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MV Air Cleaner Service

Some military vehicle service and maintenance tasks are so mundane, boring, or dirty that we often tend to neglect them, or at least keep putting them off until we're in the right mood or the stars line up. Ironically, it is often the dirty, boring, and relatively simple maintenance duties that do the most in keeping our MVs on the road, in good mechanical condition, and ensure that our vehicles have a long and dependable lifespan.

Some military vehicle service and maintenance tasks are so mundane, boring, or dirty that we often tend to neglect them, or at least keep putting them off until we're in the right mood or the stars line up. Examples are lube jobs, checking and/or changing the gear oil in transmissions, transfer cases and axles, and draining, flushing, and renewing the engine coolant, as well as the brake fluid. Ironically, it is often the dirty, boring, and relatively simple maintenance duties that do the most in keeping our MVs on the road, in good mechanical condition, and ensure that our vehicles have a long and dependable lifespan. At the top of a lot of people's hate-to-do list is servicing their vehicle's air cleaner, whether oil-bath or dry-type.

Figure 1. Typical oil-bath air cleaner as used on Willys Hurricane F-head Jeep engines (CJ3B, CJ5, M606, M38A1, etc.). Complete service of these units usually requires their removal from the vehicle.


Back in the early days of Military Vehicles Magazine, I hosted a question and answer forum for mechanical MV matters. One of the points I tried to make--and still believe today--is that there aren't many "stupid questions" except the ones that don't get asked. (I slightly revised that belief after someone who was surely old enough to know better asked if his M151 "really needed a radiator?"!)

One of the letters I received back then was about an oil-bath air cleaner on a CJ3A, which, as most MVers know, is a fraternal twin of the M38. The question was: "Where does the extra oil come from to raise the level in the oil cup from 'Safe' (or 'Normal') to 'Danger' (or 'Service')?" (I know a few mechanics who would have roared with laughter at such a "stupid question.") The answer is that the oil level doesn't rise because extra oil mysteriously appears--"possibly sucked up from the oil pan," but rather because of dirt and dust particles that are trapped by the air cleaner doing its job and settle to the bottom of the oil cup.

However, this question was of special interest to me because my first Jeep was an M38, and, as a young and innocent MV owner, I recalled reading the words embossed on the air cleaner's oil reservoir: SAFE LEVEL and DANGER LEVEL. I had also wondered why the danger level was higher than the safe level. In other words, where did that "extra oil" come from?

Answering that question, and hopefully many others about air cleaners or air filters, is what this article is all about. You may even learn more than you ever wanted to know about these important engine accessories. In past articles I've mentioned various vehicular components that seem shrouded in mystery in regard to how they work or what their useful purpose might be. (Most M151s DO need radiators!) I've also written about components that seem so long-lived and trouble-free that their care and service is often overlooked or ignored until they finally wear out and fail. Wheel bearings and universal-joints are prime examples.

But, if you asked me what I thought was the most universally-neglected and least-serviced component of most vehicles, I would say the air cleaner. In fact, I would bet that the air-cleaners on about eighty percent of the world's vehicles need a complete and thorough servicing at this very moment, as well as some simple and inexpensive repairs, because they are dirty and/or leaking and not doing their jobs.


What is an air cleaner's job? It's protecting an engine, whether military, civilian, jeep, truck, farm tractor, minibike, bulldozer, or stationary, from unnecessary wear that leads to untimely wear and expensive repairs.

One of the major differences between universal joints and air cleaners is that when a universal joint fails it is often with a spectacular bang and leaves one stranded somewhere, whereas even total failure of an air cleaner--such as being so clogged with dirt that an engine won't run--is seldom a sudden occurrence. Basically, when an air-cleaner fails, an engine only wears out faster--a lot faster. This wear is accelerated even more if a vehicle is being used off-road in dusty or dirty environments.

Figure 2. Although one should always read their vehicle's manual to find the correct service instructions for any component, most air cleaners have tags or decals that specify basic service procedures and service intervals.


