mat in tn Posted May 14 Share Posted May 14 He's describing a road draft tube... How old is this engine, or the guy working on it? yes, the way it is set up right now, it is acting as a road draft tube in lieu of a pcv system. the reason that you may be seeing it near the oil fill cap is that there is a pcv valve in place even though the hose is not properly connected. this is acting like a closed valve because it is spring loaded against engine vacuum (when installed properly) therefor the crank case pressure is having to find the next best place and may even be damaging the rear main seal. at the crank level, crank case pressure can very well be pressurized oil where at the valve cover it may not. and crank case pressure increases with load. revving in neutral has very little load on the engine I will say this. the seller has a lift! this is an indicator that they know all of what I'm saying and "may" be trying to pass a problem off. if you like it enough. make an offer low enough to handle it. Link to comment Share on other sites More sharing options...
wasteland_jack Posted May 14 Author Share Posted May 14 The way Positive Crankcase Ventilation is supposed to work is that there's a line from the valve cover to the carb or intake, which puts a vacuum on the crankcase. And on the other side of the engine, the other valve cover on a V8, there's a line from the oil filler to the air cleaner, so that air sucked in there is filtered. Without that system there's positive pressure in the crankcase and blowby escapes, like you saw at both the filler and the hose at back. Even new engines have some blowby, but old engines may have a lot - and still perform quite well. So the oil leak may just be the blowby? If, say, the PCV tube is the source of the apparent oil leak, where would all that oil go if the tube was hooked up to the carb as designed? There was a significant amount of oil involved. Probably an ⅛ of a quart or more after a 10-minute drive. With (2)09,000 on the odometer, what engine seals are likely to fail soon on a straight 6? Link to comment Share on other sites More sharing options...
Gary Lewis Posted May 14 Share Posted May 14 If, say, the PCV tube is the source of the apparent oil leak, where would all that oil go if the tube was hooked up to the carb as designed? There was a significant amount of oil involved. Probably an ⅛ of a quart or more after a 10-minute drive. With (2)09,000 on the odometer, what engine seals are likely to fail soon on a straight 6? The PCV line goes into the intake and, in theory, gets burned with the fuel. But as earlier said, there's a PCV valve that has a restriction in it so it isn't a fully-open line, which would lean out the air/fuel mix too much. If the engine has a lot of blowby then the rings are leaking. That's the way combustion gases get into the crankcase, not truly seals. Link to comment Share on other sites More sharing options...
mat in tn Posted May 15 Share Posted May 15 The PCV line goes into the intake and, in theory, gets burned with the fuel. But as earlier said, there's a PCV valve that has a restriction in it so it isn't a fully-open line, which would lean out the air/fuel mix too much. If the engine has a lot of blowby then the rings are leaking. That's the way combustion gases get into the crankcase, not truly seals. also, the pcv system is set to maintain and not expected to catch up or overcompensate for excessive blow by. when it is NOT connected correctly the crank case vapors and pressure just accumulates til the next pressure relief avails itself. often it's the cork gaskets but spring-loaded rubber shaft seals are the weakest link. especially when 40 years old. remember. the old style road draft tube would not have a check valve blocking it like a pcv valve so pressure would not accumulate as much. Link to comment Share on other sites More sharing options...
wasteland_jack Posted May 19 Author Share Posted May 19 also, the pcv system is set to maintain and not expected to catch up or overcompensate for excessive blow by. when it is NOT connected correctly the crank case vapors and pressure just accumulates til the next pressure relief avails itself. often it's the cork gaskets but spring-loaded rubber shaft seals are the weakest link. especially when 40 years old. remember. the old style road draft tube would not have a check valve blocking it like a pcv valve so pressure would not accumulate as much. Correct me if I'm wrong, but isn't this PCV tube now connected to the fuel vapor output on the carb that would normally go to the charcoal canister? Link to comment Share on other sites More sharing options...
wasteland_jack Posted May 19 Author Share Posted May 19 Correct me if I'm wrong, but isn't this PCV tube now connected to the fuel vapor output on the carb that would normally go to the charcoal canister? Link to comment Share on other sites More sharing options...
mat in tn Posted May 19 Share Posted May 19 OH MY! you are correct. get that off of there! Link to comment Share on other sites More sharing options...
mat in tn Posted May 19 Share Posted May 19 OH MY! you are correct. get that off of there! the pcv draw needs to go to the plenum area of manifold vacuum. plenum so that the pcv vapors get drawn by all cylinders. as equally as possible. it normally goes as close to the carb as possible but under. the carb bowl vent needs to go to the charcoal canister to reduce latent fuel vapors. if no canister then it can be capped as the normal vent is in the venturi air stream at the top of the carb under the filter. Link to comment Share on other sites More sharing options...
wasteland_jack Posted May 19 Author Share Posted May 19 the pcv draw needs to go to the plenum area of manifold vacuum. plenum so that the pcv vapors get drawn by all cylinders. as equally as possible. it normally goes as close to the carb as possible but under. the carb bowl vent needs to go to the charcoal canister to reduce latent fuel vapors. if no canister then it can be capped as the normal vent is in the venturi air stream at the top of the carb under the filter. That was their fix for the road draft tube that was venting a great deal of oil. I've never heard an engine run so smoothly. I really wanted this truck, but I'm not sure about the work these guys did to it. Link to comment Share on other sites More sharing options...
mat in tn Posted May 19 Share Posted May 19 That was their fix for the road draft tube that was venting a great deal of oil. I've never heard an engine run so smoothly. I really wanted this truck, but I'm not sure about the work these guys did to it. there is no vacuum potential at the bowl vent. and if there was any it would be negated by fuel vapor pressure. but since the engine was venting crankcase vapor and oil then that will now be going into the carb. how much used oil do you think it will take to clog up a carburetor? then there is the heated vapor trying to vaporize the fuel in the same space. how long did it run hooked up that way? Link to comment Share on other sites More sharing options...
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