Every farm has an older gas engine in a truck or some other application that is still good but is not being used since it has some running problems. The cause may be the PCV circuit.
Anengine needs to have the crankcase ventilated. It can be thought of as opening the windows in a room to let the stale air out and the fresh air in.
The crankcaserequiresa method to relieve any pressure that is built up in the oil pan. If this pressure is not released, the engine will leak oil out of most if not all seals and gaskets. Another important reason for being ventilated is to remove the harmful blow-by gasses that leak past the piston rings and get into the oil.
When mixed with the engine oil the byproducts of combustiondegradethe lubricant and can cause corrosion of internal parts.In addition, it is imperative that any moisture/condensation that was formed be removed. If left unchecked the moisture will mix with the oil and blow-by and create sludge.
The amount of blow-by an engine has is directly linked to the effectiveness of the piston ring seal. This in turn is impacted by but not limited to:
• Piston ring design
• Cylinder wall finish
• Concentricity of the bore
• Piston-to-wall clearance
• End-gap of the piston ring
• Piston material (cast, forged,hypereutectic)
• Cylinder pressure
When anengine is running some ofthe combustion pressure leaks past the ring package andenters the crankcase.
Ametricto quantify ring seal is a cylinder leakdown test. This is different from a compression test that measures the pressure in the bore created by the piston moving in its stroke(pumping action). In contrast, a leakdown test hasthe piston in a defined static position. Air at 100 psi is introducedto the bore via a test instrument. The meter then registers the amount of leakage asapercentage.
Depending on the use, age and design of an engine,the acceptable leakage can range from5% to 20%;a high mileageorhourengine will see the latterfigure. The lower the leakdown percentage the better the ring seal. Inturn there is less blow-by.
It is important to note where the air leakage is occurring. This is done by listening. Air escaping from the induction track or the exhaust identifies poor sealing of the intake and exhaust valve(s), respectively. If airis heard in the dipstick tube or valve cover, itisfromblow-by.
Though blow-by is the main cause of pressurizing the crankcase it is not the only one. The movement of the crankshaft and especially the pistons as they travel downward are contributors to the need to have a means to ventilate the crankcase viathePCV system.
Ventilation styles
The original method to allow the crankcase to breathe was called an open system. As the need to lower the emissions on an engine producedbecame apparent,the closed system was developed.
General Motors invented the PCV system. It was determined that 20% ofthe emissions emitted from an engine and fuel system were rooted in the open ventilation design. The PCV was brought to market on GM engines for the 1963 model year. Shortly thereafter the rest ofDetroit followedsuit.
Many of us know the open system as the old blow-by or road draft tube. It was a very simple designthat employed the natural pressure differential of the air under the hood versus that going by the road draft tube. The air usually entered the engine at the highest point via a breather in thevalve cover. The clean air then coursed or pushed out the blow-by but was greatly aided by the siphoning effect of the air moving across at a perpendicular angle to the opening of the road draft tube.
The problem with this system being that at slow speeds or during idle there was no draft created and thus, little ventilation of the crankcase. In addition, the blow-by was entering the atmosphere.
Closed system
It was deemed that the blow-by gasses are a pollutant thatif reintroducedinto the combustion processcan be burned and ultimately eliminated or at the least, greatly reduced.
The early closed system was not a PCV since the letters represent Positive Crankcase Ventilation. Thefirstclosed system worked almost like the open system but instead used thelow-pressureregion in the engine’s air cleaner to create the draft and allow the blow-by to enter the inductionpath.
The GM invented PCV system took advantage of the greater depression (low vacuum) that is found in the intake manifold with it being controlled by a valve (the PCV)that employs a pintle, seat and spring to work against.
The PCVvalveismade with either a variable or a fixed orifice along witha spring-loaded plunger that is acted onby enginevacuum.
When the engine is off the valve would be closed. Then under normal or light-load driving the valve would be open sufficiently to purge the crankcase of both fumes and pressure with little restriction. Under very high vacuum such as at idle or when theengineis coasting with the throttle closed,the plunger is spring loaded in the opposite direction and the flow is limited via an internal bleed orifice.
