Sniper
Canadian Rust Bucket
In a recent post a member was asking about a stalling condition and I had mentioned checking for vacuum leaks, and in particular, check for a missing bolt in the front of the small block Chev that goes directly into the crankcase. Some members don't agree with that vacuum leak scenario, some are saying it would only leak oil since it's in the crankcase area. And some are a bit baffled how it could be all tied together. So...if your still interested, I'll try to explain this as best as I can.
First of all, an engine needs to be vented to atmosphere. In other words, vented to the outside air. Used to be in the past, it was done through a 3/4 to 1 inch pipe, (called a road draft tube) and any blowby was dumped to atmosphere. Quite often you could see an oil mist/condensation mixture and a few drops (or sometimes, a lot of drops of oil) oil, which for the most part, was returned to liquid oil as it was being condensed from a oil mist as it touched the cooler, or cold road draft tube.
Now enter the powers that be, and for various reasons, proclaim that we can't be doing this practice any longer, and lo and behold, the PCV system is born. So the Positive Crankcase Ventilation system is taking over where the road draft tube has left off. Instead of dumping the blowby overboard, it's now contained, and recirculated to be burnt along with the fuel/air mixture. Since a picture is worth a thousand words, here's a couple thousand words worth of pictures.
The first drawing shows a regular PCV system. The system will only work properly, if the engine is sealed. Whoa!! Hold on a second, you said the engine needs to be vented. How can it be sealed, and vented at the same time?
Well, it goes like this, all gaskets and seals have to be doing their job. And their job is to keep the oil and water in the proper areas, and keep the outside air...outside. No air leaking into the crankcase area from the pan gasket, timing chain cover, crank seals, or past the tappet cover gaskets, or by the intake gasket/seals. When we look at the first drawing, this is the ideal set up. Good gaskets and seals, no leaks in or out, engine running providing a good vacuum. Working our way backwards from the vacuum source we can see what's happening. The intake plenum, just below the carb base is where the vacuum source is, when the engine is operating. The vac line (quite often 3/8 steel tubing) goes from this area to a short rubber hose to the PVC check valve. The valve is pushed into a snug fitting rubber grommet that seals both the valve and the hole that it's in, in the tappet cover itself. From there, the entire internal area of the engine is subjected to a negative pressure, or vacuum. Since we have to feed that vacuum it is done in two ways, one is by a vent, and the other is by the blowby that we're trying to control. That is the whole reason for this exercise, so the blowby doesn't get dumped to atmophere. Now the vent is located on the opposite tappet cover and is attached by a hose to the breather housing and the incoming vent air has it's own small filter to control dirt infiltration.
Now, lets start at the vent end of things, fresh filtered air is drawn in through the breather housing, down the hose to the tappet cover. Under the cover, through the various holes in the head casting, (oil drain holes, hole where pushrods come up through etc.) and the air sweeps across the crankcase area picking up the blowby gasses and continues on up through the various holes in the second head. Into the tappet cover area and through the PCV valve, hose, steel tube and into the intake plenum below the carb and finally down each runner and being drawn into the cylinders as the valves open in sequence. The air/fuel mix along with the blowby gasses are burnt and flow out the tailpipe.....except for the tiny little smidge of blowby that gets to do the whole trip all over again.
So are we done? Not quite. (Don't you want to know how this ends??)
So now, lets upset the process. We have a vac leak. Small block Chevs have their own little bugaboo's and quirks. Tin tappet covers have a habit of getting a bit out of shape when over tightened, and wah-la..a vac leak and a poor running engine. The rubber seals at the ends of the intake can be a problem as well, same results if they leak. And since the entire internal area of the engine is under a vacuum, you can get the same results from a bad pan gasket, tappet cover gasket and or seal. Rear main seal, distributor gasket, even if the seal is bad, or missing from the dipstick. (Yeah, there's a little rubber seal in there too.) And last, but not least a rather large vac leak if your missing the bolt/seal at the front of the engine where it goes through to the fuel pump push rod area. After all, that area is under vacuum also.