I was once doing a demolition job in Central California, operating an ancient Allis-Chalmers bulldozer with a 6-71 GM diesel engine. It was August, the dust was as fine as talcum powder and billowed around in a blinding blizzard as the bulldozer's tracks churned it up. I had to wear goggles and a bandit bandanna just to keep my eyes open and breathe. The engine was fitted with a pair of oil-bath air cleaners, and I had to service them at least three times a day. I could actually feel the gradual loss of power as they clogged up with dust. I could also see the engine begin to produce black smoke from too rich a fuel mix as it struggled to breathe.

Most people won't encounter a situation this extreme with their MVs, but an average day spent in dusty off-road conditions with your M37 or M151 will usually necessitate air cleaner service. At the very least, and even if you never take your vehicle off the pavement, a dirty air cleaner is wasting fuel and causing needless wear in your engine. How?

Since the engine is not getting enough air, there is an excess of fuel, which washes lubricating oil from cylinder walls and dilutes the oil in the crankcase. In addition, an air cleaner with a dented or damaged case that doesn't seal properly, a rusted case with pinholes, or loose, leaky, or deteriorated hoses, fittings or gaskets, is also causing unnecessary wear by letting the engine suck in particles of dust and grit. These not only muddy up your nice golden oil, but dust and grit mixed with oil makes a very efficient polishing compound that will quickly polish things inside your engine that shouldn't be polished any more than they already are.

There are countless sizes, shapes and styles of vehicle air cleaners. Some mount directly to an engine's carburetor or intake manifold, while others are located elsewhere, either within the engine compartment or out on a vehicle's fender or cowl. However, there are actually only two basic types of air cleaner: the oil-bath, and the dry or replaceable-element type. Some heavy equipment such as mine trucks or bulldozers may have air cleaners that are combinations of both types; and there are also pre-cleaners, which are commonly used on newer 2-1/2-ton and larger military trucks.


We will also check out Positive Crankcase Ventilation (PCV) systems and their components in this article because the care and service of these are often essential to the efficient operation of an air cleaner, and therefore to the life of an engine. Many crankcase breathers are miniature air cleaners all by themselves and require the same service and attention as the main unit.

In addition, most of the components of a deep-water fording system are either part of, or an extension of, a basic PCV system; and since most common M-series military vehicles, such as the M38, M38A1, M151, M37, M715, M35, etc., are fitted with these systems--whether or not a deep water fording-kit has been installed--one should be aware that their many small hoses, tubes, fittings, gaskets and connections provide many performance-robbing or potentially engine-damaging air or vacuum leaks.


There has been much research in the last thirty years in regard to the comparative efficiency of the two basic types of air cleaner: oil-bath, and dry-type. Generally-speaking, the dry-type is the most efficient at filtering out dust and grit...but only if properly serviced and maintained. The main problem with dry-type air cleaners is that the owners of vehicles so-equipped often try to save a few bucks, either by not replacing the filter elements when they need replacement, or by improperly trying to clean, and sometimes damaging them, so they leak and let dirty air into the engine. Personally, I prefer oil-bath air cleaners, since the only expense is a pint or two of new oil, plus the time it takes to service the unit.

While it would not be a stupid question if one were to ask how often they should service their vehicle's air-cleaner, I have yet to find a manual that doesn't specify normal service intervals, as well as something to the effect of "SERVICE MORE OFTEN, OR DAILY, UNDER SEVERE OPERATING CONDITIONS." Indeed, such instructions are usually on a tag or decal on the air cleaner unit. However, it is your engine, so use your own common sense, taking into account how frequently you drive your vehicle and under what conditions it operates, to judge when should inspect and service its air cleaner. At the very least, get into the habit of checking your air cleaner half as often as you check your vehicle's water and oil, especially if you live in a dusty environment.

