[FullyZooked]

Hey guys,

Doing some tweaking to my Weber 32/36 this arvo. Just finished running my fuel return lines for the primary and secondary fuel supply.

I started her up without the air hat on and brushed my hand past the pipe that comes up off the inlet manifold. It has VERY strong Vacuum?! This is the pipe that runs from the inlet manifold and connects to the airbox along with the rocker cover breather.

Now, what I don't understant is why there is vacuum here? It must be manifold Vacuum as its super strong and with the engine running and I stick my finger over it, it lowers the idle. Now as I uderstand it, there is to be NO unmetered air to enter past the carb.

Sooooo I plugged the sucker up and went for a drive. Power is much much more responsive and pulls harder just on the primary?

What is this valve/breather and why is it there? Is it the PCV Valve?

[suzkboy]

i dont know what it is for but it would probably have been making your car run leaner. what size jets are in your weber? got any flat spots at all going on to the secondary's

[FullyZooked]

Still yet to check Jet sizes on the weber.

My main concern at this stage is this inlet manifold breather/vacuum robbing beast.

Your are correct in saying it would make my engine run lean. REAL LEAN!

Im looking for someone to say "thats not meant to draw vacuum thats the XXXX Valve..." then I say yea! And block that shit off asap!

[joeblow]

Thats the pcv valve, which sucks air from the crankcase etc and burns it up again.


If you are after even better performance....and a bucket load more reliability chuck a vit carby on it.

[FullyZooked]

joeblow wrote:

Thats the pcv valve, which sucks air from the crankcase etc and burns it up again.


If you are after even better performance....and a bucket load more reliability chuck a vit carby on it.

I wouldnt have thought the PCV valve would pull full vacuum at idle? I think it is faulty?

Is it safe to block this up or is it best to replace?

I would prefer to block it but don't want to run the risk of blowing out seals. How I understand it is that the PCV uses manifold vacuum to suck engine blowby back into the engine via the manifold. So blocking this wouldn't be an issue as the rocker cover breather is still open in the air hat?

If this Valve is indeed faulty and pulling 100% Vac all the time it would no doubt be the cause of my poor starting when hot and crap throttle response/fuel mileage.

Can the PCV valve god please reply and confirm

[FullyZooked]

Google is good...

The PCV valve is only one part of the PCV system, which is essentially a variable and calibrated air leak, whereby the engine returns its crankcase combustion gases. Instead of the gases being vented to the atmosphere, gases are fed back into the intake manifold, to re-enter the combustion chamber as part of a fresh charge of air and fuel. The PCV system is not a classical "vacuum leak". All the air collected by the air cleaner (and metered by the mass flow sensor, on a fuel injected engine) goes through the intake manifold. The PCV system just diverts a small percentage of this air via the breather to the crankcase before allowing it to be drawn back in to the intake tract again. It is an "open system" in that fresh exterior air is continuously used to flush contaminants from the crankcase and into the combustion chamber.

The system relies on the fact that, while the engine is running under light load and moderate throttle opening, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.

Sooooo. my PCV valve is shagged.

[Jackson]

How do you gather that? The PCV on my toyota engine runs to the base of the carb under the butterflies. I think there is some sort of valve in the crankcase that controls air flow.

But you had yours disconnected so it was just somewhere else for the manifold to suck from. If I pull mine off the same thing happens.

[FullyZooked]

Because my PCV Valve is pulling 100% intake manifold vacuum at idle.

I have just learnt that a PCV valve should only pull minor manifold vacuum at idle.

As the throttle increases manifold vacuum drops and crankcase pressure increases opening the PCV valve simultaniously thus letting more crankcase fumes/pressure into the intake manifold.

Making sense?

[skyman]

Quote:

I wouldnt have thought the PCV valve would pull full vacuum at idle? I think it is faulty?

You would be correct. They are a 3000 mile part and fail all the time. if your rig runs better with it plugged, replace it,and make sure it is plumbed in right.

we use the pinch test, pinch the line going to the PCV, it the engine runs better, replaced PCV.

If they are not plumbed right pressure builds in the engine and pushes oil past your seals. if completely blocked a short freeway run can push all your oil out and seize the engine (happened here with a CV carb and a poorly plumbed PCV.

[Jackson]

At idle manifold vacuum is at its maximum as your butterflies are closed. At the instant you open the butterflies manifold vacuum should drop but steadily rise back to maximum.

So if your PCV runs to the manifold, yes it will have alot of vacuum at idle.

Besides none of this matters... The PCV doesn't suck in air from outside the air system. It pulls air from inside the air system and re-routes it momentarily then back to the manifold anyway.

[FullyZooked]

Jackson wrote:

At idle manifold vacuum is at its maximum as your butterflies are closed. At the instant you open the butterflies manifold vacuum should drop but steadily rise back to maximum.

So if your PCV runs to the manifold, yes it will have alot of vacuum at idle.

Besides none of this matters... The PCV doesn't suck in air from outside the air system. It pulls air from inside the air system and re-routes it momentarily then back to the manifold anyway.

I don't think you understand. A stuck open/faulty pcv valve WILL pull unmetered air into the intake manifold resulting in a valuum leak.

Best thing about the Vitara air box and PCV is that the pcv runs into a T peice with the engine breather and then into the air box. This allows one to disconnect the PCV valve and block it without worrying about venting crank case pressure as it is still connected to the airbox.

[Jackson]

Your right I don't understand

I don't even get how the whole system could function with the manifold vacuum t'd into the breather? As soon as the PCV valve in the crank shuts from the vacuum, the manifold could just suck air through where the breather runs into the airbox?

[FullyZooked]

The PCV valve on the Vitara is bolted into the inlet manifold...

[Jackson]

Ahh... Now I am with it.

[stocker]

PCV - Pressure Control Valve.

At x Pressure, is meant to release gases ie: oil vapor. doesn't sound right that its sucking air. Replace it

[JrZook]

stocker wrote:

PCV - Pressure Control Valve.

At x Pressure, is meant to release gases ie: oil vapor. doesn't sound right that its sucking air. Replace it

Errrrrrrrr no...................

Positive Crankcase Ventilation valve.

What I did was block it off completely and just route the rocker cover breather into the top of the carb/air filter hat.

[honks]

is there any harm done to the engine disconnecting it and hooking the beather straight up to the air filter or is pcv just an emmissions thing. Because i just noticed that mine will suck and stick to my finger at idle but looses vacum when revs rise is this right.

[fritter]

honks wrote:

is there any harm done to the engine disconnecting it and hooking the beather straight up to the air filter or is pcv just an emmissions thing. Because i just noticed that mine will suck and stick to my finger at idle but looses vacum when revs rise is this right.

Can anyone answer this?

Mine does the same.

[JrZook]

Nope no harm done

[fritter]

JrZook wrote:

Nope no harm done

Cheers.

[Jester]

some PCV valves arent really valves more a fixed orifice which is what my understanding is of the vitara system on the carby