The Stovebolt Site Forums The Shop Area Engine Shop PCV system

I have a 58 235 in a 54 1/2 ton. The fumes from the road draft tube get bad in my garage and when the truck is sitting still at idle. I would like to plug off the road draft tube and put a pcv valve system in it. I think this would cure the problem. I have a 2-1 offy manifold that has a spot for a fitting to direct manifold vacuum and currently a twist- on breather in the valve cover. Has anybody done this and if so what can you suggest? thanks

You can cut the road draft tube off, weld up the hole, and fit the can-shaped part that plugs into the block with a rubber grommet to accept a PCV valve. Don't try the fix of putting a shallow-lip freeze plug into the block with a PCV fitted into it- - - -the valve will suck up oil from the crankcase as well as vapors. You'll need a vent of some sort in the valve cover area, such as an oil filler cap with a mesh screen vent built in, or a clean-air inlet that draws air from the clean side of the air filter. (best choice) The PCV will need a 3/8" diameter hose connected directly to the intake manifold, and you'll need to adjust the idle speed and mixture to compensate for the increased airflow into the engine. The engine will last a lot longer, due to the more efficient removal of corrosive crankcase vapors, and less dust and dirt pulled into the cramkcase by the open vent system.
Jerry

I have the type of road draft tube that has the oil fill cap on the can portion. It is a 1/4 turn twist on. I currently have a twist on breather/fill cap in the valve cover. Did GM ever offer a twist on valve cover breather with a pcv grommet in the top? That would make it easy, then I could keep the road draft tube in place and just jam a plug in the bottom of the tube where it dumps out by the oil pan. Trying to keep it at least looking a little original. thanks hot rod.

nevermind , I answered my own question. My flaps who is awesome had everything on the shelf as usual. We're all hooked up, I will road test and tweak it over the next few weeks. Raining now so she's put to bed for the day.

Good luck with that setup. You need to provide a vent of some sort to allow air INTO the engine. Generally, it goes into the valve cover, and pulls back out of the crankcase somewhere, but you might be able to create some sort of metered opening in the draft tube by welding the pipe partially shut, and using the airflow backwards to the normal route. (In at the crankcase, out at the valve cover) Definitely don't block off the breather tube altogether, as you'll create a vacuum in the crankcase that will cause problems. The idea is to flow air through the engine, not seal it off and make a suction on it like a can of soup!
Jerry

I agree that a PCV system is a good idea and will help with the problem. What may need to be addressed is the fact that you may have too much blow-by which is caused by worn rings. If the engine is tired the PCV system can only provide some relief but may plug up in a short time.
Fred

the rings aren't worn , it is a fresh rebuild. I am going to leave the road draft tube open as hotrod suggests so that the engine can breath. I still have some smell coming out the tailpipes but that may be more from the 2-1 bbl setup that is so hard to get perfect although the plugs look great.

you said you have no blow by issues. the road draft tube doesnt work while standing still. the truck needs to be moving in order for it to work. so either you do have excessive blow by or its the accumilated exhaust fumes building up in your garage.

Most road draft tubes have a rear-facing tapered cut at the end of the pipe that is supposed to create a low pressure area from the air passing under the vehicle at speeds above 30 MPH. That's why the tube is long enough to hang down into the airstream. That's how crankcase ventilation was accomplished in the days before PCV systems were developed. The slots in the valve cover on the 216 or the ventilated oil fill cap with the horsehair packing in it gave air an entry point into the top side.

I'd suggest plugging the end of the road draft tube to prevent having two opposing vacuum sources, and drilling about a 1/2" hole in the tube about 6" up from the bottom, out of the airstream. That will allow air to enter the crankcase without creating a resistance to the airflow caused by the low pressure area at the bolonga-sliced end of the road draft. Sneaky guys, those oldtime engine designers!
Jerry

That will allow air to enter the crankcase without creating a resistance to the airflow caused by the low pressure area at the bolonga-sliced end of the road draft

More rocket science?
The low pressure area at the road draft tube doesn't "create a resistance", it's what causes air to enter the crankcase. "Resistance" would be a high pressure area.
Making a hole elsewhere in the tube means ruins that function, so the fumes just do whatever they want.
Yes, that's much better.

You do have a day, job, don't you?

Read the whole thread before you comment. In the meantime, don't eat beans- - - - -you'll blow your brains out. You do live under a bridge, right?
Jerry

Your remark is what is called "unresponsive": you said something, but it's not an answer.
How is low pressure at the road draft "resistance" to air entering the engine?
Your answer: "I don't know, so being rude is my only recourse".
don't eat beans - is that a crushing retort in your schoolyard?

It's OK, I have a very limited interest in what is essentially beating up children.
Please continue to dazzle your audience with more nonsense, I promise I won't waste any more time pointing out your mistakes - it would take forever.

Wow!

As a rookie here, I appreciate conflicting opinions. It helps me learn and gives me two perspectives. However, when the post start becomming personal, I start discounting the information. Please understand that I want to learn. I do not know what you know. Thanks for keeping it clear and factual.

Dale

dont discount this.
For this system to work, the air must be moved rapidly through the crankcase and out of the road draft tube.The lower end of the road draft tube is designed to act like a suction pump to get this rapid circulation of air.Notice that the lower end of the road draft tube is cut at an angle.Have you ever stuck your hand out of the window of a fast moving car? The wind blowing past your handwas a pretty powerful force, wasn't it?If a vehicle with a road draft tube was moving down the road, the wind would be rushing by the end of the road draft tube at a pretty good clip. With the road draft tube cut at the angle shown, the wind moving by itwould create a suction (known as a partial vacuum) at the end of the road draft tube. This will draw the airand vapors out of the crankcase. This system is known as a nonpositive crankcase ventilation system because it will not work unless the vehicleis moving forward.

A positive crankcase ventilation system operates anytime the engine is running and does not depend on vehicle movement. Notice that the air enters the crankcase through a breather and goes out through a tube leading to the intake manifold. Anytime the engine is running there is a low-pressure area in the intake manifold. Theair from the crankcase containing the blowby gases and fuel and water vapors is drawn through the intakemanifold to the combustion chamber. There, the combustible gases are burned and discharged through theexhaust system.

this http://www.tpub.com/content/armyordnance/OD1001/OD10010120.htm shows the picture. it is also in the maintenance manual i believe

51 ashton has told you the correct way that the system works without getting all bent out of shape.
Good job.

I have a different setup, no draft tube but a filler tube with an breather/filter at the top with an oil cap on it. I like the set up better than a draft tube, one less place for oil to drip out of and rev up my land lord! :P
The valve cover had a pipe fitting on it that used to go to a vac line for the governor (long since gone). I replaced the fitting with a PCV valve and ran a fitting into the back of my air filter like many modern vehicles do. Since I converted the oil bath air filter to take a paper element filter there is more than enough room in there for the fittings, and really, unless someone is looking close it's not terribly obvious there's something not quite right.
http://inlinethumb49.webshots.com/40624/2720844220049210764S600x600Q85.jpg
http://inlinethumb46.webshots.com/40877/2858472740049210764S600x600Q85.jpg