For most common MV's with oil-bath units, checking the air cleaner is usually as simple as unscrewing a wing-nut or loosening a clamp. We will get to the service of a few different models a bit later, but for a simple inspection all it generally takes is a peep into the oil cup or reservoir to make sure the level is at the safe or normal line and that the oil is still fairly clean and not black or muddy. If you want to be daring you can also dip a finger into the oil cup to feel if there's a layer of crud in the bottom.

It's an even simpler process to check the dry-types--just take out the element to see if it looks dirty. (It was probably white when new.) Tap it gently to see if dust falls out. An old shade-tree test was to hold the element up to the sun to see if light came through: if it did, then supposedly the element was still okay. 

Figure 3. Refilling an oil-bath air cleaner's oil reservoir (CCKW). It is vital to the operation an oil-bath air cleaner that the oil be maintained at the proper level, as well as kept clean. Note the the "wire wool" element in the upper section of the unit, as well as the sealing gasket on the unit's cover.


On the other hand, sometimes a clean oil reservoir or a spotless dry-type element can indicate that an air cleaner isn't working properly, or that there is a major leak somewhere in the air intake system. For example, if you drive a lot where it's dusty, and yet after repeated inspections you keep finding that the oil in the cup is clean and there's never any crud in the bottom, or that your dry-type element never seems to get dirty, it's time to suspect an air leak.


It might be helpful to understand how the two basic types of air cleaners work. The smaller, light-duty dry-types are the simplest: the element strains all the air before it enters the engine just like a coffee filter strains the grounds out of coffee before it enters your cup. The element catches and traps dust and grit particles down to a certain specified size, which is usually stated in microns. Naturally, the smaller the particles the element can trap, the more efficient it is in protecting an engine from wear. The downside is, since engines require large amounts of air to run, a small air cleaner can't filter as finely and still provide enough air. In other words, when it comes to air cleaners, bigger is usually better.

Up until around the late 1920's, many cars and trucks either didn't have air cleaners--though there were various aftermarket accessories such as "Air Maze"--or they had some sort of wire mesh screen that kept rocks and small animals out of the engine. While this was pretty rough on the engines, especially considering that most roads weren't paved in those days, the engines themselves were loose and primitive in regard to tolerances and materials and were not very long-lived anyhow. Those who speak with fondness of "the good ol' days when things were built to last" seem to forget that most engines wore out long before reaching 100,000 miles and often needed ring and valve jobs, as well as rod and main bearing replacement, at 20,000 mile intervals. Early aircraft engines often lasted less than a hundred hours.

As the filter element clogs up with trapped dirt particles, less and less air gets through to the engine. Fuel mileage begins to suffer, and lubricating oil is washed off cylinder walls by an excess of fuel. Often, you can save a few bucks and extend replacement intervals by gently tapping the accumulated dust out of a dry type filter, or by gently blowing it away from the inside out with low pressure air. Be careful not to bend or distort the element or it won't seal in the air cleaner's case. And don't use so much air pressure that you blast holes in the element's filter material. Whatever size hole you accidentally punch or blow through the element's paper or cloth will let the same sized particles of dirt get sucked into your engine.

Most light-duty dry-type filter elements should never be washed. And, unless it is recommended by the manufacturer, never try to make a dry-type element "more efficient" by oiling it. While there are some dry-type aftermarket air cleaners--usually made for off-road use--that do require oiling, they are comparatively rare. Oiling any other type will only reduce the air flow, just as if the element was dirty.

Of course, the manufacturers of dry-type filters want you buy a new element every time it gets dirty, so the elements are not made to be cleaned, nor will endure many cleanings. They are also not made to last a long time, and the rubber or vinyl will shrink so the element won't seal inside the air cleaner's case. An aged element that stays clean after repeated inspections over many months has probably shrunk--assuming there is no leak in the air intake system--though I suppose you could resort to padding it with weather-stripping if you can't find a replacement right away, or are just plain cheap.

While there are companies that specialize in washing filter elements for larger, heavy-duty air cleaners, I have found that such reconditioned elements never seem to be quite as good as new ones. And, when trying to clean and recycle your own element, one reaches a point of diminishing-returns where a few dollars saved now may end up costing a lot more on engine repairs in the future. Be aware that even though an air cleaner unit may be be in perfect condition and fully functional, it can't do its job if there are air leaks somewhere else, such as loose mountings, clamps, fittings, or deteriorated hoses and gaskets. 

