95Cobra302 said:
I bought one of those ford racing breather caps with the open filter element mainly for cosmetic reasons, but have heard of some people saying it may cause problems because of letting un-metered air into the crankcase. Does anyone with knowledge on this know any more info? Would it be better to just put my stock oil fill cap on?
Below is how the PCV system works and why it's bad to run an open breather cap on a closed crankcase. I used a FRPP open breather cap because I had oil coming out of some gaskets. The cap solved the crankcase pressure, but was a big vacuum leak. I changed back to a closed cap and put new PCV valve, PCV grommet, and PCV filter on and the oil stopped coming out of the gaskets as I found my PCV filter screen was clogged.
I too like the way the breather cap looks, so I too a vacuum cap the same size as the hole in the breather, slathered it with RTV, plugged the hole in the cap with it, then filled the open space with RTV and let it cure for one week. This way the cap was still closed and didn't create a vacuum leak in the closed crankcase but still look kickasstic! Or you could get one of the blingy closed oil fill caps from FRPP.
How the PCV system works
One an EFI engine and some later carburated engines, the PCV (positive crank case ventilation) system draws air from the crankcase into the intake stream and into the combustion chamber where it is burned in order to reduce emissions. This system does more than reduce emissions, it positively ventilates the crankcase prolonging engine life, whereas running a breather cap system even with the PCV valve capped or filtered to the outside is a passive system rather than a positive system like the PCV, and is not as efficient at ventilating the crankcase as the PCV system as our newer emissions friendly engines do not have road draft tubes like older muscle cars did in order to create the positive ventilation effect that the PCV valve does. Also remember that the PCV system is an emissions component, and if it is deleted or changed some states may fail the visual portion of your emissions test.
Why not to run a filtered breather cap
One should never run a breather cap on an EFI engine with a functional PCV system (with the oil filler neck capped or connected) as the breather cap allows unmetered air from the engine bay to enter the crankcase and then the intake charge via the PCV valve which creates a lean condition. This lean condition may cause detonation\pinging (a common problem with SN95s in stock form) to occur, and it is important to note that detonation is not always audible so you may not know that you’re pinging. This lean condition is created because the PCV system draws fresh metered air into the crankcase via the oil filler neck hose in order to replace the oil vapor laden air that the PCV valve drew out of the crank case into the intake charge. This is a 1:1 swap so the air fuel ratio is affected, and prevents a vacuum from forming in the crank case when the PCV valve draws the air out of the crankcase. If one runs a filtered breather cap the the intake charge still has 100% if the intake charge’s air as no air was diverted to the crankcase, and now the additional air from the crankcase is added to the intake charge leaning the air fuel mixture.
Capping the filler neck hose
One should not cap the oil filler neck to throttle body PCV hose in order to reduce oil in the intake charge as that hose is bi-directional (with the correct non breather oil cap or even with a filtered breather cap). The PCV system draws fresh metered air into the crankcase in order to replace the oil vapor laden air that the PCV valve drew out of the crank case into the intake charge. This is a 1:1 swap so the air fuel ratio is not off and prevents a vacuum from forming in the crank case. If the PCV valve cannot keep up with the pressure inside of the crankcase the extra pressure can be relieved via the oil filler neck hose. Capping off the PCV oil filler tube with the proper oil cap prevents the extra crankcase pressure relief, contributes to a vacuum in the crankcase, and contributes to a lean condition as no air is diverted to the crankcase from the intake charge while air is added to the intake charge from the crankcase. Capping the oil filler neck with a filtered breather cap allows unmetered air to enter the intake charge from the engine bay via the crankcase, through the PCV valve.
One Possibility for filtered breather caps
If one is dead set on running breather caps, you should cap off the oil filler neck\TB PCV connections and put a small breather on the PCV valve (capping off the intake vacuum ports that the PCV hose used to lead to) as well as on each valve cover. This system is not as efficient as running the PCV system but does prevent excess crankcase pressure from blowing out gaskets and causing oil leaks. This is not recommended as this is not a positively vented system, it only relieves excess crankcase pressure and doesn't ventilate the crankcase with fresh air extending engine life.
I hope this helped.
Tim
It also says it is a twist in. My valve covers take a push-in, so should I order the push in version (part#m-6766-g302)???
