greasemonkey

Does anyone know what the grey and black plastic gadget about an inch up the pipe from the middle carbon canister connection is supposed to be?

Mine's making a noise very much akin to a leaking dumpvalve under boost, so something inside is obviously vibrating. I can blow through it both ways, so if it's supposed to be a one way valve, it's bust...

I presume it's in the line from the throttle body purge, although I can't follow the pipe under the manifold. Either way, if someone can tell me what this thing is supposed to do I can either content myself that there's nothing major amiss, or replace it...

tweenierob

It is a one way valve, it is an extra precaution when on boost so that you do not over pressurize the cannistor.

Rob

RICH WILD

I removed the carbon cannister and this one way valve and left the pipes dumping to atmosphere.

I found that when filling up with fuel, there was no longer that hiss you get when you open the fuel cap.

Replaced the valve but not the cannister and the hiss returned.

IIRC the Subaru workshop manual states that this valve is to vent excess pressure in the fuel tank into the carbon cannister, though in what situations you'd get excess pressure I don't know.

I'll try to find the exact section again to check.

Cheers

Rich

RICH WILD

Just found the relevant bit in the manual

It's apparently a 2 way valve (that's something I've just learned too)

it says:

The 2 way valve is located in the evaporation line between fuel tank and canister. When evaporation pressure in the fuel tank exceeds the specified value, the valve is pressurised to push the spring. This lifts the valve seat so that the evaporative gas is discharged into the cannister.
When evaporative gas pressure in the fuel tank drops below the specified value, the valve moves to close the valve seat. However, air which enters from the inlet port of the cannister is introduced into the fuel tank through the pin hole located at the end of the valve.

johnfelstead

the carbon canister is there to assist emisions control. It prevents the hydrocarbons from fuel vapour being emited freely, there is a purge valve on the engine that operates when the engine is up to working temperature to burn of the excess fuel the canister traps.

On later model Impreza's you will throw a CEL if you remove the carbon canister, just like you get a CEL if you leave the filler cap removed and start the engine. Thats certainly the case on US versions with their emisions laws.

greasemonkey

Cheers fellas. Rich, I suspect Rob's on the money in thinking this is supposed to be a one-way valve - as mentioned it's on the purge line from the throttle body, not from the fuel tank, and it's got the arrow on it.

Obviously is bust, as it's allowing full boost pressure into the canister under WOT. Will have to check and see if there's a purge solenoid further up the line that should be closing, but for the time being I've just replaced it with a piece of machined alu to block off both sides. Thus, the can still deals with the fuel tank vapour - as long as all that compressed air hasn't already b*gg*red it!

RICH WILD

Have found the diagram from the workshop manual which shows the central pipe on the cannister goes back to the fuel tank via the 1 or 2 way valve thingy.

The purge solenoid is connected to the outer right hand pipe (standing in front of the car)

Can't get the pic to post up here though.

Greasemonkey, mail me and I'll send you the diagram.

I'd like to know which is right as well, so see what you make of it.

Cheers

Rich

greasemonkey

Rich, I've looked at the 97/8 service manual and that does indeed show the central tap to the carbon can being the fuel tank takeoff.

However, on my car, this is definitely the right hand (as you look from the front) tap. If you take the pipe off sniff, there's beaucoup whiff de gasoline. Centre one must be from the IC side of the inlet manifold, as it sends out a small amount of air at idle, rising to a lot of air when you rev it.

RICH WILD

Hmmm weird.

Which model is your car then? Wonder why it's different?

Rich

greasemonkey

Mines theoretically an MY00. It's got a really weird spec, so there are a few good reasons why the emissions control could be different, but logically I don't see why this particular section of the car should be arranged in anything other than the same way as other 99/00 cars.

RICH WILD

Saw a P1 engine bay last night and it has a little grey and black plastic thing in the right hand pipe that is about as thick as the pipe itself and also has an arrow on it.

Mine (MY97) doesn't have that it only has a larger grey and black valve in the central pipe.

Is it the little one that you are talking about?

That would explain my confusion. Sorry if I've been leading you up the garden path but mine doesn't have that on for some reason.

Cheers

Rich