Figure 4. Heavy-duty oil-bath air cleaner as used on many large vehicles with diesel engines. The "hood" may also contain a pre-cleaner. All such units should be checked regularly to be sure the oil cup is tightly sealed and there is no damage or distortion to the unit's case.


The simplest dry-type air cleaners are the ones that mount directly to the top of an engine's carburetor or intake manifold. Assuming the air cleaner's case isn't be nt, damaged, or rusted full of holes, and the filter element is in good condition and sealing properly (and is also the correct element for that unit), the most likely place for an air leak would be the gasket where the filter case meets the carburetor or manifold. A shriveled or missing gasket will let in an amazing amount of dirty air into an engine, especially if the filter element itself is dirty. Like some people, air will take the path of least resistance, and would much rather enter the engine through a leak than go to all the bother of being filtered first. It's a simple job to make a new gasket from gasket paper; and smearing the gasket with wheel bearing grease will provide a better seal. A lot of dirty air can even get in through the hole on top of a filter case--usually where the wing-nut goes to hold the top of the filter in place --but a rubber washer will solve that problem.

If your air cleaner's case has any extra fittings or hoses--such as for a PCV system--you should make sure they are firmly fastened and tightly sealed. Such fittings are one of the few places on an engine where the use of silicone gasket sealer is a good idea. You should also check to be sure the hoses are in good shape and securely attached at both ends, preferably with clamps.

Oil-bath air cleaners work on a different principle from most light-duty dry-types. While oil-bath air cleaners usually have an element of steel wool, or what may look like Excelsior or seat-cushion stuffing, this element is often not removable from the filter case. This material is actually the second-stage, or final scrubbing, of the air cleaning process in an oil-bath unit. Most of the actual air cleaning in an oil-bath unit takes place when the air flow momentarily slows and reverses direction above the oil in the reservoir or cup. Air is sucked in and down from the top of the filter case. It is then drawn horizontally across the surface of the oil, where it slows down a little. As it slows, most of the larger and heavier particles of dirt and grit fall into the oil. Then the air is pulled up toward the wire wool (or "stuffing") and more dirt particles fall back into the oil. Finally, the smallest and lightest particles are caught and trapped in the oily wool (or "stuffing") before they can get into the engine. This is why it is vital to thoroughly wash the element whenever one services an oil-bath air cleaner. Many people are not aware that simply cleaning the oil reservoir and adding new oil is only doing half the job when servicing such units. And, because it's a messy and often time-consuming job, many people put it off way too long.

Naturally, as dirt particles sink to the bottom of the oil cup, the oil level rises, which sometimes mystifies people, as mentioned at the beginning of this article, or confuses them into thinking that more oil is somehow being pulled in from the engine. From my own experience on everything from M38 jeeps to Euclid mine trucks, I would say that if you wait for the oil to reach the danger or service line in the reservoir then you've waited too long to service your air cleaner.

Oil is relatively cheap--even at today's grossly inflated prices--and you don't have to use the same multigrade or expensive brand in the air cleaner as you may be using in your engine. Read your manual, of course, but in most cases, except for severe cold, the cheapest 30-weight will work just as well in your oil-bath air cleaner as five-dollar-a-quart synthetic. Remember that the purpose of oil in an oil-bath air cleaner is only to trap dirt particles, not to lubricate anything. Again, read your manual. It should specify what weight of oil to use at different temperatures. While most manuals do advise using the same oil in the air cleaner as in the engine, this is generally for the sake of simplicity and convenience.

