You take your car in for an oil change. The work is almost done when the technician comes out to talk to you. He’s holding your air filter and PCV valve and recommending that you replace both because they “look dirty.” Without missing a beat, he explains how critical the air filter is to the efficient operation of your car. He tells you that a clogged air filter, or one that’s nearly clogged, can easily cost you 10 percent in gas mileage. With gas prices going through the roof, he adds, replacement will probably save you more than the cost of the filter. Plus, a dirty PCV valve, well, that’s never a good thing. Then he waits for your decision. It’s tough to make up your mind about an expensive air filter and PC-whatchamacallit valve when you don’t know what to look for.
It’s not difficult to check the air filter and PCV valve yourself. Here’s what you need to know:
Air filter check
First, ignore the dirt on the leading edge of the air filter pleats. All air filters accumulate dirt on the leading edges in as little as a few thousand miles. Yet most last for about 12,000 miles. You want to know how much dirt has penetrated deep into the pleats. To test the true condition of your filter, hold a shop light behind it. See how much light passes through the inner pleats and compare yours with the three sample photos (after Photo 2). The filter labeled “replace” is totally clogged and cost the owner a fortune in wasted gas. The filter labeled “borderline” shows a clogged area, but the rest of the filter has decent light transmission. It’s borderline, and the owner could probably squeeze 2,000 to 3,000 more miles out of it. It should be replaced at the next oil change interval. The last filter shows how much light passes through a new filter.
The PCV story
The PCV (positive crankcase ventilation) valve is a one-way valve that recycles crankcase gases back into the engine to burn. A plugged PCV valve can result in a rough idle and poor mileage. Worse, it can cause costly oil leaks. Always follow your manufacturer’s replacement recommendations. And never replace a PCV valve simply because it "looks dirty." All used PCV valves look dirty. Photos 1 and 2 show two ways to check its real condition.
Video: How to Test and Replace a PCV Valve
What is a PCV valve? Do I really need to change it? Rick Muscoplat, an automotive expert at The Family Handyman, will tell you what a positive crankcase ventilation (PCV) valve does, how to test it, and, if needed, how to replace it. PCV valve replacement is very simple.