Alan_Mac 48 Report post Posted January 25, 2019 3 hours ago, 1969_Mach1 said: At high engine RPM the PCV will not flow much so crankcase vapors also get pushed out through the breather. That is normal operation on all cars since the advent of a PCV valve. That is why crankcase breather filters use to get replaced as a normal tune-up item. I have a pair of Scott Drake valve covers with a oil filler tube like shown in your picture. They are ready for the garbage can. They don't hold the filler cap on very well compared to twist on and the other type that push into a grommet. Connecting the breather cap to an air cleaner will only force crankcase vapors into the air cleaner at high RPM. Then back into the motor instead of simply into the atmosphere. It won't have any effect on how well the vented oil filler cap operates. If you originally had that other vented oil filler cap Scott Drake offers, take a close look at it, the vent holes are tiny and probably will not flow enough air for motors like yours. I tried many breather caps and at one time had one on each side using two driver side Branda high valve covers with tubes. I still have a new valve cover like this is anyone needs one. They all popped up at high RPM. This arrangement fixed my problem. I expect there is enough vacuum in the air cleaner tube at WOT to aid in crankcase evacuation. Quote Share this post Link to post Share on other sites
Vicfreg 771 Report post Posted January 25, 2019 The whole idea of this PCV plumbing is simple. One side is the crankcase air inlet, the other side is the crankcase air outlet. On the air inlet air side, since it is going through the engine, Ford decided to use the main air inlet (air filter) as a point to take the PCV inlet air from. On the outlet air side, to apply some motive force (suction), the PCV is hooked to Carb Vacuum. The PCV valve (check valve) is there to prevent a backfire from from getting into the crankcase and causing a big bang... My experience with a carbureted 90's 5.0 in my 1968 Mustang was that I initially did not run a PCV, and had one breather open to atmosphere. It always pushed oil out, in particular after a high RPM downshift. 2 breathers did not change this problem. Running a PCV to the proper carburetor port fixed the problem for good. I hooked the other breather to the air cleaner to keep the oil vapor from accumulating on my valve covers. Problem solved. Quote Share this post Link to post Share on other sites