The second picture shows the path when air is drawn in through the open bolt hole in blue. Yellow dots are motor mount holes, the red one is the one that goes right through to the inside of the engine. It may leak oil when you shut down and things are draining back to the pan, but it won't leak if it's running and there is a vacuum, it will just slurp any oil back in.
First of all, an engine needs to be vented to atmosphere. In other words, vented to the outside air. Used to be in the past, it was done through a 3/4 to 1 inch pipe, (called a road draft tube) and any blowby was dumped to atmosphere. Quite often you could see an oil mist/condensation mixture and a few drops (or sometimes, a lot of drops of oil) oil, which for the most part, was returned to liquid oil as it was being condensed from a oil mist as it touched the cooler, or cold road draft tube.
Now enter the powers that be, and for various reasons, proclaim that we can't be doing this practice any longer, and lo and behold, the PCV system is born. So the Positive Crankcase Ventilation system is taking over where the road draft tube has left off. Instead of dumping the blowby overboard, it's now contained, and recirculated to be burnt along with the fuel/air mixture. Since a picture is worth a thousand words, here's a couple thousand words worth of pictures.
The first drawing shows a regular PCV system. The system will only work properly, if the engine is sealed. Whoa!! Hold on a second, you said the engine needs to be vented. How can it be sealed, and vented at the same time?
Well, it goes like this, all gaskets and seals have to be doing their job. And their job is to keep the oil and water in the proper areas, and keep the outside air...outside. No air leaking into the crankcase area from the pan gasket, timing chain cover, crank seals, or past the tappet cover gaskets, or by the intake gasket/seals. When we look at the first drawing, this is the ideal set up. Good gaskets and seals, no leaks in or out, engine running providing a good vacuum. Working our way backwards from the vacuum source we can see what's happening. The intake plenum, just below the carb base is where the vacuum source is, when the engine is operating. The vac line (quite often 3/8 steel tubing) goes from this area to a short rubber hose to the PVC check valve. The valve is pushed into a snug fitting rubber grommet that seals both the valve and the hole that it's in, in the tappet cover itself. From there, the entire internal area of the engine is subjected to a negative pressure, or vacuum. Since we have to feed that vacuum it is done in two ways, one is by a vent, and the other is by the blowby that we're trying to control. That is the whole reason for this exercise, so the blowby doesn't get dumped to atmophere. Now the vent is located on the opposite tappet cover and is attached by a hose to the breather housing and the incoming vent air has it's own small filter to control dirt infiltration.
Now, lets start at the vent end of things, fresh filtered air is drawn in through the breather housing, down the hose to the tappet cover. Under the cover, through the various holes in the head casting, (oil drain holes, hole where pushrods come up through etc.) and the air sweeps across the crankcase area picking up the blowby gasses and continues on up through the various holes in the second head. Into the tappet cover area and through the PCV valve, hose, steel tube and into the intake plenum below the carb and finally down each runner and being drawn into the cylinders as the valves open in sequence. The air/fuel mix along with the blowby gasses are burnt and flow out the tailpipe.....except for the tiny little smidge of blowby that gets to do the whole trip all over again.
So are we done? Not quite. (Don't you want to know how this ends??)
So now, lets upset the process. We have a vac leak. Small block Chevs have their own little bugaboo's and quirks. Tin tappet covers have a habit of getting a bit out of shape when over tightened, and wah-la..a vac leak and a poor running engine. The rubber seals at the ends of the intake can be a problem as well, same results if they leak. And since the entire internal area of the engine is under a vacuum, you can get the same results from a bad pan gasket, tappet cover gasket and or seal. Rear main seal, distributor gasket, even if the seal is bad, or missing from the dipstick. (Yeah, there's a little rubber seal in there too.) And last, but not least a rather large vac leak if your missing the bolt/seal at the front of the engine where it goes through to the fuel pump push rod area. After all, that area is under vacuum also.
The second picture shows the path when air is drawn in through the open bolt hole in blue. Yellow dots are motor mount holes, the red one is the one that goes right through to the inside of the engine. It may leak oil when you shut down and things are draining back to the pan, but it won't leak if it's running and there is a vacuum, it will just slurp any oil back in.
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