is connected to the Canister.

[reb]

The vent valve is located at the charcoal canister and is either open or closed. The purge solenoid is located by the front valve cover. It basically is a small pump the moves gas vapor from the canister into the throttle body to be burned along with the pressurized gas. When active it will act as you have observed.

[myfirstbonnie]

Can you post a picture of what you are referring to and where it is located?

Do you actually have a problem or code, or just asking a general question?

[justdave]

>Thank you "reb". We thought that little monkey was 'acting like a pump', and you have confirmed it is supposed-to.
>Now have a new question, due to the 2nd part of your reply: "vent valve located on the Canister...". We always were told that at-least two solenoid-activated EVAP system valves, including the vent valve, were located back near or on the fuel-tank. So, question is, does this Bonne have any more EVAP valves back there?? (Or are we just 'lucky', and all of them are up front under the hood...)
>The car never generated any EVAP DTCs. Am posting a link to our purge valve pix. We bought a new one, 'cause of the heavy task it has accomplished for 163,000 miles of use, we figure it is 'tired out'...and it was only $25. http://www.autozone.com/autozone/parts/ ... 24311_0_0_
>FYI, the new purge valve has not 'solved' other problem with this car (posted in another MAP/MAF topic)...but at least we do not have to worry about IT any more. Next we will be pulling out the Canister and Vent valve to check them, and all the lines/connectors to/from them.

[reb]

Pressed into a hole on top of the gas tank is the FTP- fuel tank pressure sensor. The fuel tank sending unit also is located on top of the gas tank. This is where the fuel vapor vent line originates from running to the charcoal canister. The gas cap also has a built in spring loaded "vent" valve. All the above are part of the EVAP system.

[justdave]

Thanks, reb. That is another nightmare we are not looking forward to checking, since we already know there is severe corrosion back under there from the car's origin in Florida, and it's years up here in Northeast snow-salt. Good to know there are no more "valves" back up there, though. [Already knew about the pressure sensor, and the 'safety release' feature of gas caps.]
>Guess we can consider this thread closed. Thanks for the help!
*If i find anything interesting, of a technical nature, as i check the remaining parts, i'll post it.
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Update 12-11-2012: while using a Vacuum Gauge to isolate EVAC system problems, discovered that just one(!) GM rubber connector that is dried out from being under-hood for over 5 years, and that 'looked' perfectly fine...was actually causing a hard-to-spot 'small' vacuum leak of about 1/2 inch Hg.-----If you are using a "Pro' large-dial Gauge with a full 270 degree dial movement from 0-to-30 inches Hg, such a leak is easier to spot; if using the usual inexpensive Gauge available everywhere (sometimes having a combo fuel-pressure section on the dial) with only a 180 degree dial movement from 0-to-30, it can be tough to spot such a small leak; keep tapping the Gauge with a finger for better accuracy.-----Can recommend to everyone that you actually "remove" ALL rubber connectors and flex them thoroughly with your fingers "everywhere" to discover hidden cracks and crumbling rubber areas.-----It turned out that just one 'tiny' vacuum leak, combined with the lower-than-'normal' vacuum produced by my 164,000 miles engine...was enough to have been causing DTCs un-related to the EVAP system! (In my case, i chose to replace both the black nylon tube and it's old rubber connector...with a length of properly fitting reinforced rubber hose. *Do NOT use the cheap standard rubber hose that has no reinforcement braid within it; it may collapse under vacuum, or get pinched shut too easily at bends.)

[eazhar]

The vent valve is located at the charcoal canister and is either open or closed.

Wait is the vent valve (the one that connects out of the middle port of the cannister) located under the hood or by the gas tank?