turboguzzi Posted July 12, 2006 Share Posted July 12, 2006 due to high oil consumption in, been reading the posts regarding the PCV valve here as well as the service manuals..... and something just doesnt make sense. Subaru seems to use a system unlike anything I've seen. the pcv valve attaches to the manifold after the throttle body. that indeed means that when coasting downhill there is a very large vaccum sucking the cranckase vapors into the engine AND new air enters the engine throught the valve covers tubes thus creating a strong air flow that no doubt, has big chances of picking up aerosoled oil so it simply burns.... I am baffled! No other engine that I worked on (and I worked on a few) has the pcv attached AFTER the throttle body under FULL engine vaccum or either "fresh air" intakes. Most engines simply suck whatever vapors come out without having to apply vaccum. maybe just some, i.e. the low pressure created downstream of the air filter. has anybody tried to modify the subbie system to a more common layout? planning on doing this for the sake of science. any warnings? thanks TG Quote Link to comment Share on other sites More sharing options...
Setright Posted July 12, 2006 Share Posted July 12, 2006 Ermm, the "backwards" system used on Subarus is MUCH better than what other brands use. Where does the blowby get blown by to ?? The crankcase. Where does the Subaru system draw these potenially harmful vapours from? Straight from the crankcase. What does it replace the vapours with? Fresh, filtered air. FANTASTIC The system you seem to prefer pulls the blowby up threw the cylinder heads, introducing harmful vapours to the valvetrain. Why do that? The PCV valve shuts at high vaccum, and it only opens wide at full throttle. If it doesn't shut at idle the engine will choke. I suggest replacing your PCV valve, it's probably got some gunk stuck in it, that means it doesn't seal properly. Quote Link to comment Share on other sites More sharing options...
Wayne Boncyk Posted July 12, 2006 Share Posted July 12, 2006 I agree. The only down side to the Subie design is that if/when the PCV valve gets contaminated and stuck partially open, then you will have excessive oil sucked into the manifold and burned during periods of high manifold vacuum. But, if operating properly, it is "cleaner" than the older designs. Quote Link to comment Share on other sites More sharing options...
turboguzzi Posted July 13, 2006 Author Share Posted July 13, 2006 OK, thanks for the info. Had another look at the manual and seen that the pcv valve does indeed close the passage at high vaccum. What confused me is that in other cranckase vent systems, there is simple a one-way valve going out of the cranckase. will check this afternoon to see if stuck. Is it a must to change? And BTW, shall I find it screwed to the manifold somwhere? cheers TG Quote Link to comment Share on other sites More sharing options...
Dr.Outback Posted July 13, 2006 Share Posted July 13, 2006 Usually it's on the right side of the manifold with a large diameter vacuum line and a clip style hose clamp. Use a 3/4" wrench. Quote Link to comment Share on other sites More sharing options...
blitz Posted July 13, 2006 Share Posted July 13, 2006 TG, I'm curious which vehicles don't use maifold vaccum? Every vehicle I've ever owned used manifold vaccum to run the PCV system. The only difference is that I've never had one that drew the blowby fumes straight out of the crankcase with fresh air piped-into the top of both valve covers like Subaru does. All my V-8's had fresh air entering the valve cover on one cylinder bank and drew the fumes out out of the valve cover on the opposing bank. All my inline engines had fresh air entering one end of the valve cover and drew fumes out of the opposite end of the valve cover. **** ******! Quote Link to comment Share on other sites More sharing options...
turboguzzi Posted July 18, 2006 Author Share Posted July 18, 2006 Thanks all for the replies. Pulled out the valve and it seemed very dirty asnd clogged. any problems washing it with gas? why is everybody suggesting getting a new one? blitz, most systems I know off use the light vaccum downsrtream of the air filter and hook up BEFORE the throtle body, mostly to the center of a typical pancake air filter cover . if you hook up after the throtle body you have to control a huge vaccum and indeed subarus depend on that valve working properly. But then, i live in Italy and emission systems might be different here than in the US. TG, I'm curious which vehicles don't use maifold vaccum? Every vehicle I've ever owned used manifold vaccum to run the PCV system. The only difference is that I've never had one that drew the blowby fumes straight out of the crankcase with fresh air piped-into the top of both valve covers like Subaru does. All my V-8's had fresh air entering the valve cover on one cylinder bank and drew the fumes out out of the valve cover on the opposing bank. All my inline engines had fresh air entering one end of the valve cover and drew fumes out of the opposite end of the valve cover. **** ******! Quote Link to comment Share on other sites More sharing options...
hohieu Posted July 18, 2006 Share Posted July 18, 2006 OK, thanks for the info. Had another look at the manual and seen that the pcv valve does indeed close the passage at high vaccum. What confused me is that in other cranckase vent systems, there is simple a one-way valve going out of the cranckase. will check this afternoon to see if stuck. Is it a must to change? And BTW, shall I find it screwed to the manifold somwhere? cheers TG Cleaning it with gas or a carb cleaner will work fine -- so long as you can hear the internal piston slide freely back and forth when shaken. Quote Link to comment Share on other sites More sharing options...
blitz Posted July 18, 2006 Share Posted July 18, 2006 blitz, most systems I know off use the light vaccum downsrtream of the air filter and hook up BEFORE the throtle body, mostly to the center of a typical pancake air filter cover . if you hook up after the throtle body you have to control a huge vaccum and indeed subarus depend on that valve working properly. But then, i live in Italy and emission systems might be different here than in the US.Interesting TG. Well, if what you say is true, then it's a fact; systems are apparently different worldwide. In the US, PCV systems were introduced in the mid-60's (I don't know the exact year) first in California, then the rest of the country a few years later. All systems here to the best of my knowledge have always used manifold vaccum regulated by a PCV valve. Prior to PCV systems, a "road draft tube" was used to draw fumes out into the atmosphere as the car moved down the road. Quote Link to comment Share on other sites More sharing options...
turboguzzi Posted July 22, 2006 Author Share Posted July 22, 2006 did an intresting test. with the engine running I disconnected the hose to the valve and found that at idle, there was a considerable vacum at the valve entrance. cleaned the valve from all the gum and still, it wont shut down ermetically. According to the replies here, the valve should shut down fully at high vaccum but it doesnt. Should it? Maybe its worn? Did anybody ever noticed if a new valve does shut the flow fully? Thanks Quote Link to comment Share on other sites More sharing options...
ferret Posted July 22, 2006 Share Posted July 22, 2006 The PCV Valve is a one way check valve. It's purpose it to allow air to flow FROM the crankcase to the intake so the oil bypass fumes can be burned up and not released to the atmosphere. By being a valve it closes when there is pressure, not vacuum in the intake such as an engine backfire or misfire. At that time the intake can have a flame in it and the valve stops the flame from igniting the oil fumes in the crankcase thereby destroying the engine. Granted backfires are not as common on todays electronically controlled engines, but they do occur. Quote Link to comment Share on other sites More sharing options...
edrach Posted July 22, 2006 Share Posted July 22, 2006 If you wind up replacing the PCV, use a genuine Subaru one. The Board is rife with comments about "after-market" valves not working properly. Quote Link to comment Share on other sites More sharing options...
SaltCar Posted July 23, 2006 Share Posted July 23, 2006 How much is a PCV at the stealership these days? Anyone have a favorite thread on PCV replacement and passage cleaning? Quote Link to comment Share on other sites More sharing options...
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