This is done as to not allow the PCV to alter the air/fuel ratio greatly and impact how the engine idles andruns correspondsto the throttle beingreopened whenthecoast downsituationends.
An additional function of the PCV system is to eliminate any moisture build up (condensation) that can form in the engine,especially one thatexperiencesshort-cycleuse.
The PCV system can be thought of as a controlled manifold vacuum leak. It needs to balance the bleeding of the vacuum from the intake manifold with the ability to purge the blow-by and pressure from the crankcase.
Factory designed PCV systems also employ a baffle under where the valve resides to not allow the high velocity airflow to suck up engine oil.
The M/EWagner difference
The factory PCV system is a wonderful design.Yet, it is oftenthe root cause of problems. It shows itself as an engine prone to oil leaks and oil vapors under the hood along with upsetting the way the engine performs.
If I had a dollar for every carburetor that was replaced or blamed foradriveabilityor idle issue that wascreatedby the PCV,I would be a rich man. How can this be? Let me explain.
When an engine is developedthe manufacturer goes through great lengths to design and calibrate theflow of the PCV system for thatapplication,and the systems works superbly.
Now the owner goes to the auto parts store to buy tune-up parts and purchases an aftermarket PCV. The outside looks the same and it fits but the flow rate for blow-by and vacuum is not correct for the engine.
It is impossible for an aftermarket companyto make a valve with the proper flow rate forevery engine. What theymarket isa generic design that they package into a different enclosure so that it mounts properly.
Sometimes youwin,and all is well. Most times there is a negative impact on the engine with the onlyquestion being how greatandhow does it present?
When it comes to older engines found on the farm, purchasing an original valveis no longer possiblefor tworeasons: the engineis long out ofproduction; the amount of blow-by and engine vacuum are different sincethe engine has now many years of use on it.
Excessive crankcase pressure can be seen with oil pushing out of a breather.
The symptoms of theincorrectflow rateor vacuum transition point of aPCV valve are oil leaks, oil in the air filter, leaks/fumes around the valve cover, persistent rear main seal issues, poor idle quality, the carburetor’s lack of adjustability,tip-in stumble/hesitation,pingingandpoor throttle response.
The PCV valve ifnot correct also impacts the carburetor calibration beyond idle. It must be recognized that the circuits of a carburetor employ a building block approach; the idle circuit has impact on the main metering circuitand so on.
If you have a PCV valve with an excessiveamount of flow (vacuum)ordoesnottransition in a linear manner,you can spend a good deal of time trying to compensate for this by playing with thecarburetor and ignition timing to no avail.
If youhave an engine thatis suffering from any orallthe aliments I listed previously,then there is a very good chance that the PCVvalvethat is installed has the wrong flow rate or vacuum response/transition.
When Ifound out about the M/E WagnerDF-17tunablePCV valve Ibecameextremely excited. It is a part that the industry should have invented forty years ago and never did.
The DF-17 is beautifully machined in the USA from aircraft grade aluminum. The real story is that it offers almostinfiniteadjustabilityforflow rate and vacuum transition response.
The design replaces the pintle/plunger with two check balls.It can also be configured into a fixed orifice mode for engines that have very low or unstable vacuum.
It hasthe unique ability to offer whatis known in engineering as twodegrees of freedom (adjustability). The idle flow rateand idle to cruisetransition vacuum level can becalibrated separately.
It is important to add that the developmentof this valve was not a hit-or-missventure. Itwas rooted in a major investment inlaboratory testingthat wassupplemented with countless hours of on-engineresearch.
The DF-17 is designed to fitevery domestic engine that has been built the last 60 years.It is very easy to calibratewith a simple vacuum gauge. M/E Wagner provides in-detail and easyto understand instructionsand live technical support.
Retailing for $129.00theDF-17 at first blush may sound expensive for a PCV valve.I can saywith confidence that it is one of thebest investmentsthat you can make to an older farmengine;ending ventilationrelated oil leaks, dirtyengineoiland poordrivabilityonce and for all. You can put that engine back into service, where it belongs!
M/E Wagner
570-899-4544
