Thread: Purge Valve Electrical Conundrum

I'm working through a P0440 issue that I've been putting up with for a couple of years. I'm finally tired enough of clearing the code to solve the problem.
I've already confirmed that the pressure sensor is good. I've smoked it and replaced the gas cap due to a small amount of smoke that appeared near the cap.
Here's where it gets odd to me and suggests there's something that I don't understand.
I took the purge valve out and tested it on the bench and confirmed that it opens when there is 12v applied.
I check the harness and confirmed 12v between the two wires in the plug when the ignition is on.
When I connect the valve to the wiring though, it does not engage. If I connect the 12v line from the plug to the valve along with a ground from the chassis, the valve engages.
How is this possible. In my brain, if I get 12v between the two leads on the connector, the valve should engage regardless of where it's coming from.

I feel like I'm not thinking this through properly, missing something, or don't understand the operation.

Can anyone tell me what am I missing here?

iirc that solenoid is PWM (pulse width modulated) on the ground side. it'll get grounded with little on time, most DVOM's cant react quick enough, if you were to put it on an occiliscope/DSO youd see a square wave pattern if im not mistaken.


to test operation youd want a bi directional scantool to tell the pcm/bcm to command it on to test.

Ah. That would make sense. Thanks. I'll work out another way to test it but it seems like the valve works at any rate.

vent valve near the charcoal cannister/fill neck tend to get grit into the foam filter and stick open/seep...if the foam is even still there...

the purge solenoid is the main issue, since th eearly days of the charcoal cannister these have been getting plugged/stuck open with carbon granules that escape

put the purge on the bench and actuate it with a 9v and tap its inlet and outlet sides on a piece of white paper and look for dark gray grains of grit....even if ya dont see em i still like to put a small carb fuel filter inline between the purge solenoid and the cannister line

and if its stuck open/jammed then a smoke test will only fill the intake manifold after about 30minutes to an hour before youd see the smoke at the TB or otherwise

You know, it's great to have kids that share your hobbies and interests but it really sucks when you come home find they've torn their engine's down and you can't get into the garage until they get it back together.

Anyhow, thanks for the info Turbochard400sbc. I still plan to pull the vent valve but I may have sorted this out. It appears to have been a combination of bad vacuum and bad ignition coil. I started having hard acceleration issues a while ago so, while checking my purge valve again, spent some time figuring that out. That turned out to be a bad coil. I also found some tiny cracks in a couple of vacuum lines and fixed those up. Since replacing the coil and fixing the vacuum leaks, I haven't had a code. It's been about 150km / 4 hours of run time which should have been enough to kick the code.

Thanks again!