PCV System Questions

[raoul.duke]

I've been reading various threads on this site regarding PCV systems and I'm curious about mine- I have been having problems with oil blowing by the passenger side intake manifold bolts and have retorqued/ resiliconed/ replaced gaskets/ etc. with no luck. Truck is a '72 with a 350.

Everything that I have seen on this site shows the PCV to be on the driver's valve cover into a vacuum port on the carb and a breather on the passenger valve cover.

The way my truck is set up by the PO there is a vacuum hose from a vacuum port on the carb to a fitting (open elbow, not PCV valve) on the top of the intake manifold right behind the carb, breather on the passenger side vc, nothing but an oil fill cap on the driver vc.

Could this be my blowby problem? Should I cut in a PCV valve in the driver's vc? If so, what is the top port in the intake manifold behind the carb for/ should it just be capped off?

Gracias.

[JimKshortstep4x4]

If I understand your arrangement there is no PCV valve in the system so by all means set your system up like the factory had it. The PCV system is a flow through system where clean air is sucked through the engine and burned after picking up the blow by. Normally the PCV is hooked to the front of the carb on a 4 bbl carb and plugged into the driver's side valve cover. The passenger's side valve cover should have a hose with a spark arrester in it hooked to the air cleaner. If there is no PCV valve or adequate breathers on the engine, pressure will build up adding to the oil leakage problem.

Jim

[raoul.duke]

OK, noted- but what purpose does the vacuum line to the top of the intake manifold serve?

[cdowns]

you should be using a good quality thread sealer with teflon to seal the threads on the intake bolts i've found that silicone has poor to none thread sealing ability// i'll beadblast the threads clen with carb cleaner put thread sealer on threads allow to set 15-20minutes b-4 installing// i'm guessing if you take your bolts out you'll find no silicone on the threads nothing on the threads means it just peeled off when you installed them and therefore no sealing for oil leaks

[sneakysnake]

Originally Posted by raoul.duke

OK, noted- but what purpose does the vacuum line to the top of the intake manifold serve?

The vac port is for the brake booster and auto trans or other vacuum needs . If you have this hooked to your valve cover then your probably leaning out your fuel mixture from the carb.If you have excessive blowby you may have a stuck ring.

[raoul.duke]

got it. Is it advisable to install a collar and PCV valve in the valve cover anyway even though the motor didn't originally have one? Any disadvantages to this?

[piecesparts]

A motor needs some form of evacuation of the burned oil fumes. The removal is very important or the pressure can build up and destroy seals in theengine. The earlier motors only had the open breather cap on the fill tube that mounted in the intake manifold (283/327 era), then came the first rudimentary evacuation system that tapped off of the rear of the intake manifold and took the fumes from the lifter valley and drew air in through that very same breather cap on the fill tube. That was followed up by the systems that we see so much of today. The PVC tube going from the valve cover to the base of the carburetor and the other valve cover with either a breather cap on it or a tube going from it to the air cleaner to provide a fresh air source to the crankcase.

It does not matter as to which valve cover you mount the PCV valve into, but you should have one for various reasons, HOWEVER, it is important that you provide a fresh air source to the crankcase, when you run a PCV valve to ensure that you don't pull a vacuum on the crankcase. (Damage seals of cause advers oil consumption). The previous owner had a hose connected to the intake manifold was not accomplishing a thing. running a vacuum source to a vacuum source and not pulling ANY crankcase fumes off of the motor. This is undoubtedly a good source of your oil leakage. The crankcase is running at a POSITIVE pressure, not good./

I would look at purchasing a quality set of aftermarket valve covers, PCV valve and either a breather cap or a connection to my air cleaner and remedy that problem very quickly, If I were you.

[69rut]

If your vacuum hose is hooked up to the intake port, as shown in the previous post, you have NO crankcase vent. This could be the problem. Of course if you have a worn block, you will have blow by. The air in the cylinder blows by the piston and rings because the bore is too large. This will also cause the problem you have because the engine vacuum is too low to pull the excess gas out.

You can run a vent with or without a PCV valve. The air inlet should be from the air cleaner base or a breather cap, through the engine (valve cover), and then to the carb base. The pcv valve will restrict the flow according to the vacuum of the engine. If you mash the gas the vacuum drops and the PCV valve closes.

The purpose of the crankcase vent is to purge excess gas and water vapor that build up inside the engine. The carnkcase has alot of air moving around in it and the heat makes water vapor. Plus any gases that pass the rings. The crankcase vent pulls this stuff out and burns it in the cylinder.

[bryanw1968]

The are 2 main purposes of the PCV valve.

1. Safety. If the motor back fires and the vacuum temporarilly stops the valve inside the PCV valves closes to prevent an explosion in the crank case.

2. Emmisions. By sucking out all of the blow by and unused gas vapors you have a cleaner burning, more efficient engine.

You can run a motor with out the PCV valve by just having 2 free flow open breathers to evacuate blow by. HOWEVER, I wouldn't recommend it as it makes a pretty big oily mess.

[raoul.duke]

OK. Looks like I'll pick up a new breather w/ a PCV valve in addition to my open breather I already have.

[bryanw1968]

Originally Posted by raoul.duke

OK. Looks like I'll pick up a new breather w/ a PCV valve in addition to my open breather I already have.

I personally run a PCV valve to the back on the carb on one valve cover and have a direct tube connected from the other valve cover to the underside of the air cleaner. That is how they come stock and gives you a "balanced system". But the way you're going to run it is perfectly fine also. Good luck and I hope that stops your oil leaks. However, once you blow oil past the front/rear of the intake it still might leak a little bit since oil has now gotten past it.

B-dub

[raoul.duke]

I have an open element air cleaner so I have to go the breather route. I'll probably pop the intake and replace the gaskets and bolt sealant just for peace of mind.....

[bryanw1968]

Originally Posted by raoul.duke

I have an open element air cleaner so I have to go the breather route. I'll probably pop the intake and replace the gaskets and bolt sealant just for peace of mind.....

Heck just fix the PCV situation and see if you get lucky first.

[piecesparts]

I have open air cleaners and I connect the breather tube to my cleaners. Most of them come with an adapter for just such a situation, but you can make one out of plastic pipe fittings from the local Farm/Hardware store. This keeps the nasties off of my engines.