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Funcruiser · 03-19-2023, 10:15 PM

So I'm getting ready for a short trip to the drag strip next weekend and while doing some routine maintenance i noticed that there is negative pressure at the oil fill tube while the engine is running. The idle will dip slightly when the cap is removed and return to normal idle. It's enough vacuum to hold a piece of paper in place. Not enough vacuum to deform the paper.

Is this normal? My thinking is yes. The PCV should provide some amount of vacuum. I have just never noticed this before.

It's a 2003 Suncruiser with a 8.1 Vortec 120k miles on it, runs great and does not consume oil.

Thanks

twinboat · 03-19-2023, 10:46 PM

Yes, normal. The PCV valve is always drawing a vacuum in the crankcase.

PCV testers sit on the oil fill and test the pull.

yeloduster · 03-20-2023, 08:09 AM

There should not be a vacuum in the crankcase. The crankcase ventilation system has an orifice built into the intake manifold. The orifice uses intake vacuum to draw crankcase vapors into the engine air intake. The engine then burns these vapors.

In order for the system to work there has to be a clean air intake into the crankcase. I went and looked at my 8.1L engine but can't see where filtered air is introduced into crankcase. Maybe someone else can enlighten us.

If you have a vacuum in the crankcase it indicates that the intake air is plugged.

Funcruiser · 03-20-2023, 11:42 AM

Quote:

Originally Posted by yeloduster

There should not be a vacuum in the crankcase. The crankcase ventilation system has an orifice built into the intake manifold. The orifice uses intake vacuum to draw crankcase vapors into the engine air intake. The engine then burns these vapors.

In order for the system to work there has to be a clean air intake into the crankcase. I went and looked at my 8.1L engine but can't see where filtered air is introduced into crankcase. Maybe someone else can enlighten us.

If you have a vacuum in the crankcase it indicates that the intake air is plugged.

Its not at the crankcase but at the valve cover where the oil fill tube empties into the head.

wolfe10 · 03-20-2023, 11:57 AM

Quote:

Originally Posted by Funcruiser

Its not at the crankcase but at the valve cover where the oil fill tube empties into the head.

From an "air movement" standpoint, what is the difference?

PVC valve does use engine vacuum to "vent" the crankcase to reduce air pollution from oil vapor. It is now burned in the engine, not vented to atmosphere.

twinboat · 03-20-2023, 01:12 PM

The intake air of the PVC system is the 3/4 inch hose, from the opposite valve cover, to the small filter tucked up in the side of the air filter housing.

It's a bit restricted by design, so opening the oil fill cap let's more air in the system. That causes the lean condition and rise in RPMs.

Some GM engines use a different system but it still puts the engine under negetive pressure.