Crankcase Breather & Dipstick Blowout?

[TimpanogosSlim]

When the engine cools down, guess what, air flows into the engine from the breather. When you decelllerate without depressing clutch, the pressure in crankcase drops dramaticly, and air flows into the engine from the breather.
Keith

Disagree.
pressure may indeed drop, but it is still pressure nonetheless. It is not a vacuum.

I can imagine NO scenario where air would be moving INTO the crankcase through the breather. Beek is right, that engine-builders dream of vacuum in the crankcase. Some methods of artificially inducing vacuum have existed for years (60 or more), but even under those situations, you are drawing air OUT of the crankcase, in order to create a vacuum.

Air does not travel into the crankcase through the breather.

I would have to agree that no running engine is going to draw air into the crankcase.

When a hot engine is shut off and cools, the air in the crankcase will also cool and thus occupy less volume, so some tiny amount of negative pressure will be created, but it's not significant, and the engine isn't running.

[majicwrench]

Appreciate all the thoughts...I may have to hook a vacuum/pressure gauge up to something's crankcase and take a picture:)
But on decell/engine braking, there ain't no pressure in the crankcase, and air heads back down the hose.
Keith

[timinator]

Appreciate all the thoughts...I may have to hook a vacuum/pressure gauge up to something's crankcase and take a picture:)
But on decell/engine braking, there ain't no pressure in the crankcase, and air heads back down the hose.
Keith

Installed a vacuum gauge to the breather hose from the valve cover. The gauge showed a pressure drop of 1 in/hg at idle in neutral with a warm engine. At 50 mph in 2nd going down hill with the throttle closed it was 3 in/hg. Sorry to take away your fun, but just wanted to see for myself. I thought it would have been a bigger depression at 50 mph. I made no attempt to try and improve the seal of the hose connections at the valve covers or the oil cap gasket. That might increase the depression some what.

[131]

Installed a vacuum gauge to the breather hose from the valve cover. The gauge showed a pressure drop of 1 in/hg at idle in neutral with a warm engine. At 50 mph in 2nd going down hill with the throttle closed it was 3 in/hg.

Great to see some science, not a large sample size, but food for thought, not what I would have expected to see, well done.

[majicwrench]

TIminator, ah ya beat me to it. But not sure I follow your results, certainly should not be any pressure drop at idle in nuetral. Were you hooked up to a fiat or somethng else??
Keith

[timinator]

Highest vacuum happens with closed throttle plates. Used a 454 with idle manifold vacuum at 18 in/hg. Did not check manifold vacuum at 50 mph with throttle plates closed. Just revving up the engine and letting the throttle snap shut it goes to 25 in/hg. Not sure if this relates exactly to the Fiat carb with its rotating restriction on the throttle plate shaft, but willing to bet it does.

[majicwrench]

Well, we are not directly discussing manifold vacuum, but crankcase pressure/vacuum. And a 454 has a PCV valve, so that is what creates a vacuum in the crankcase. Fiat has no PCV valve.
Keith

[ROOK1]

I am not a mechanic or a appliance repair man or even a magician, I'm in finance, so here is my .02. I thought crankcase pressure was caused by blow by past the rings and I believe everyone here will agree that is the case. That being said, compression happens both on acceleration as well as deceleration since timing of the valves and/or stroke does not change when we lift our foot. So, there will always be higher pressure on the top of the piston than there is in the crankcase and with the blow by agreed by all above) there will be pressure generated in the crankcase whether we are accelerating or slowing down.

To sum it up. Clean the blockage from the vent hose and the dipstick should stay in place.

[131]

I thought crankcase pressure was caused by blow by past the rings and I believe everyone here will agree that is the case. That being said, compression happens both on acceleration as well as deceleration since timing of the valves and/or stroke does not change when we lift our foot. So, there will always be higher pressure on the top of the piston than there is in the crankcase and with the blow by agreed by all above) there will be pressure generated in the crankcase whether we are accelerating or slowing down.

On over run, going down a hill etc, the engine is pulling against a closed throttle plate, this is where you have maximum cylinder vacuum, which is why it pulls oil down the valve guides if the seals are worn, then blowing smoke on acceleration. So there are times where cylinder pressures are lower than the crankcase.

[majicwrench]

131 is quite correct, and that has been my point all along, on decell/engine braking the whole proccess is reversed, there is a massive vacuum in the cylinders that the pistons are trying to move down against, thus slowing the vehicle down. That massive vacuum, like was said above, is why they tend to suck oil vapors out of the crankcase past the valve seals and the piston rings, thus creating airflow into the crankcase thru the breather hose.
Keith

[So Cal Mark]

all well and good, but Fiat does describe their system as a pcv system. The small vacuum hose attaches to the air cleaner nipple and the carb. The carb connection is a vacuum source that is modulated by throttle opening. It's all described in the factory shop manual

[timinator]

Well, we are not directly discussing manifold vacuum, but crankcase pressure/vacuum. And a 454 has a PCV valve, so that is what creates a vacuum in the crankcase. Fiat has no PCV valve.
Keith

Well actually we are discussing manifold vacuum. The depression in the manifold is what draws crankcase air through the pcv system. At wide open throttle when the manifold vacuum is as little as 0.01 in/hg the pcv system is ineffective and the crankcase volume can go to atmo or above. Agreed that the Fiat pcv doesn't look like a Chevy pcv, however it does function the same.

Just for fun google pcv. Turns out they were not used for emissions originally but for allowing tanks to drive through water higher than the engine block. Ever heard of a Cadillac valve?

[majicwrench]

Fiat "pcv" functions nothing like a chevy pcv. Chev (or any typical old american car) is flowing air thru the crankcase, in normal operation -cruising- air passes thru the breather ( usually on one valve cover), thru the crankcase ( picking up fumes as it goes) thru pcv valve (usually on other valve cover), into intake manifold. Fiat has no way for air to really flow "thru" crankcase.
Timinator, Crankcase "volume" has nothing to do with it. Not sure I understand reference to volume.
I said fiat has no PCV "valve", if they wish to call it a PCV system that's fine .
Keith

[So Cal Mark]

Fiat does have a pcv valve, it's just in the carb rather than a separate valve. But hey, believe whatever you want about how the system works.

[BEEK]

all i can say is block off the breather totally. and see what happens, does it suck the seals in or does it blow oil out everywhere!