The basic service procedure for most oil-bath air cleaners is to wash out the cup or reservoir with solvent, and rinse out the wire wool (or "stuffing") with the same type of solvent. For most smaller oil-bath units, such as those that mount atop the carburetor, this is a simple task; and the entire unit can be removed from the vehicle and washed. For larger units, or on some jeeps, the steel wool or stuffing can be removed for cleaning. On other units, including some jeeps, one may have to dismount the entire air cleaner from the firewall, fender or cowl to wash it. Again, since this is a time-consuming task, a lot of people don't do it... or don't do it often enough. However, there is no other way to properly and thoroughly service such air cleaners.

The steel wool or stuffing should either be allowed to dry completely before reassembly, or be gently blown dry with low pressure air. It should then be squirted with fresh oil, but don't overdo it, especially if you have a diesel engine, or the engine will suck in the oil and run rich--maybe race--for a few moments after startup. Likewise, if you have a diesel, and have used gasoline to wash the filter element, be very sure the element is dry before start up.

Lastly, the oil cup or reservoir should be filled with fresh oil up to the Safe or Normal line. Don't overfill it, especially on a diesel, or you'll encounter the same situation of an over-rich air mix as mentioned above. Even a gasoline engine may smoke and run rich for from sucking in excess oil.

As with carburetor-mounted dry-type cleaners, check and maybe replace the mounting gasket and/or wing-nut washer to prevent any possible air leaks, and examine, tighten or replace any attached hoses or fittings. On canister-type oil-bath units, check the seal (often a large O-ring) where the oil cup joins the main body. And, make sure the clamps or sealing ring hold the cup tightly in place, because an air leak here is a serious problem, messing up the air flow so the unit can't do its job.

Many large dry-type air cleaners use the same reverse-direction principle as the oil-bath types in addition to replaceable filter elements. In these units, the larger and heavier particles of dust and grit fall to the bottom of a dust cup or collector before the air is filtered by the element. Naturally, one has to check and clean out the cup or collector at frequent intervals. Keeping the cup clean is more important for a dry-type unit because there is no oil to hold the dirt.

Some heavy-duty dry-type units do have an oil reservoir. Service for these is common checks the dry element for dirt and condition, cleans or replaces it if necessary, then and cleans and refills the oil reservoir just like an oil-bath unit.

No matter what type or size of air cleaner your vehicle has, always be sure to check the case for damage, distortion, or rust holes, and replace all questionable gaskets and seals. The same goes for any attached hoses and fittings.

Some super-duty air cleaners have pre-cleaner devices. These are usually at the top of the case, and catch rocks and small animals before air enters the main unit. Pre-cleaners may operate on the reverse-direction principle. Again, always read your manual, but the servicing procedures for pre-cleaners are usually obvious, and it is important to clean them often.

On many diesel or multifuel-powered 2-1/2 ton and larger military trucks there is a service or restriction indicator. These devices are supposed to pop a red flag to warn when the filter element is dirty and needs to be replaced or cleaned. They work on vacuum--as the air filter gets dirty, the engine has to suck harder--but I have found most of these gadgets to be about as useful as a solar-powered flashlight. Many give false alarms; some don't work at all; and just as when waiting for the oil in your air-cleaner's cup to reach the danger or service level, if you wait for one of these things to go off you've waited too long anyhow.

At this point we have inspected, serviced and/or repaired our air-cleaner units, and hopefully fixed any leaks in their cases, fittings, hoses or tubes. Let's move on to inspect, service, and possibly repair the other components of our engine's air intake system. As mentioned earlier, some engines have a separate crankcase breather as shown in the illustration. This may be a mushroom-shaped cap--often where you pour in the engine oil--or it might be a can-like thing packed with "stuffing". You should thoroughly wash out the stuffing in solvent, squirt it with fresh oil, and reinstall the unit on the engine. Naturally, you'll also inspect any attached hoses, fittings or connections, and replace any questionable gaskets or seals. Even if your engine just has a simple tin cap to add oil, the cap should have a gasket or dirty air will be sucked into the engine. Always check the condition of this gasket, replace it or make a new one yourself if it is damaged or shriveled. Smearing it lightly with wheel bearing grease will provide a better seal. Some engines may have a miniature oil-bath or dry-type air cleaner for a breather; and the service and/or repair of these is the same as for the main unit. 

