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PCV system questions

353 Views 4 Replies 4 Participants Last post by  hotrodjohn71
Hi group, let me see if I can articulate in this in a way that's understandable.

I have a stock 1965 Pontiac 326 engine. The engine has the PCV valve in the Valley Pan like all those Pontiacs do. That PCV valve vacuum hose goes directly to my manifold vacuum.

I just noticed that both valve covers have no vent or vent hose at all. Only the Passenger side has a provision hole, but it has a rubber plug.

My (stock) air cleaner assembly does have a hard mounted fitting for a hose attachment, but it is capped off.

It seems that there is no fresh air circulation.

What are the consequences of this setup?
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This is fine many were built this way. In some respects it’s a good idea in that it only permits blow by to be recirculated to the intake, blow by being mostly unburnt mixture. The internal baffle design is intended to separate oil mist from blow by so little to no oil goes into the induction system. But some very small amount of oil does vaporize and pass through.

The PCV valve only system keeps a bit of reduced pressure in the crankcase which helps seal the rings tighter, Pontiac being a user of chrome rings which can withstand this without additional wear. The down side is as manifold vacuum decreases with throttle opening so does flow through the PCV valve so crankcase pressure will go up and the ring seal amount from lower crankcase pressure decreases.

The systems that also free vent to the air cleaner allowing the WOT throttle blow by to vent into the air-cleaner thus back into the combustion side of the intake but with a carb more fuel is added to an airstream already carrying fuel mixture as crankcase blow by. This leads to other problems largely worked by leaning carb mixtures which again leads to other AFR feedback problems.

Plus at engine operating temperatures there is always some oil component in blowby because oil does some evaporation at these temps making it difficult to completely eliminate some oil contamination. You find this condensing on the filter element, carb or throttle body.

There really is no need to circulate fresh air through the crankcase this is a leftover from the road tube era where fresh air was taken in usually at the valve cover or valley then venting it under the engine somewhere to make a mess on the bottom of the vehicle. So this was mostly a method to smear and dilute blow by as it was released to the atmosphere and of course the oils of those days were mostly unable to deal with sludge and varnish reactions so to reduce those reactions was to dilute those chemicals as much as possible. Todays oils and fuels either manage these reactions or just don’t carry or create the reactants that make this goo. To goo up an engine these days you have to really ignore doing oil and filter changes where back in the day if you weren’t doing oil and filter changes on a 3,000 mile interval the engine quickly became sludged up inside.

We had a neighbor who thought oil and filter changes were baloney cooked up be the oil industry. He drove up to our place to talk the problem of declining oil pressure out on the highway with my father who told me to remove the valve cover which was close to impossible as it was so sludged that the only space was where the rockers and springs were actually moving. So when he got on the highway the oil being pumped to the top end drained back so slowly that eventually there wasnt enough oil in the crankcase to feed the pump it was in the rocker covers and valley. Then overnight it would drain back into the pan where firing up the next morning everything appeared normal at least for a while.

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