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Meet The Purge Solenoid

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How the Purge Solenoid Works in the FS engine:

The vapor line begins its life at the top of the fuel tank where vapor gathers. The vapor line runs (next to his best-good-friend the fuel line) all the way to the front of the car. After that the fuel line and vapor line part ways and wave goodbye never to see each other again. In the engine bay the vapor line connects to the charcoal canister. From the charcoal canister it connects to a metal pipe sitting on top of the intake manifold. I'm sure you're all familiar with those little pipes. On the other end of the first-most-forward-pipe it goes into the purge solenoid valve. From the purge solenoid the vacuum line is routed back into the 2nd-most-forward-pipe on top of the intake manifold and it ends its life cycle plugging into the side of the throttle body. The purge solenoid valve is fed battery voltage and is an open or closed switch.

Conditions of Operation

The purge solenoid valve is controlled by duty signals from the PCM to perform purging of the charcoal canister. Purging is done when these conditions are met:

(1) After warm-up

(2) Driving in gear

(3) Accelerator pedal depressed

(4) Heated oxygen sensor functioning normally

When the conditions are met the solenoid is sent 12v by the PCM and the solenoid (electromagnetic switch) opens and begins venting fuel vapor. The intake manifold vacuum sucks vapor out of the line all the way back to a 2 way check valve which is located between the charcoal canister (front) and fuel tank (rear). At this time I don't know the exact mounting location of the check valve. All vapor is sucked out using manifold vacuum so the purge solenoid relies on the amount of vacuum inside the intake manifold to work correctly. The vapor is mixed with the air in the intake manifold before finally being fed into the combustion chamber along with fuel from the injectors.

Does the purge solenoid valve create vacuum?

No, actually the only thing that creates vacuum are the pistons in your engine when they go down. When the pistons go down vacuum is created inside the entire intake manifold. That's where manifold vacuum is created. The purge solenoid uses vacuum from the intake manifold. A blown intake manifold gasket and old charcoal canister are the worst enemies to the purge solenoid valve.

Does the purge solenoid valve suck vapor from the fuel tank?

Honestly I haven't figured that out yet. It's possible. I still have to figure out what manifold vacuum psi is compared to the check valve psi.

Will disconnecting the purge solenoid have any detrimental impact in the short or long term?

No, other than reduced gas mileage (MPG). You shouldn't notice any drivability issues. If you do notice a drivability concern then it's possible that you have multiple issues working in concert. A broken or disconnected purge solenoid is basically the same thing as leaving your gas cap loose which you can do as a replacement for a bad purge solenoid. Just to be safe please note the potential dangers of venting vapor from your gas cap. The purge solenoid is basically the on/off switch for a fuel vapor recapture system. Instead of venting to the atmosphere the fuel vapor is sucked into the engine as a separate system.

Does the charcoal wear out?

The job of the charcoal canister is to scrub fumes before they enter the intake manifold. Like any charcoal media it's effectiveness is reduced over time. Same concept as charcoal filter in a fish tank. It needs to be changed every so often because the charcoal isn't effective forever. Charcoal has a shelf life. According to the manual the entire vapor system from the fuel tank to the purge solenoid is to be inspected every 100,000 miles. Replace parts including the charcoal canister on an as needed basis.

Does a blown intake manifold or throttle body gasket affect the purge solenoid?

Yes, any unmetered air will reduce the effectiveness on all systems which depend on manifold vacuum. The purge solenoid relies on manifold vacuum to do its job so any loss of vacuum lowers its effectiveness.

Note: the Purge solenoid in the 1998-2002 Mazda 626 was moved to the top of intake runner #4. The design and diagnostic procedures are exactly the same only the location moved.

Diagnostic Videos:

img-341461-1-F3-138.jpg

img-341461-2-F3-139.jpg

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Excellent explanation of the Purge solenoid and how it works/what it does.

I can't wait for the pics and video.

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Posted video explanation and a comprehensive diagnostic video. Covered just about everything i could think about concerning that little bugger. Next up on le menu... the PRC solenoid.

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If there's a problem with the purge solenoid valve being stuck open, I'm assuming it would show up in the fuel trims?

 

I've read in the 98 WSM that a malfunctioning psv can cause rough idle and lack of power on acceleration and was wondering how that works.

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I highly doubt that even the most veteran mechanic would be able to detect a difference in fuel trims from a stuck open purge solenoid on the 626.  The amount of additional fuel vapor is so little that it likely wouldn't show up at all.  The Mazda 626 is different from other vehicle manufacturers (US Domestics) that use much larger evap hoses and purge solenoids making detection via fuel trims a valid and recommended method of diagnosis.  They are so small on the 626 that it's not worth pursuing because one can easily misinterpret the data, results would be inconclusive and would need more investigation regardless, therefore it cannot be relied upon.  Attempting to use fuel trims as a diagnostic tool for a purge solenoid issue on a 1993-1997 Mazda 626 is specifically and officially advised against in the work shop manual.  It requires a physical inspection and jumper exactly as shown in my video. 

A stuck open purge solenoid is not a vacuum leak.  The air is not escaping the system out of the intake or anywhere else. All of the incoming air has already been accounted for by the MAF/VAF.  There is a safety vent to prevent suction from caving in the fuel filter or fuel tank.  No worries.

For a 1998-2002 model an expensive bi-directional scan tool can be used which of course I did not cover in my video because I do not own one.

If there is a vacuum leak in the rubber hose to the purge solenoid it will throw off fuel trims just like any other vacuum leak.  There is nothing special about that, a vacuum leak is a vacuum leak.

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We have an ultra low mileage 1996 626 with a V6. Can anyone confirm where the Purge Solenoid Valve is in the larger engine? I think we found it, but would love to have confirmation since my son and I aren't exactly experienced mechanics. We've got a P0443 and it is causing us to fail inspection. We're thinking about changing out the valve. Thanks for any help you can provide.

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