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PCV SYSTEMS: MAINTAINING THE FLOW BY SAM BELL Like other well-established technologies, PCV systems can be taken for granted. But PCV faults can cause a wide range of driveability symptoms, as well as oil consumption and/or leakage problems. arly internal combus- matter how well-built, contributed their tion engines, like the share of this type of pollution. steam engines which Road-draft tubes were an early at- preceded them, were tempt to cope with the increased messy and smelly con- crankcase pressures that accompanied traptions. They routine- ever-higher engine speeds and loads. ly leaked oil and spewed noxious fumes These tubes were fitted onto the tops of Einto the atmosphere. Many features we the engines, extending up several inches this advance in engine design, crankcase now take for granted were missing, in- before looping down to an open end fumes still accounted for nearly half of a cluding, in many cases, even valve cov- positioned somewhere in the “draft” or typical engine’s hydrocarbon emissions ers and crankshaft seals! Early “engi- relative low-pressure area beneath a and were a major factor in the forma- neers” often subscribed to the belief moving vehicle. The pressure differen- tion of sludge (see the sidebar on page that oil leaks simply assured a full flow tial between the two ends of the tube 36) and the breakdown of engine oil. of lubrication to the bearings. Gaskets allowed the relatively pressurized During the economic boom that fol- and seals, as anyone who grew up ser- crankcase gases to flow downward into lowed the end of World War II, an ever- vicing English cars knows, were meant the vehicle’s slipstream. The short up- burgeoning vehicle population, coupled simply to limit oil loss, not prevent it. ward section of the tube kept the heavi- with increasing urban growth and heavy Blowby gases, including substantial est oil droplets from following along industrial emissions, led to increasing quantities of water, escaping past the with the flow. Breathers of one sort or smog formation in many areas of the piston rings would diffuse through the another had to accompany the draft country. Testing and analysis eventually engine oil and escape into the atmos- tube to allow makeup air to flow into confirmed that automotive crankcase phere. Worn or poorly built motors the engine. These were often simple fumes were also a significant contributor, spewed more of these hydrocarbon- mesh openings integrated into oil filler and support for industry regulation grew. laden fumes, but all early engines, no caps or air cleaner housings. Even with As it happened, an existing technologi- 30 August 2013 are produced in varying amounts under different operating conditions. At idle, when manifold vacuum levels are high and loads are low, crankcase fumes are at a relative minimum. Under these conditions, the high vacuum sucks the restrictor to nearly close the valve. Makeup air is supplied to the crankcase via a breather hose, “positively” displac- ing the crankcase fumes—hence the cheery “positive” of the PCV name. Under midrange throttle openings, engine loads and speeds are higher, and more blowby gases are produced. Man- ifold vacuum, however, is lower, so the spring tension can largely counterbal- ance the pressure differential, thus al- lowing increased flow through the valve. As before, the breather supplies clean air to displace the fumes. At very high throttle openings, intake manifold vacuum drops to near zero. Spring tension then holds the valve open for maximum flow, just as it does with the engine off. Even so, the amount of blowby gases needing evacu- ation often exceeds the flow capacity under high-load conditions. At such times, a substantial portion of crankcase gases may flow backwards from the en- gine through the breather and into the air filter housing or intake snorkel. (This accounts for all those oily breather filter elements we changed back in the not- so-good old days of carburetion.) Some vehicle designs opt for a fixed- orifice PCV system. Here, there are no moving restrictor pellets and no springs, but simply an opening con- necting the crankcase to the intake Photo: Sam Bell Photo: manifold. Such systems usually incor- cal solution to the problem had been de- How It Works porate an oil separator. veloped for a somewhat different appli- The photo on page 34 (top left) shows a Separation anxiety, as I like to call it, cation during the war. This was the PCV cutaway view of a typical PCV valve. has led some manufacturers to opt for (positive crankcase ventilation) valve, The larger end is typically fitted into a more complex PCV system designs incor- originally developed to prevent water grommet, often located in a valve cover, porating oil separators, whether coupled from traveling back into the road-draft through which the blowby gases will be with a conventional PCV system or with a tubes of tanks fording