Posted by Clint Dixon on Monday, September 05, 2005 at 6:16PM :
In Reply to: Maybe. PCV= Positive Crankcase Ventilation, posted by chriscase on Sunday, September 04, 2005 at 5:24PM :
"The smaller tube was originally to vent the crankcase into the manifold when fording. Later it became a pollution control device called a PCV. It contains a valve, the pcv valve, that can go bad and cause a vacuum leak."
That information is somewhat true, in a round about way. But, the reason that a positive crankcase ventilation system was a standard feature for our POWER-WAGONS is because the road draft tube would simply not work when these trucks were used as they were designed.
Road draft tubes will only draw corrosive contaminating gasses and vapors from the crankcase while a vehicle is operated at road speeds. The tube requires a steady flow of air across its open lower end in order to create a vacuum within the tube. It is this vacuum that draws the gasses out of the crankcase. The crankcase is replenished by fresh air through a filtered cap sealing the oil filler tube.
The POWER-WAGON was designed to be used as a portable power plant. It was designed to be able to withstand long periods of time being used to power farm and industrial equipment where the truck would remain stationary. Since the engine would be required operate for hours at a time, with no significant airflow past the road draft tube, the special system was developed to allow manifold vacuum to draw contaminated air from the crankcase. The oil bath air cleaner mounted to the carburetor filtered the incoming air, replenishing the crankcase. What we now know as the PCV valve was actually called a "Donaldson Valve" on the POWER WAGON. It is simply a one-way valve that will not allow airflow back into the crankcase from the manifold. This prevents the gasoline and air mixture from entering the crankcase during times of little or no vacuum. It also prevents a "backfire" from entering the crankcase.
Even when moving, the POWER-WAGON benefits from positive crankcase ventilation. These trucks were meant to carry heavy loads while crawling along formidable terrain at a slow speed. Again, not enough airflow past a road draft tube to properly ventilate the crankcase.
A vacuum leak in the "pvc" valve on our POWER-WAGONS will not effect the combustion chamber mixture. This valve is designed to always be held open as long as there is sufficient vacuum from the manifold. Crankcase air is always flowing through it and into the combustion chambers as long as there is vacuum present to pull it. This flow is part of, and required for, proper gas/air mixture within the combustion chambers. In effect, it is always “leaking” air into the manifold. The only way that a "leak" could develop that would adversely effect the mixture would be if the valve was unscrewed partway, or the gasket sealing the plumbing to the crankcase was bad or missing. In those cases, quantities of outside unfiltered air could be pulled into the manifold causing the mixture to be too lean.
The correct carburetors for these trucks were designed to be an integral part of the crankcase ventilation system. Plumbing actually connects the carburetor to the oil filler tube allowing clean filtered air to be drawn into the crankcase.
Junior
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