positive vacum in crankcase while running

Old 12-27-2009, 03:43 PM
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Join Date: Aug 2007
Location: Colebrook Connecticut
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Default positive vacum in crankcase while running

while running I pulled oil fill cap on valve cover and whooosh!
btw engine has no oil leaks but I had a breather on passenger side and engine had leaks all over.
car runs fine as is and seems to run a lot better than with a breather.
I think its sealing rings better.
any thoughts?
found this interesting too.
So the PCV valve is doing two things. It is maintaining a constant light vacuum level in the crankcase, and it is modulating air flow to exactly match the crankcase ventilation volume and blow by volume combined at all times. The proper description for this device is then "vacuum modulated flow control valve". Or if you think in terms of crankcase absolute pressure you can call it "pressure modulated flow control valve". For the common folk we just call it the PCV valve.

There are a few unnatural operating conditions for this valve. First, if you remove the oil filler cap when running you lose the crankcase vacuum. With increased absolute pressure in the crankcase the PCV valve will open wider in attempt to process this perceived excess "blow by". When the valve cannot increase flow enough to restore crankcase vacuum, the best it can do is to go wide open. When allowing maximum flow of air through the PCV valve the engine idle will speed up a little, and the fuel mixture will go lean, and it should have a slightly rough fast idle.

Second, putting your hand over the oil filler port and closing off the valve cover inlet vent will stop all ventilation air flow through the crankcase. This will cause higher vacuum in the crankcase that will make the PCV valve go completely closed, except it may stay open just a crack to process blow by gasses. Stopping the fresh air ventilation flow will make the engine idle slightly slower, and the fuel mixture will be a little rich, so it should then have an abnormally slow rough idle.

Third, if the valve cover inlet vent should happen to get clogged up during normal operation, the results will be similar to the second case with rich running. This can foul spark plugs and carbon up combustion chambers as well as accumulating water and other nasty things in the crankcase. Resulting damage could lead to a large repair bill. This could happen if the inlet filter in a vented oil filler cap becomes clogged. That is why service instructions call for changing the vented type oil cap periodically.

Fourth, if the piston rings or cylinder walls are badly worn, beyond the normal expected range of normal engine life (or if the piston rings are stuck in the grooves), the blow by volume may be more than the PCV valve can process. In this case the PCV valve will be wide open and there may also be positive pressure in the crankcase. This would result in reverse flow through the valve cover inlet vent and possible expulsion of oil past the normal engine seals. Then you can hang up your jock and go home while you contemplate the cost of the required engine overhaul.
this could explain why some have a problem with PCV sucking oil?
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