purge valve
 

howdy, first post on this form. 2008 GMC Sierra 5.3. P0496 and found purge valve open. Replace from autozone, 120 miles later, same code. replaced under warranty, code went away, 150 miles later, valve stuck open again.So that's 3 valves bad. anyone had this before. reading 14.volts at connector when truck running. I can feel switch click but stays open. thanks

Well ... unless you have something electrically shorting the switch to command it "open" it's a normally closed pulse width modulated "valve". If you use a scan tool to control the valve, it will present you with control from 0%-100%. 100 being full "purge" flow. Typically when they're stuck open, your fuel tank pressure sensor shows vacuum when the truck is started. That's 99% of the time how I check these. The other time is to physically blow through the valve with weak air pressure (your lungs :P). Like I said, normally closed. Otherwise, while the truck is running, the computer commands the valve to around 16% during a warmup cycle. So ... difficult to tell unless the operation of the valve is understood.

I'd say best test is to unscrew/unplug the valve from the truck, put your mouth on it and blow through it. If you can blow through it, she's bad. If not, then look elsewhere.

Thanks Null, didn't know voltage varied, thought either "0" or 12 volts. I had 12 volts
at start up. Bought two auto zone valves and they locked up to open position after I
ran truck for a while. Would unplug and valve stayed open. Bought OEM, probably same manufacture, CEL remained. 200 miles later, CEL went out. Drove another 50 miles and had Truck inspected for registration and it passed. Amazing. Guess I should consider PCM if it goes bad again. Thanks for good people like you out there.
Any other thoughts would be appreciated. God Bless U and USA

Voltage testing on a circuit like this can be confusing sometimes. I've found that the PCM on a truck like yours puts out a "bias" voltage to simply test if the circuit is open/shorted. It uses that bias voltage, probably a very low reading of maybe 1.5v, to monitor voltage drop on the supply wire. So if the sensor is unplugged, you'll get a different DTC than if the valve is stuck open. That's how the computer differentiates between a mechanical part failure and an electrical failure.

That in mind, taking arbitrary voltage readings can be extremely confusing sometimes because the computer might switch between bias voltage and full component voltage from the internal impedance of your volt meter. So typically I used connected circuit testing methods for a more predictable reading. That means usually I use a small sewing needle to back probe the connector while it's plugged in.

Or, what's even easier than that, is to forget all the fancy computer tricks. I use a and a few jumper wires and pins to simulate the purge valve being plugged in and use a scan tool to command the purge valve from 0-100% and watch the noid light get brighter/dimmer respectively.

You can also use a high impedance test light instead of a noid light.

Lots of food for the brain here - I find this stuff fascinating