Just curious on how the PCV system works. Why does a good PCV valve "rattle" when shaken?

Also, I seem to have a leak in the line that goes from the front of the intake manifold to the PCV valve - if I listen carefully to that area, I can hear a slight but constant "hissssss". How would this leak affect how the PCV system works?

Would it cause oil to shoot out of the oil dipstick? My oil comes out of the oil dipstick and from the area where the tube from the TB goes into the passenger side valve cover.

I am hoping somebody can explain the PCV system to me so I can better understand what is going on and how the leak would affect PCV/vaccum - I would really appreciate it!

Thanks,

 

PCV = positive crankcase ventilation

The vacuum pulls air from the crank case, or, underside of the pistons. Basically it keeps pressure from building up behind you pistons when they are on the down stroke. Without it, you could get serious blowbye, and in extreme cases, I could see it causing oil to work back up your dipstick tube. High performance engines will sometimes use an external vaccuum pump to ventilalte the crankcase, and I read in CHP magazine that a 550hp motor picker up another 25hp just by installing the pump.
The 'rattle' is the check valve in there that prevents oil from being sucked up through the hose and into your intake manifold.

 

Air enters a port in front of the throttle body blades, and flows under the top cover plate on the throttle body, to the large port on the passenger side of the TB. From there is flows through the long tube to the passenger side valve cover. This air is pulled through the crankcase and lifter valley by vacuum from the PCV valve. The air enters a metal shield under the intake manifold, and goes into the end of the PCV valve that is poked into the drivers side of the intake manifold - note that this end of the PCV valve is exposed to the lifter valley, not the inside of the intake.

The PCV valve is under a vacuum, supplied by the line from the intake manifold plenum. On the early LT1's, that's the short curved tube that runs directly back into the side of the intake manifold from the PCV valve. On the later models, there is a line that runs from the front of the intake manifold to the PCV valve.

The valve has a spring in it to control the amount of vacuum the crankcase can be exposed to. Actually, the "ball" in the valve (what rattles when you shake it) is "suspended" between 2 springs that alter its position based on the realtive pressure in the crankcase and the intake manifold to control flow. Also acts as a check valve to prevent a manifold backfire from blowing into the crankcase.

So... the intake manifold vacuum is pulling air through the crankcase. If you have a leak in the hose from the valve to the front of the intake, you have a vacuum leak. It is causing the PCM to correct the A/F ratio using the long term fuel correction multipliers. It also means that the vacuum is being "lost" before it reaches the PCV valve, and the crankcase gasses are not being removed from the crankcase. Pressure can build up, and blow oil out the dipstick, or blow the hose on the passenger side valve cover off.

If it happens a lot, you may have excessive "blowby".... the combustion gasses that are leaking past the rings. In the event this happens, the system is designed to have the pressure in the crankcase relived by letting the gasses flow "backwards" in the hose to the throttle body. That is when you start to see oil under the cover plate. To get significant pressure in the crankcase, as you appear to be getting, either the line to the TB is plugged up, or your rings are really worn.

First, fix the vacuum line from the PCV valve to the TB. Then with the engine idling, take the oil fill cap off. Hold you hand over the opening and feel for any "puffing" of air and vapor out of the fill tube. Then put your ear near the opening and listen for a "chuffing" sound, both of which would indicate piston ring problems.

 

hmmmm so THATS what happened to my dads old LTD ford country squire

 

Injuneer, thank you!
you've answered every question i've had running around in my head. I've been worried about blowby for a while now, I was hoping that the pcv leak would explain by poor engine performance/oil problems.. I guess I'll fix that leak in the line, and hope for the best!

thanks to everyone that replied.

 

I wonder if you could use the AIR pump for that purpose? Stick the vaccum hose on the hole, in lieu of the PCV valve, and program AIR to run constantly? hehe.

 

No... the AIR pump is a "loose" vane type pump and can not pull a lot of vacuum - I thnk maybe 5"Hg. Two possible sources are a vacuum pump that was installed on the ZR1/LT5 engine, or an aftermarket vacuum system from Moroso, or similar. The aftermarket systems run $600. To be effective, I think they need to pull about 15-18"Hg. The main purpose is to stabilize the rings, when very thin, or very low tension rings are used to minimize friction. Pretty much a "race car" only sort of application.

 

well injuneer, i've fixed the vacuum lines to the pcv system.

I've opened the oil cap/dipstick, and I have puffs of very very thin white smoke coming out.. hard to see but definitley there. I put my ear to the oil fill opening, but did not hear any noises.

I can't exactly tell what's going on.. I'm thining maybe my leak in the pcv hoses caused too much crankcase pressure, which blew out the intake manifold rtv sealant?

 

ok let me get this ....if i was to take a peice of paper and lay it on the oil fil tube , should it suck air or blow air?

 

I believe it should be relatively neutral on a healthy engine. A test you can do is to block off the vent hose to the valve cover and put a vacuum gauge on the dipstick tube. Run engine at 1500 rpm for 30 seconds and read the gauge while at 1500. If there is vacuum, you're ok. If there is no vacuum, the cause may be due to gasket leaks. If there is pressure, it may be a bad PVC valve, poor vacuum to the PCV or excessive blow-by.

 

ok well mine sucks air pretty good at idle ....havn't tried ur little test yet ... what could b the prob?

 

TTT

 

This method works great. It's the best way you can check the PCV valve. I had mine go on my Z. The wierd thing was, if you shake it, it would still rattle like it should. But when I tested the crank case pressure I was building pressure that I shouldn't have been. So dont depend on the "rattle" cause it could still be bad!!