PCV Valve

[BichoBII]

When i bought my 84 BII there was no pcv valve, just a breather, so decided to install one, but when installed i notice vacuum compressed spring to the point valve closes, this happens on and of the valve cover, so i assume its not blowby. I installed a carb jet inside the hose to reduce vacuum to the pcv, now it doesnt close, but i have the feeling something is not right. Btw it has been durasparked. Any thoughts? Thanks in advance!

 

[kimcrwbr1]

Remove the jet in the vacuum to the pcv valve. Where are you connecting the hose to the PCV to it should have full manifold vacuum. To test the PCV valve just stick your finger over the end of it to feel for vacuum pressure. If the engine runs good just stick it in the valve cover and let it do its job.

 

[BichoBII]

It had full manifold vacuum, sucked my finger really hard, the problem is at idle vacuum is too high, or valve too weak, that inner spring is compressed and valve is blocked, reducing idle speed till it dies

 

[benhojo]

Where did you get the pcv? Maybe its to large, or they sold you the wrong part. If its getting vacuum and sucking open then it's installed correctly. Either go exchange your part or check your breather not allowing fresh air in..

 

[BichoBII]

Thanks for the response, i got the valve at a parts store here in ecuador... it does suck the valve open, but if i shake it a bit it opens all the way, compressing the spring and "closing" the valve, my feeling is that it is too small... i tried a chevy 350 pcv valve and it handled the vacuum, but i was worried if the stock pcv was not working well there might be some kind of problem. By the way the engine has been "durasparked" by previous owner, was there any kind of vacum control in the stock system? maybe i need an older 2.8 pcv valve...

 

[benhojo]

The original 2.8 pcv with the feedback carb should have had two vacuum line connectors on it. I for the life of me can't remember where the top one went but bottom got full vacuum. You would just need to plug the top vacuum connector with a vac plug,if durasparked. Also the 350 pcv would be way to big, would make your mixture way too lean-sucks too much air from the crankcase. Point of pcv valve is to suck moisture or blow by/fumes from the rankcase. As rpm increases vacuum pulls the valve further open to compensate for the increase in blow by...

 

[BichoBII]

isn't vacuum reduced with throttle opening-increasing rpm?

 

[benhojo]

No manifold vacuum should rise as rpm increases=the cylinders are requiring more air=creates more suction...

 

[adsm08]

No manifold vacuum should rise as rpm increases=the cylinders are requiring more air=creates more suction...

You speak of thing that you do not understand.

Air moving does not create vacuum. Pressure differences create vacuum. Vacuum is just an area of pressure that is less than atmospheric.

In a gas engine atmospheric pressure is available at the outside of the throttle plate, all the time. The plate itself is a restriction when closed or at part throttle. As the throttle plate opens up it becomes less of a restriction to air flow and so the pressure on the outside of the plate and the inside of the plate begin to equalize.

If you get a set of AP (absolute pressure) guages that run from about 30 PSI to 20 in HG, leave one open to atmosphere and hook the other to the manifold of a running engine and watch them as you slowly bring it up to WOT, you will see the one hooked to the engine will go from about 15" hg at idle up to about 10-13 PSI WOT. Got try it. Even if you can only get one gauge and hook it to the intake. The other gauge would have read about 14.7 PSI the whole time.


The one and ONLY time that manifold vacuum will increase as engine speed increases is during engine braking because the throttle plate is closed and the wheels are driving the engine faster through the transmission.


Also, there are certain times when you do not want PCV flow. The PCV valve is just a check valve. It allows gas to pass when certain conditions are met. Idle is one time when you don't want PCV flow. Blow-by gasses are basically the same thing as EGR, just coming from a different source. EGR at idle makes a gas engine run poorly if it doesn't stall it outright. PCV valve is supposed to get sucked closed at idle.

 

[dangerranger83]

I used a pcv from a 351w off a 80s Ford since mine was allowing oil to come out my dip sick. Never had a problem after that.

That is when I had a 2.8 under the hood.

Sent from my rooted SPH-D710 using Tapatalk 2

 

[BichoBII]

PCV valve is supposed to get sucked closed at idle.

So what im describing is normal? Vacuum at idle should be so high that compresses the pcv valve inner spring, therefore closing the valve, and when opening throttle vacuum decreases, so valve is opened and it starts sucking?

If that is the case maybe i didn't have enough vacuum to keep valve closed at idle, it did closed but not always, making idle iregular. I recently replaced carb base gasket, and noticed vacuum advance increased a bit, maybe now vacuum is enough to keep valve closed at idle...