rivers and streams. evacuated for introduction into a central fixed-orifice system. Oil loss is an ongoing Eventually, engineers at GM’s Cadil- location in the intake plenum. Each end problem for many PCV systems, and lac Division, which had built Army of the valve features a carefully sized such separators are a logical means of tanks during WWII, adapted the PCV orifice, with a tapered pellet riding be- combating it. Most consist of a molded valve to automotive use, supplanting the tween. A precisely calibrated spring acts plastic container with one or more inter- recombustion tube. Through the Auto- to help regulate the flow as speeds and nal baffles. They’re usually attached at mobile Manufacturers Association, GM loads change. Obviously, any change in the lower end directly to the block via an offered the technology, royalty free, to orifice size, pellet weight, spring tension externally sealed tube positioned well all U.S. automakers. It was quickly or mounting orientation can alter the above the normal oil level. The upper adopted by all, just a year ahead of valve’s performance characteristics. end of the separator assembly may be fit- scheduled requirements. As you might expect, blowby gases ted directly to the PCV valve or plumbed August 2013 31 PCV SYSTEMS: MAINTAINING THE FLOW that bypasses the combustion chamber via the crankcase. If the breather hose is located downstream of the MAF sensor, throttle valve deposits are again likely. In either event, if the breather configu- ration has been altered relative to the MAF sensor, a new fuel map will need to be programmed in. Manufacturers and aftermarket suppli- ers have tried a number of solutions, with mixed success. One of the most interest- ing adaptations is the Krank Vent Crank Case Vacuum System, available at http:// www.et-performance.com/turbo.html. This approach uses two additional check valves plumbed into the PCV and Photos: Sam Bell Photos: breather system hoses and claims to pro- This dismantled Saturn PCV valve typi- duce and maintain a vacuum in the fies conventional valve construction. The crankcase. Other designs may make use smaller port (upper right) is connected to intake manifold vacuum, which tries of heated separators, fixed orifices or I drew a grid on this shop towel to high- to suck the tapered pellet (lower left) to even dual-stage ventilation valves. light the standing center pressure wave an almost-closed position at idle. As Flow rate through an orifice is pro- and surrounding “donut” depression manifold vacuum drops, the precisely portional to the size of the restrictor and typifying normal PCV operation on a calibrated spring (center) counterbal- the pressure differential between the Corolla engine. Try the experiment de- ances the effect of the vacuum, opening two ends. The actual flow rate can be scribed in the text so you can see how the valve to allow more flow. measured in liters per minute at a speci- the system reacts to altered conditions. fied pressure drop, but as technicians to it. Some manufacturers utilize a fixed- we’re usually more comfortable think- I discovered that PCV flow at idle can orifice restrictor instead of or in addition ing in the more familiar terms of grams account for upwards of 10% of total to a conventional variable-flow valve. Any per minute (gm/min.). flow, while at 2000 to 3000 rpm it might oil droplets incorporated into PCV flow I set out to make a few rudimentary account for a greater mass but a smaller eventually wind up as soot particles that measurements by monitoring MAF flow percentage of total flow. can foul exhaust oxygen or air/fuel sensors rate at idle with the PCV system in nor- Many PCV systems will last the life of or degrade catalytic converter substrates. mal operation, then with both the PCV the vehicle with no need for special atten- Of course, adding a turbo or super- hose and the breather hose temporarily tion. Tiny oil droplets suspended in the charger complicates the engineering blocked off. On a representative 2.0L crankcase usually keep the spring and considerably. Under boost, the pressure engine, the MAF rate dropped from pellet clean and lubricated. Sticky PCV in the intake manifold may easily exceed 2.26 to 2.05 gm/sec. with the ventilation valves usually indicate a lack of regular oil that of the crankcase, effectively stopping flow blocked. Using this and other data, changes. Gummed-up PCV valves may normal-direction PCV flow result in poor or unstable idle just when load and blowby or excessive fuel trim correc- gas production are at their tions. (These trim corrections peak. The rapidly increased can be in either direction, de- pressure within the crankcase pending on whether the valve under boost may blow heavily is sticking in a more open or oily fumes through the more closed position than ex- breather and into the intake pected.) Insufficient flow may snorkel or the airbox.

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