Breather cap question

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?

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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

my setup i feel is of concern now after reading this. i have trick flow valve covers with their oil fill tube kit and on the small neck on the side of the fill tube i have a small breather on there held on with a hose clamp. the oil fill tube has a regular push in cap and my pcv valve is hooked up accordingly. i had a vent hose going from this neck to the powerpipe but these valve covers aren't baffled and oil was making its way from the vent hose to the mass air meter so i ditched the hose and just clamped on the breather. should i ditch the breather also and just put the 1/8 plug in the side of the fill tube?

You can use the breather. I just installed one on my car this past weekend. Once you get into stroker engines and/or blower engines, you need another way to vent the engine besides the stock pcv. More air that these engines suck in = more pressure. If necessary, you have have to slightly retune the car. But in closed loop, any unmetered air that may get in can be adapted to by the eec. I have not datalogged my WOT tune yet since the breather, I will do it maybe this weekend and report back.

well tim said the same thing in another thread about the breather,and i was running one at the time.so i took the breather off and put the oil spout from tfs on and ran the hoses and stuff like it was stock,like he said to.and my car would not run,but if pulled the oil cap off it would.so i removed the stock setup hoses and stuff and the car runs fine.dont know why.but the car is put up for the winter and i will be adding another breather to the driver side vc this winter and plugging the pcv.i dont know if the reason it would not work is because of my setup ornot,see my sig.i hope this helps someone but my car will not run without a breather

im still in the process of swapping my intake. i have a breather in each valve cover and the pcv port is completely plugged on the lower. is this a bad way to do it? i would think a breather on the pcv port on the lower would spit quite a bit of oil even with the baffle. by the way, im running a stock bottom 302 right now with h/c/i and an s-trim. a 331 stroker is in the works at the moment. is it safe to keep things the way i currently have them on the new motor also?

a little tip i read about pcv valves and forced induction is to install a one way valve inline in the hose from the pcv valve to the intake, like a brake booster valve. air can only flow from the crankcase to the intake and not the other way. i did this after my friggin oil cap blew off. the pressurized boost was entering the crankcase which is not a good thing so i installed the brake booster valve and have not had a problem since. all forced induction guys should do this.

95cobradude said:

my setup i feel is of concern now after reading this. i have trick flow valve covers with their oil fill tube kit and on the small neck on the side of the fill tube i have a small breather on there held on with a hose clamp. the oil fill tube has a regular push in cap and my pcv valve is hooked up accordingly. i had a vent hose going from this neck to the powerpipe but these valve covers aren't baffled and oil was making its way from the vent hose to the mass air meter so i ditched the hose and just clamped on the breather. should i ditch the breather also and just put the 1/8 plug in the side of the fill tube?

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If you are concerned about the TFS oil fill neck try subsituting the oil fill neck (which I velieve is a push in neck) with this oil fill cap from Ford Racing. This cap is a push in cap, is for a closed PCV system, has the port to connect the tube to the TB or the CAI, and has a built in baffle, and only costs $7.50. My brother runs this cap on his 89 with great results. View attachment 497060 part #M-6766-D302 from http://www.fordracingparts.com
Tim

Green 94 5.0 said:

im still in the process of swapping my intake. i have a breather in each valve cover and the pcv port is completely plugged on the lower. is this a bad way to do it? i would think a breather on the pcv port on the lower would spit quite a bit of oil even with the baffle. by the way, im running a stock bottom 302 right now with h/c/i and an s-trim. a 331 stroker is in the works at the moment. is it safe to keep things the way i currently have them on the new motor also?

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It's not the best for the motor, but as I said in the very long post you won't have crankcase overpressure problems. The only problem for you with straight breather caps is that you aren't ridding the crankcase of the corrosive vapor build up, only relieving pressure. Even under boost, the stock PCV system, when properly maintained is better than breather caps. I understand that boost is the opposite of vacuum, but it's the pressure differential in the upper intake and not the intake vacuum that causes the PCV valve to open and suck the vapors out of the crankcase (like opening your window on the highway and it sucks the papers out of your car).
Tim

94opalgt said:

well tim said the same thing in another thread about the breather,and i was running one at the time.so i took the breather off and put the oil spout from tfs on and ran the hoses and stuff like it was stock,like he said to.and my car would not run,but if pulled the oil cap off it would.so i removed the stock setup hoses and stuff and the car runs fine.dont know why.but the car is put up for the winter and i will be adding another breather to the driver side vc this winter and plugging the pcv.i dont know if the reason it would not work is because of my setup ornot,see my sig.i hope this helps someone but my car will not run without a breather

