Eyeballing Air Intake Gear
An underhood inspection of your air system should be on your maintenance list.
TOM BERG
EQUIPMENT EDITOR
Tilt the hood and the first thing staring you in the face is the engine's air intake system. Its large-diameter pipes and big filter canister dominate the space above the engine. In some trucks the air system limits access to the block, so it's a good thing that today's long-lasting diesels don't need much work. The air system itself needs periodic attention, however, because clean air is vital to a diesel's health.
Drivers and mechanics tilt the hood to inspect the various apparatus in the engine compartment. It's a good idea to add the air intake system to the list of things to inspect. It's easy enough: On the right side of most engines, eyeball the piping and all fittings, following them from the filter canister through the turbocharger, into the charge-air cooler. Look closely for any signs of damage to the cooler's fins and other parts. Then, moving to the left side of the truck, follow the plumbing from the cooler to the intake manifold.
If the engine is cool, so are the pipes and other parts. Grab them and pull gently to be sure they're secure. If you find something loose or damaged, beware. It probably means there's a leak that can allow dirty air to be sucked into the intake under certain circumstances. Tighten or replace it before restarting the engine, or as soon as possible after you find it.
Diesels with the new exhaust gas recirculation systems will have additional plumbing and valves to look at. You might start seeing these engines in brand new trucks in the next few months, so it's a good idea to familiarize yourself with the new systems. You shouldn't have to worry about them for a while, but you never know what can happen and it's better to be prepared than not.
Providing clean air is the job of the filter element, and the air restriction gauge (most of today's Class 8 trucks have them) tells you when to change the dry-type element. Oil-bath air cleaners do not build up restriction as the cleaner becomes dirty, so they should be serviced on a time or mileage basis. Usually this is when the engine's oil is changed or a half-inch of dirt has collected in the bottom of the oil cup.
Because there are several types of filters, there are several ways to change an element. The Technology & Maintenance Council of ATA outlines them all in its Recommended Practice 301A, which offers complete information on servicing air filters. Aside from the type-specific advice, RP 301A discusses basic points about servicing dry-type air filters.
For one thing, readings from the air restriction gauge — not visual inspections — should tell you when to change an element. Visual inspections of dry elements are misleading, because an element that appears covered with dirt may not be full (and indeed, dirt particles themselves form a barrier to incoming particles). And one that looks clean may be filled with fine sand that's plugging up the paper media.
Air restriction gauges in today's trucks are commonly installed on the intake systems of heavy and medium-duty engines. They're also available for light-duty diesels. The Filter Minder-brand gauge is the most common, but any gauge's accuracy depends on where it taps into the ducting. The RP notes that bends and curves in ducting increase restriction and affect readings. The best place for the gauge's tap is just upstream of the turbocharger (or, in a naturally aspirated engine, just upstream of the intake manifold). This way, the gauge reads total restriction in the system, with nothing downstream to add unrecorded restriction.
The gauge's face should be placed so mechanics can observe it while testing the engine, the RP says. Often, though, a remote readout is on the instrument panel where caring drivers and wise owner-operators can keep an eye on it. Wherever it is, the readout gives an accurate number when a turbocharged engine has been running at full RPM under load. With naturally aspirated or mechanically supercharged engines (admittedly rare things these days), an accurate reading appears at maximum RPM (high idle). The reading stays on the face of the gauge until the gauge is reset.
Engine builders publish limits for air restriction. Like the restriction gauge, the limits are expressed in inches of water. In a heavy duty diesel, the limit is usually 20 to 25 inches; check your builder's literature for the actual limit on your engine type. It is at this top limit that the element must be changed.
Because restriction causes loss of power and fuel economy, fastidious owners and maintenance people might want to change it sooner. Bear in mind that restriction builds slowly for the first 80% of the element's life, then quickly in the last 20%. If you feel the "proper" change time is at that 80% point, calculate what the gauge should read at that point. With a published 25-inch restriction limit, it would be when the gauge says 20 inches.
How much fuel economy is lost through air restriction? With a turbocharged diesel, from about 0.4% to 0.5% at 15 inches to as much as 1.5% at 25 inches, according to RP 301A's chart (which is based on average data from engine builders). Against this slight loss in fuel economy must be balanced the cost of buying and installing the element.
Remember, too, that anytime someone opens up the filter canister, he risks "dusting" the engine by spilling dirt into the intake. It's better to leave it buttoned up and read the gauge before changing the element.
