If you take the plunger out of one of today's fuel injectors and hold it in your hand, the warmth of your hand will make the metal expand so you can't get the plunger back inside the barrel.

That's according to Paul Bandoly, manager of technical service and customer training at Wix Filters, as he explains just how tight the tolerances are in today's high-pressure fuel injection systems.

Those tolerances aren't going to get any less stringent under new federal fuel-economy/greenhouse gas regulations. That means the cleanliness of the fuel going through those tiny openings is more important than ever.

"The more filtration you can get, the better off you are," says Darry Stuart, a past chairman of the ATA's Technology & Maintenance Council who works with fleets on maintenance issues through his company, DWS Fleet Management. "You start screwing up your injectors, you're talking $5,000 to $10,000 a truck."

With that in mind, following are six ways to keep fuel clean and dry.

1. Buy a quality product

"I'm not saying the brand names are the best, but I've seen city fleets buy aftermarket filters that were so cheap, when they had issues, they'd find out they had collapsed," says Mike Dobbs, operations and service manager at the Waco, Texas, branch of WheelTime Network member Stewart & Stevenson.

You can't tell if a filter's the right one simply by appearance. Two filters could use the same can and the same gasket; you could cut it apart and the media looks the same to the naked eye. But the structure of the media and its capabilities can be vastly different. You don't want to use a filter designed to trap 10- to 15-micron particles on an engine that needs control down to 2 or 4 microns.

That's why you need to use a product recommended for the application, and check regularly with your filter maker and engine supplier for any changes.

Marty Barris, director of product management for liquid filtration at Donaldson, says one thing to ask a potential filter supplier is whether filters have been tested in real-world conditions, not just in labs. Engine vibration, the pulsation of the fuel-injection system, the surges and pressure changes from stopping and starting - all can affect filter performance and longevity.

"In many cases, buying a quality filter is a relatively small investment compared to the one they've made in that expensive engine or truck," Barris says.

2. Watch out for water

Filter makers and maintenance experts say with the advent of ultra-low-sulfur diesel, the industry has seen more problems with water in fuel.

Water, of course, promotes corrosion - and it increases wear on injectors and other fuel system components because it does not have good lubricity.

"We see it as an increasing challenge to effectively separate water" from diesel fuel, says Donaldson's Barris. "We know that with the advent of ULSD, because of the changes in refining and the need to add lubricity additives, which are essentially surfactants, it has become more difficult to separate water than the historical method."

Wix's Bandoly says water is attracted to some of the additives refiners are using to restore the lubricity that was removed along with the sulfur in the fuel.

Not everyone agrees that ULSD is an automatic water-magnet. The issue has also come up in conjunction with reports of corrosion problems in fuel tanks, both on and off the vehicle, especially steel.

Lorri Grainawi, director of technical services for the Steel Tank Institute, says the manufacturing process used to make ULSD results in diesel fuel that actually holds less water in suspension than older higher-sulfur fuels.

"At the present time, there are quite a few theories on why these corrosion issues are happening, but there is not enough data to truly determine the cause," Grainawi says.

An alliance of companies and trade associations called the Clean Diesel Fuel Alliance is looking into it. The alliance includes the Steel Tank Institute as well as groups such as the American Petroleum Institute, the American Trucking Associations, the Diesel Technology Forum, the Truck & Engine Manufacturers Association, the U.S. Environmental Protection Agency and the U.S. DOT.

The research has been ongoing for almost two years; the group hopes to have more conclusive results by the end of the summer.

Whatever the causes, it's a good idea to do whatever you can to keep water out of the fuel. Consider spec'ing a fuel-water separator. Water that does separate in the fuel tank can be drained out via a plug in the bottom. Stuart, who says such plugs are becoming less common, advises his customers to use a special vacuum/filter system to pull the watery fuel off the bottom.

3. Change filters when needed

The signal that there are too many contaminants in the fuel is often a plugged fuel filter.

"To be honest, I'm quite happy to have that happen," says Stuart. Why? Because it means the filter's doing its job and not letting those contaminants into the combustion system where they can do damage.

Some truck owners want to change the micron ratings of the filters they're using so they don't have to be changed as often, he says. That may save you some filter changes, but it's going to cost you in shorter engine life in the long run.

As fleets extend oil drain intervals, they may find those intervals become longer than the fuel filter is designed to last.

Bruce Stockton, formerly head of maintenance at Con-way Truckload and now a fleet maintenance consultant at Stockton Solutions, tells about his experience:

"Before leaving Con-way/CFI, we finally spec'ed Davco secondary fuel/water separators that were also heated to bridge the gap between PM services of 40,000 miles and avoid pulling trucks in for just fuel filters," he says. "We also started a program of carrying extra filters on each truck and training drivers how to change the fuel filters."

It worked, he says, and he continues to recommend the program to his fleet clients.

On the other hand, there's some indication that situation could change. The Detroit DD15 on the new Cascadia Evolution boasts an improved fuel filter module with two filters - and Freightliner says they have a 100,000-mile filter change interval.

Some filter makers offer filter restriction indicators, which measure the pressure drop across the filter and let users know if it's reaching a predetermined level.

"My experience is a lot of smaller fleets or individual users like vacuum or restriction gauges," says Dan Stibel, fuel filtration product manager at Racor. "With the increase in the cost of fuel systems and stuff, we're seeing some increase in systems that actually monitor and notify the customer. They all work very similar to an air filter gauge. It's just a matter of whether you actually have to look while it's running or if you have some telltale in the vehicle."

Others, however, are skeptical of the benefits of pressure gauges for fuel filters.

4. Don't pre-fill

Unless the factory-recommended service procedure tells you to, never prefill a fuel filter. All that does is give your engine a shot of unfiltered fuel when you start it up.

"It may be a lot easier to start the truck if you prefill the fuel filter, but you're not dealing with 1970s injectors," Bandoly says.

One exception, he says, might be filters that come with a special funnel that only allows the fuel to go into the intake side of the element to prevent this from happening.

5. Beware of biodiesel

Unlike the mystery over water in ULSD, it's well-known that biodiesel holds a lot more dissolved water.

Donaldson's Barris says straight biodiesel can hold several thousand parts per million of dissolved water before it reaches the saturation point - the point at which the water separates from the