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If the TFS oil spout didn't work for you, check out the FRPP cap that I listed below. I have a number of people I know running them with great results. As I said earlier, I used to be a breather cap man and I got lucky because my PCV filter was almost completely clogged which is why I didn't get a huge lean condition from the PCV (that was from the EGR delete without turning it off in the computer...I'm an EGR convert now too!). The car did run rougher and more ill mannered with the breather cap on (vacuum leak). I hooked up the PCV system with the Steeda Oil Seperator and the car ran like a dream!
Tim

I read all these threads and now I'm confused. Maybe you can help. I just installed a 306 w-h/c/i. I used a regular breather. Should I use this ford part? Do I have a vacume leak because of the open breather? thanks 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)???

18mustangs said:

If you are concerned about the TFS oil fill neck try subsituting the oil fill neck (which I velieve is a push in neck) with this oil fill cap from Ford Racing. This cap is a push in cap, is for a closed PCV system, has the port to connect the tube to the TB or the CAI, and has a built in baffle, and only costs $7.50. My brother runs this cap on his 89 with great results. View attachment 497020 part #M-6766-D302 from http://www.fordracingparts.com
Tim

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I used another FRPP number for my breather, same deal tho...Just run a 3/8" line from the cap to your CAI...just like the stock oil fill line used to run to your factory rubber inlet...

part#m-6766-g302 is like mine, push in type for a "closed" crankcase...depends on your style of VCs..


Another shot...just run a line to your CAI...I only run one breather...the drivers side is blocked...

FRPP doesn't have screw in with a vent line on it, looking at the catalog....Maybe someone else does??

What kinda VC's??? You may be able to push in a rubber grommet and then switch to a push type...

Wow that was a long post. I have run a breather on my stang for two years now, and i dont have a lean problem, and no pinging, or other adverse affects. That is way too in depth thinking of a post. My car idles rock solid and has no lean ness or pinging.

Edit: Also the engine running lean is even more of a moot point because i forgot about adaptive control of our engines. The amount of change that adding a breather would add could easily be compensated by the cars computer.

ok, let me see if I have this right, I can run a breather cap without the line from it to the throttle body pipe and the pcv valve and everything will work ok? I'm just wanting to make sure I got that right. reason I'm asking is I just put a set of valve covers on. (cal customs from the 60s) and the valve covers use twist in type caps. I was going to run a breather on one side without a line from it to the throttle body.

RFMustangGT said:

ok, let me see if I have this right, I can run a breather cap without the line from it to the throttle body pipe and the pcv valve and everything will work ok? I'm just wanting to make sure I got that right. reason I'm asking is I just put a set of valve covers on. (cal customs from the 60s) and the valve covers use twist in type caps. I was going to run a breather on one side without a line from it to the throttle body.

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You can get twist in caps like the cap I posed earlier in the FRPP catalog to fit your 60's era valve covers. Contrary to what others may say (I've run open breathers and closed and stopped after learning how exactly the PCV system is set up and works) you cannot use a PCV valve and the open filtered breather cap. The reason is the PCV draws air from the crankcase, and with a closed cap the air from after the MAF is diverted to the crankcase to prevent a vacuum and to put fresh air into the crankcase in place of the corrosive vapro laden air drawn out by the PCV. This is a 1 for 1 swap so the air fuel ratio is not thrown off. If you run an open filtered breather cap the air is drawn out of the crankcase, and unmetered air is drawn into the crankcase along with 100% of the intake charge. This extra air leans the air fuel ratio. The computer can only compensate so much with the adaptive fuel strategy (adaptive fuel limits) , if the A\F ratio goes too far out of tolerance you're pinging.
Tim

18mustangs said:

If you run an open filtered breather cap the air is drawn out of the crankcase, and unmetered air is drawn into the crankcase along with 100% of the intake charge. This extra air leans the air fuel ratio. The computer can only compensate so much with the adaptive fuel strategy (adaptive fuel limits) , if the A\F ratio goes too far out of tolerance you're pinging.
Tim

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I see where your coming from Tim. I have learned a lot from playing with my Tweecer for over a year and one thing is the the adaptive strategy for the computer is pretty good, and it allows for a lot of change. But what your saying does make sense. My car is running damn good so im not gonna change anything, but good lookin out.

If you have a datalogging wideband and a tuning device you can fix any lean issue the breather may give you. I picked up a breather for my car last weekend like I said because it seems the stock pcv is marginal for stroker engines and blower engines so we need something extra. I have not done a datalog pull yet but I will try to this weekend. My idle and cruise a/f were still 100% ideal after the breather swap