[Twilightoptics]

Question about PCV valves. I don't know how disruptive vacuum leaks are on Carbs, but on a MAP efi it's pretty disruptive!

So I've got my PCV valve plugged into one valve cover, and hooked to the plenum with manifold vacuum.

I've got a breather on the other valve cover.

Isn't the pcv sucking air into the plenum? If so, doesn't that make it a giant vacuum leak? I know the normal EFI setups have a purge canister that is hooked to the other valve cover and the throttle body, making all air in vacuum non outside. It's all "metered" as it was coined to me before.

Just trying to understand more about it, and my lean problem. (see other post.)

How do you know if your PCV valve is fuctioning? Is it the manifold vacuum that sucks the valve open, or the crank case pressure that pushes it open? Right now my manifold vacuum holds it open. If I rev the engine a bit as the vacuum drops the valve closes.

That normal?

Thanks guys
-Paul

 

[bracketchev1221]

Yes basically the pcv valve should be held by engine vacuum. This was something that I wanted to try. With an engine with moderate to low vacuum at idle 7-10 inches could you use a pcv as a poor mans vacuum pump. I tried it once on a 465 hp small block but it sucked in the cork valve cover gaskets. Maybe the solid core ones would have worked. But on a street car didn't they just have a pcv, and the filler cap that was sealed. If the engine is healthy with minimal blowby there should be no real crankcase pressure. Does this make sense or sound like it could work??

 

[zwede]

Yes, a PCV system is a controlled vacuum leak, but a speed density EFI system doesn't care as long as the air leaks into the plenum and is evenly distributed to all cylinders. It doesn't matter if the air "leaks" pas the throttle plates or through the vacuum fitting to the PCV. Air is air and as long as it goes into the plenum the EFI "knows" about it through the MAP sensor that measures the pressure in the plenum.

The comment you had of "metered air" applies to MAF systems. All air entering the engine MUST go through the MAF sensor or you'll have all kinds of problems. With MAF you should not run a breather element on the other side of the PCV system. Instead a hose must go to the front of the TB, but after the MAF so the MAF measures the air properly.

Now the PCV valve: At idle you should be able to pull the PCV out of the valve cover and the internal plunger should be fully sucked up into the shell. Some valves are designed so they oscillate at idle (rattle back and forth) and that is OK too. Rev the engine to 2000 and the plunger should be firmly sucked up into the shell.

I had a tricky issue with my PCV system. The passage in the throttle body was tiny although it went to a 3/8" barb. The tiny passage restricted flow to the point the PCV valve plunger was not sucked shut. This made it flow way to much and it sucked oil out of the motor. The oil caked on the valves and made the engine run like crap. After I drilled the passage out the plunger clacked shut and the system worked.

/Markus

 

[Twilightoptics]

It's definitely stuffed up the shell at idle and high vacuum. Vacuum is pretty low from the cam, map is about 60-70in gear. Out of no vacuum at 100.

Looks like that's not my problem then. Thank's for the info guys!