crankcase pulling vacuum

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Apr 30, 2019
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kansas
My hyundai has started pulling vacuum through the crankcase. There is no noticeable air being sucked through the intake yet when in try to pull the dipstick or oil cap it sucks down back to the crankcase with considerable force. What on earth could be the cause of this?
 
does it run like always, or are you having running problems? Why are you taking the dipstick or oil fill cap off while the engine is running? There should be some crankcase vacuum as that is what the pcv system does.
 
does it run like always, or are you having running problems? Why are you taking the dipstick or oil fill cap off while the engine is running? There should be some crankcase vacuum as that is what the pcv system does.
It has developed a high idle and it seems as though it does not intake air through the actual intake anymore. It also rev hangs quite bad where it did not used to.
There also used to be a bit of noticeable blow by but that has changed into a rather violent suction.
 
I've heard of blow-by, but never suck-by. Only thing I can think of is that your pcv circuit isn't functioning property, and intake vacuum is making it into the crankcase. Or, your intake tract is severely plugged (air filter) and engine vacuum is making it past the rings, but if the intake as that plugged, it wouldn't run. Both seem too weird to be true, but weird stuff happens every day.
 
I've heard of blow-by, but never suck-by. Only thing I can think of is that your pcv circuit isn't functioning property, and intake vacuum is making it into the crankcase. Or, your intake tract is severely plugged (air filter) and engine vacuum is making it past the rings, but if the intake as that plugged, it wouldn't run. Both seem too weird to be true, but weird stuff happens every day.
i have taken the crankcase vent tube out of the intake to check this and it has no effect, there still is no air sucked through the intake.
 
I've heard of blow-by, but never suck-by. Only thing I can think of is that your pcv circuit isn't functioning property, and intake vacuum is making it into the crankcase. Or, your intake tract is severely plugged (air filter) and engine vacuum is making it past the rings, but if the intake as that plugged, it wouldn't run. Both seem too weird to be true, but weird stuff happens every day.
I would bet on the intake vacuum getting into the crankcase. Is the pcv valve built into the engine? I am most familiar with the old style that had the pcv valve on a hose on the valve cover. The newer engines that have it internal, something may have come undone, and feeding full vacuum to the crankcase.
 
i have taken the crankcase vent tube out of the intake to check this and it has no effect, there still is no air sucked through the intake.
If there is no air traveling through the tube between your air filter housing and the throttle body, you have an impossibly large intake manifold leak.
 
So you're saying that with the oil cap installed the engine will not run because it cannot obtain any air? Or that there is still some air passing through the intake?
 
So you're saying that with the oil cap installed the engine will not run because it cannot obtain any air? Or that there is still some air passing through the intake?
there is not any noticeable air passing through the intake. Whether or not the intake is installed does not affect idle whereas pulling the dipstick or engine cap causes idle to jump by 1.5k rpm.
 
Actually owned a car, one of my Northstar Caddy's in fact, that would not run well with the oil cap removed. Seems that the crankcase was required to be sealed so that excess air wouldn't get through the internal PCV valve.
I'm thinking you have some other issue not related to the dipstick.
Maybe it's the mass air flow metering system.
 
The whole point of the PCV system is to put the crankcase under a vacuum so that blowby gases (which contain a lot of HC pollutant) are pulled into the intake manifold and mostly get burnt.

An abnormally high vacuum in the crankcase might result from the atmospheric air inlet being blocked or the PCV valve stuck open.

On a speed density system any air leak will cause idle speed to increase, while on a MAF system unmetered air entering will cause the idle to lean out resulting in rough running or stalling.
 
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