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What exactly does the purge control solenoid do and how does it know when to do it? It seems to be part of the PCV system but what would happen if it wasn't working?

it will cause a CEL. as far as i know it is related to emisions and wont hurt the car. both my cars have had bad PCCS and have had no ill effects as far as i could tell.

one car had no change when i replaced it and the other (the SS) is getting around 17-18 mpg but i dont think that has anything to do w/it as i just got 22mpg on the highway w/some high rpm second gear forest road driving thrown in there for a few miles.

Thank you

It's not part of the PCV system. It's part of the evaporative emissions control system.

Gasoline evaporates, right? And you don't want its vapors escaping into the atmosphere, because that would a) pollute and b) be a waste of gas.

So, we have this canister behind the passenger side headlight. It's full of activated charcoal, and has a hose leading to the fuel tank. The fuel tank itself is sealed at the cap, so all the vapors can only escape through this hose. They get to the tank and are adsorbed by the charcoal.

So, you have to eventually purge the canister by letting the engine consume the fuel vapors. This is done through the purge control solenoid valve. When the ECU opens it, the vapors can flow out of the canister, through the solenoid, and into the little nipple at the top of the throttle body.

Normally, the ECU opens the valve when the engine is at low or medium load but not at idle. At idle, it could screw up the air/fuel ratio. And at high load there wouldn't be enough manifold vacuum to suck the vapors up. And of course, if you left the solenoid open on boost, you'd end up pressurizing the canister and forcing the fuel vapors out of it, which is no good.

So you can imagine what happens if the solenoid fails by sticking either open or closed. If it fails closed you basically just don't get purging, and if it fails open you potentially get some screwy air/fuel mixtures off boost (the fitting on the throttle body is on the inlet side of the throttle plate when at idle, though, so it doesn't interfere too much with idle) and potentially have fuel vapors running around your engine bay on boost.

If the solenoid fails electrically, and ceases to present a load to the ECU, the ECU throws a trouble code for it and stops trying to drive it. I don't think it does anything else.

Umm, thanks Vikash. Lol, that was what I was after, an explanation of how it works and it's relative importance to fueling. I did realize that it was connected to the purge canister but I didn't know how important it was and I thought it was tied into the PCV. You see, I noticed one time that when the hose on the solenoid was disconnected that my car wouldn't idle or run as well. I didn't know it was a solenoid until I looked it up in he Haines yesterday at Brent's house.

If I'm seeing things correctly then one hose connects to the throttle body and one to the purge canister, yes?

Yep, you have it right. The hose going to the canister is actually two hoses and a hard pipe though:

http://www.graphics.cornell.edu/~v/vacuum/

Ok, I get what the PCS does, but what does the Auxilary Purge Valve do?

Does anyone know if the Liberty RS came with this charcoal canister crud? I don't think it did, but I'm not sure, I heard it didn't have a purge control solenoid so I assumed it didnt have the canister either. I really should be asking this question at rslibertyclub but oh well...

Edit: I just did a quick search on rsliberty club and found out the liberty does have a PCS and Charcoal canister, looks like I'll be looking for a new pcs...

The auxilary purge valve is just another way for the vapors to enter the intake track. I believe the aux. valve is opened with positive manifold pressure.

My two '91 Legacys have the same problem. The part is destined to fail every 7 or 8 years, and rather pricey. I went to a junkyard a few years ago and checked 8 or 10 of the cars and only found 1 that was still working, and it failed within 6 months.

The ECU keeps track of two things related to the valve. 1) Whether or not it is open or short circuited. 2) Whether or not the fuel tank pressure changes when the aux valve is opened with engine running. A problem with the gas cap or fill neck will also give this indication. You can't trick it by replacing the valve with a resistor and either plugging the hoses or connecting them.

It you don't like the Subaru price, buy any other car's valve with the right tubing barbs.

Or just ignore the little light. The reformulated gasolines have much lower vapor pressures than they did in '73 when these first appeared and the charcoal canisters are probably obsolete.

That's not correct -- our ECUs don't monitor tank pressure. All they monitor is whether there's a reasonable load presented to the purge control solenoid's driver. A resistor will indeed work.

The problem with ignoring the light is that then you're not alerted to other engine management problems.

One thing you can do to extend its life is relocate it to a place where it is not subjected to as much vibration and heat. On one of my cars I mounted it right by the ABS hydraulic unit, next to the charcoal canister. I should take some pictures.