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PCV Valves: Cheap and easy (but not flawless) emissions control

by Walt McCrystal

One of the earliest and simplest emissions control devices was positive crankcase ventilation (PCV), which doesn’t cost much, doesn’t hurt performance, and requires little attention. The system works simply by taking the unburned fuel vapors from the engine’s crankcase and directing them back into the airstream to be burned.

pcv valve

Originally, companies listed annual PCV valve changes in their owner’s manuals; now, they generally don’t. PCV valves usually don’t wear out, and are probably replaced far more often than they need to be as a matter of “preventative maintenance.” They can last for years and still do their job just fine.

The usual test for PCV valves is to shake them and see if they “rattle,” but some modern ones are spring-loaded and don’t “rattle” when shaken, so that PCV “test” can be invalid.

1981 PCV valve

A good first test is to see if you can feel a strong intake manifold vacuum at the tip, with the engine at idle. Blocking the breather hose for a moment and then releasing it should show a vacuum has built up inside the engine crankcase. It should “whoosh” into the engine slightly when released; or hold a piece of paper, the stiffness and weight of a business card, at the end of the breather hose with suction.

If the crankcase pressure flow is the opposite way (blowby is being pushed into the breather filter, making it wet with oil), then the PCV apparently can’t keep up with the blowby. It can also leave an oil or moisture stain of the air filter. This may be a PCV, engine wear, motor oil quality, or low vacuum problem.

1984 PCV valve

I have seen the PCV iced shut with engine condensation or restricted by heavy engine sludge. Some engines have heated PCV hoses to keep condensation from freezing. The PCV valve will close under turbo boost if there is an intake backfire events, because if a backfire flame were allowed to enter a crankcase, it could cause an explosion.

Staff: It’s easy to find PCV valves on the earliest engines, sitting on their own, out in the open, in the middle of the valve cover. On modern cars, they can be much harder to find and change, buried under the noise cover and then hidden towards the back of the valve cover and taking a nontraditional shape. On a 2006 minivan, they are on the back cylinder bank, underneath the cowl. In modern Pentastar-based cars, it can be hard to find, as seen in this video (which calls it a “pollution control valve.”)

Known PCV issues and solutions

In the early 1980s, Mopar released a PCV valve that was heavily spring-loaded in the closed position on the carbureted cars. In the colder winter months, when engine crankcase blowby was heavier and the choke was on for longer periods of time, the high moisture content of the blowby vapors often iced up, blocking PCV flow, and leaving a stain on the air filter. In the shop, it became known as the “no-flow” PCV.

evaporative emissions control system

The PCV for the straight six and V-8 engines had too much flow and would cause rough idle and stalling if used on the four-cylinders, so it wasn’t a good option. The 2.2L turbos used the 6 and 8 cylinder style PCV without issues [Hemi Andersen added: the multiple-port injection system on the turbo engines meant that extra flow from the PCV valves was not a problem].

Chrysler released a PCV hose heater kit for the four-cylinders which included a metal tube and a bracket held on by an exhaust manifold nut; it used exhaust heat to warm the incoming PCV vapors and hopefully prevent icing. It was an improvement, but didn’t fix all the problems. The “no-flow” PCV valve was eventually discontinued, and a more “moderate-flow,” softer-sprung PCV valve was introduced. That seemed to be a happy medium, reducing rough idle/stalling problems, and keeping the air filters pretty much stain-free.

Hemi Andersen added: The Volkswagen and 1.7 liter SIMCA/Peugeot engines on the Horizon and Omni had different valves, as well. The PCV valves for the 6 and 8 cylinder engines had too much flow and would cause rough idle and stalling if used on the carbureted four-cylinders; yet another “moderate-flow” valve was used with throttle-body injected (TBI) 2.2 and 2.5 engines.

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