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Common Filter Questions


11/14/2006
By Larry Carley

Automotive technology rarely stands still. Even filter technology is in a constant state of change. This month, we look at the facts on filters.
 


QUESTION 1: HOW OFTEN SHOULD I CHANGE MY OIL FILTER?
Often enough to prevent premature engine wear and damage. That means replacing the filter before it loses its ability to keep the oil clean and before it plugs up. As a filter becomes saturated with dirt, soot and sludge, it builds up resistance to the flow of oil. This creates a backup of pressure ahead of the filter and a drop in oil pressure downstream of the filter. Unless the filter is soon changed, it eventually reaches the point where the pressure differential across the filter exceeds the limit of the relief valve (typically 10 to 12 psi). When this happens, the relief valve is forced open so unfiltered oil can bypass the filter to prevent oil starvation. Any dirt or wear particles in the oil will then go straight to the bearings.

The basic idea is to change the oil filter before the media becomes clogged and causes a problem. Most professionals recommend replacing the filter every time the oil is changed. The standard oil change interval recommended by most vehicle manufacturers today is 7,500 miles — for "normal" driving, which does not include short trip stop-and-go driving (especially during cold weather), towing a trailer, prolonged idling, sustained high speed driving during unusually hot weather, or driving on gravel roads. Because of this, many motorists are actually "severe service" drivers who should have their oil and filter changed every 3,000 miles or every three months.

Diesels and turbocharged engines should also have their oil changed every 3,000 miles. Diesels produce more acids and soot in the crankcase, while turbocharged engines expose the oil to much higher operating temperatures inside the turbo.

Older, high mileage (over 75,000 miles) engines should also have their oil changed at 3,000 mile intervals because of the increased blowby caused by wear.On vehicles with oil reminder lights, the oil change interval may be estimated mathematically based on hours of operation, temperature, driving speeds and other operating conditions and/or by an oil sensor that measures the electrical resistance of the oil.

MY OWNERS MANUAL SAYS I ONLY HAVE TO REPLACE THE OIL FILTER AT EVERY OTHER OIL CHANGE. WHY SHOULD I CHANGE IT EVERY TIME?
Don't be cheap. Most oil filters cost less than five bucks. How much does it cost to replace or overhaul and engine these days? $4,000? $5,000? $6,000 or more? Spend the few extra bucks to replace the filter when the oil is changed.

Another good reason for replacing the filter every time is because the filter holds anywhere from a pint to a quart of dirty oil. Most engines have a four quart crankcase capacity. So if the filter is not changed or drained, the dirty oil inside the filter will contaminate the fresh oil and reduce its lubricating qualities and potential service life.

AREN'T ALL OIL FILTERS THE SAME?
Absolutely not. The media inside a filter may be resin-impregnated cellulose fibers, synthetic fibers (glass and polyester), or more commonly, a composite mixture of natural and synthetic fibers. Pleat density of the filter media and how it is supported and sealed to the end cap will differ depending on the brand and cost of the filter.

Overall filter performance depends on three criteria: resistance to flow, capacity and efficiency.
Resistance to flow is the drop in oil pressure that occurs as oil flows through the filter. A good filter should pass oil easily without creating an excessive pressure drop, yet offer enough resistance to trap contaminants.

Capacity refers to the amount of junk a filter can hold before it plugs up. Higher capacity means longer filter life. Long-life filters typically have more square inches of surface area to trap and hold contaminants.

Efficiency is a filter's ability to trap contaminants. Efficiency generally works against capacity, however, because the better a filter traps particles the more quickly it plugs up. Efficiency actually goes up as the filter becomes dirty because the accumulation of debris helps block and trap additional debris. But this also increases the rate at which the filter plugs up.

Premium oil filters typically have a higher efficiency rating than standard oil filters, and typically capture up to 95 percent of all particles down to 10 microns in size. A standard filter, by comparison, might only trap particles 40 microns and larger in size (a human hair is about 75 microns in diameter).

When evaluating filters, engineers talk about things like the "BETA ratio" (SAE J1858) that compares filter efficiency to the micron size of the particle it traps. The BETA ratio is a better indication of overall filter performance than the SAE HS 806 standard single pass filtration test because this test only shows how much total dirt the filter can trap, not the size of the particles. Particles under 40 microns in size contribute just as much, if not more, to engine wear than the larger particles. Unfortunately, filter manufacturers do not publish BETA numbers for their products, making it difficult for consumers, countermen or technicians to compare one filter to another.

WHICH TYPE OF OIL FILTER IS BEST FOR MY ENGINE?
Premium filters are better than standard filters. A "long life" filter is a good choice for someone who tends to be forgetful about regular oil changes, while a "high efficiency" premium filter is best for someone changes their oil regularly and wants maximum protection for their engine.

WHAT MAKES AN OIL FILTER PLUG UP?
Sludge. Sludge forms when moisture accumulates in the crankcase, especially during cold weather stop-and-go driving when the engine never gets warm enough to boil off the water. Dirt, soot and metal wear particles also contribute to clogging, but generally don't build up fast enough to pose a real threat to the filter unless a vehicle is operated off-road, run without an air filter or has open crankcase breathers.

I HAVE AN OLD CAR THAT USES A LOT OF OIL. AS LONG AS I KEEP ADDING OIL, WHY CHANGE THE FILTER?
An engine with a lot of wear will experience more blowby than a newer engine with good compression. That means even more dirt, soot and moisture will accumulate in the crankcase. If the oil and filter are never changed, even though oil is added regularly, sludge will build up and shorten the remaining miles left on the engine. And once sludge plugs a vital oil line, it's the end of the line.

