Ford Mustang Forums banner

1 - 6 of 6 Posts

·
Registered
Joined
·
962 Posts
Discussion Starter #1
Now and again you’ll hear someone having pcv issues and they’ll ask if they can use breathers on their efi valve covers. The answer is generally no because unmetered air will get into the combustion chamber and cause it to run lean, idle bad, etc.
I’ve been running a breather oil fill cap in conjunction with the pcv and a catch can on my mustang for years. I haven had any issues. I can’t tell a difference with the breather on or with the regular cap on.
So I got to thinking, I remember a time or two when I’ve added oil to an efi vehicle while it was running, and there was no change in the idle or any noticeable difference at all when the cap was off. If the oil fill hole isn’t big enough to “let unmetered” air in, surely a gause filter wouldn’t be any worse.
Today I tried to let some unmetered air into my DD’s crankcase to see what would happen. The outcome was not very exciting.
So, “unmetered air” entering the crank case and causing problems, myth busted?
https://m.youtube.com/watch?v=RjQH5x3RWzs
No excitement. There wasn’t even any oil flinging out. The oil on the plastic cover is from an oily funnel. I changed oil yesterday.
 

·
Registered
Joined
·
962 Posts
Discussion Starter #2
Sorry to bum out the self tuners and such. Unmetered air through the crank causing problems? I don’t think so Tim. Uncle Mike says it’s ok to use breathers.
 

·
Registered
Joined
·
2,518 Posts
I tried the same thing on the dyno during tuning as well as some cruising around town. I figured I'd at least have some surging at low cruising speeds. I saw no changes in AFR, but the ECU did appear to be adjusting fueling with a wider swing. I also felt no surging while cruising. When I asked Decipha if my ECU would compensate for the unmetered air; he said "Yes."
The only change I made was a breather that screwed into the oil filler tube in place of the stock cap. I'm no rocket scientist, but I thought I'd share.
 

·
Registered
Joined
·
2,037 Posts
Come on man... The filler neck vent will release positive crank case air, not suck it in. Leaving this port completely open (or in your case using a breather for a cap) doesn't create a vacuum. Instead you would stand a much better chance of oil dropplets being expelled if anything. Why do you think many cars that hit the stripe with some money in them run vacuum pumps? to ensure that there is negative air pressure in that area while helping to seat the rings and reduce the amount of blowby present in the crank case.

Rock your chrome screw on breather instead of the plastic oil cap and be happy
 

·
Registered
Joined
·
21,745 Posts
With a pcv, at idle the crankcase is lower than atmosphere

If you open the oil fill

Air will enter, simple

If the fuel trims change that is verification

Also remember what ever it draws through the oil fill, it wont enter the air filter

There is a trade off, therefore its unmetered, not ADDITIONAL air
 
1 - 6 of 6 Posts
Top