CAN I TELL IF MY FILTER NEEDS CHANGING BY LOOKING AT THE OIL?
There's no way to judge the condition of the oil or filter by appearance alone. Even new oil quickly discolors once it's in an engine because of oxidation and contamination by blowby particles too small for the filter to remove. Oil analysis is the only way to tell if the filter is still doing its job. Unless somebody operates a fleet of vehicles and is trying to economize, it's cheaper for the average motorist to just change the oil and filter on a regular schedule than to mess around sending in oil samples for lab analysis.

WHY DON'T THEY PUT THE OIL FILTER WHERE I CAN EASILY REACH IT?
Good question. Because engineers don't have to service the cars and trucks they design. The filter is located where it is so the engine can be easily installed on the assembly line. Many engines today are "corporate" engines and are used in a range of makes and models. The filter location may not be too bad on one vehicle, but not so good on another.

I have a Ford Windstar with a 3.8L engine. Ford uses this engine in a number of its vehicles. The filter on my van is located at the right front corner of the engine, above the subframe crossmember. It’s barely accessible, even with a cap style adapter that fits over the end of the filter and a six-inch extension on my ratchet wrench. There's no way to get a band style filter wrench on it, and I can barely turn it with my finger tips. And when the filter finally comes loose, the oil dribbles down on the crossmember and runs in three or four different directions, making it impossible to catch all of the oil with a single catch pan. The end result is usually a royal mess on my garage floor and oil all the way up my arm. I curse the idiot who designed that filter location every time I change the oil.
Ford isn't the only vehicle manufacturer with filter accessibility issues. My daughter's Saturn isn’t any better, and there are dozens and dozens of other examples on most brands of vehicles where the filter is in a hard-to-reach location or dribbles oil all over other components when it is removed. The truth is service accessibility is usually a secondary consideration to cost.

One way to make filters easier to change is to not overtighten the filter when it is installed. The gasket on a spin-on filter should be lightly lubed with clean oil prior to installation, then hand tightened only 3/4 turn after the gasket makes initial contact with the filter seat.

HOW OFTEN SHOULD I CHANGE MY AIR FILTER?
Don't wait until the engine is gasping for air. Change the filter before it becomes overly restrictive. A dirty air filter will hurt fuel economy, performance and emissions.

Replacement intervals for air cleaners depend entirely on the driving conditions and miles driven. The factory suggested replacement intervals for most vehicles today range from 30,000 to 60,000 miles. But a vehicle that is driven mostly on rural gravel roads might need a filter change every 5,000 to 10,000 miles. The best advice is to inspect the filter at every oil change and replace it as needed.

The same advice goes for cabin air filters. Many late model vehicles have an air filter that cleans air entering the passenger compartment. The filter may be located in the cowl area at the base of the windshield, or in the HVAC unit behind the glove box. Many motorists don’t realize their vehicle even has such a filter, let alone how often it should be changed.

Cabin air filters with an active layer of charcoal to absorb odors have a service life of about one year, so an annual change is recommended. Ordinary dust-only cabin air filters typically have a recommended replacement interval of every two to three years — but this depends on operating conditions. Checking the filter once a year is a good idea because if the filter becomes plugged it can restrict airflow through the air conditioning system, heater and defrosters.

HOW DO I KNOW WHEN THE AIR FILTER NEEDS TO BE CHANGED?
The most accurate means of determining an air filter's true condition is to measure the drop in air pressure across the filter. Some diesel-powered trucks and off-road vehicles have a restriction gauge that warns the driver when the filter needs to be changed. But visual inspection is still the most common method for determining the filter's condition. Removing the filter and holding it up to the light will usually reveal whether it's clean or full of dirt.

WHY CAN'T I JUST CLEAN AND REUSE MY OLD AIR FILTER?
The only air filters that are reusable are oil bath filters (from cars back in the 1950s, or off-road vehicles), or those that have an aftermarket washable foam, cotton or screen filter. Ordinary pleated paper filters can be damaged by water or solvents. Filter life can be extended a bit by blowing compressed air backwards through the filter. This will dislodge the big pieces of dirt, but not the microscopic particles that are embedded in the filter's pores. If the filter is visibly dirty, replace it. Don't waste your time trying to blow it out.

HOW OFTEN SHOULD I CHANGE MY FUEL FILTER?
The fuel filter should be changed often enough to prevent fuel line plugging. The life of the fuel filter depends on the cleanliness of the fuel, the condition of the fuel system and the number of miles driven. One tankful of dirty gas can plug a brand new filter as can a fuel tank that has rust and corrosion inside it, so specific mileage recommendations are not mentioned in many owners manuals.

Some vehicles even have a "lifetime" fuel filter located inside the fuel tank on the fuel pump assembly. But that doesn't mean the filter will never plug for the life of the vehicle. It may go 100,000 miles, or it may not. It all depends on the cleanliness of the fuel that goes into the tank and the condition of the tank itself. Plastic tanks don't rust but they can still be contaminated by dirt (or microbes in the case of diesel fuel).

For preventive maintenance, many technicians recommend replacing the filter every two or three years. The filter should also be replaced if the fuel pump has failed and is being replaced.

DOES MY TRANSMISSION HAVE A FILTER?
Automatic transmissions have a filter or filter screen located inside the pan that covers the transmission valve body. It is seldom changed because most motorists don’t have their transmission fluid changed. Today's long life transmission fluids can supposedly go upwards of 100,000 miles without maintenance, but it all depends on operating conditions and heat.
Transmission experts say the filter should be replaced when the transmission fluid is changed. They say it should be done every 50,000 miles for preventive maintenance, or every 24,000 miles if the vehicle is used for towing.













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