Figure 5. Extra heavy-duty dry-type air cleaner. Note the dust cup or "collector," the removable top cover to replace the element and the restriction indicator. While some heavy-duty filter elements can be washed or blown out with compressed air, one should use care not to damage or distort the element.


Before we go any further, it would be wise to check what should be an obvious place for an air leak, yet a place that is often overlooked or not considered when trying to track down such leaks. This is the engine's intake manifold, and the gaskets where it attaches to the engine block. Generally speaking, if your engine never seems to idle quite right no matter how many times you've tuned it up and/or replaced or rebuilt the carburetor, and/or stutters or stumbles during acceleration, you should suspect an air leak in or around the intake manifold. Check other obvious places for leaks, such as the carbeuretor-to-intake manifold gasket, and the carburetor itself for any shrunken or leaky gaskets, or even a missing plug. Don't forget the throttle valve butterfly shaft, because the bushings are often worn on vintage vehicles and will leak a lot of air. Many "professionally rebuilt carburetors" come out of their boxes with worn out butterfly bushings that were never replaced! Waterproof M-series carburetors usually have seals on the butterfly shaft, but these may begin to leak over time.

Of course, check all the vacuum lines, hoses and fittings that go to your windshield wipers, as well as the wiper motors themselves for leakage. Old vacuum wiper motors often leak at the switch valve, usually accompanied by a slight hissing sound. NOS wiper motors are in plentiful supply for most common collector MVs and are not too expensive, so your best fix would be to replace them. While you don't have to worry about lots of dirty air getting into your engine through tiny leaks such as these, your engine will probably never idle quite right.

Other related areas to check are the vacuum side of your double-acting fuel pump--if your vehicle is so-equipped--and its hoses, tubes and fittings; as well as the vacuum line to your distributor, if equipped with a vacuum-advance. Also check the vacuum-advance mechanism itself: the diaphragm ages, fatigues, and may begin to leak, which will also mess up the spark-timing. Try sucking on the vacuum advance hose or tube to see if the mechanism leaks or if it is actually working. Most distributors have a mechanical advance in addition to a vacuum advance, and you would not be the first person in the world to discover that you've been driving for years with no vacuum advance! 

Figure 6. Typical military vehicle air restriction indicator. One should not trust such devices. Some give false alarms, while others simply don't work. In most cases if one waits until one of the gadgets goes off to service their air cleaner, one has usually waited to long!


You should do a thorough visual examination of your engine's air intake manifold for cracks, or loose or missing fittings. Just as with a vacuum advance, you would not be the first person to discover that you've been driving with either a loose manifold (where it attaches to the engine) or even with a plug missing from the manifold and leaking air. It is a good idea to check both the air intake and exhaust manifold mounting bolts about every six months on a daily driver, and tighten them if necessary. The constant expansion and contraction from heating and cooling will eventually loosen the nuts, bolts or studs...even if they appear to be rusted solid.

If you have an M715, be aware that its intake manifold is fairly delicate and is easy to distort or crack by over-tightening the mounting bolts.

Intake and exhaust manifold-to-engine-block gaskets don't last forever; and while replacing them is not especially fun, it's neither a complicated nor expensive job. Use either a thick type gasket sealer or high-temperature silicone for intake and exhaust manifold gaskets.

If you have a G.M.C. 270 or 302 engine (CCKW, DUKW, M211, etc.) or a Chevy 235 engine (G-506) pay special attention to the locating rings that fit between the intake manifold and the engine block. These little rings are more important than they may seem. Over the years, after many owners, and after many engine rebuilds or repairs, these rings are often lost or discarded. They are also easy to bend or crush. Without them it is difficult to properly install the intake/exhaust manifold assembly so it doesn't leak. Furthermore, the assembly will often shift over time and begin to leak without these rings. You can make satisfactory replacements by cutting stainless steel hose clamps, but pay close attention to size if you have to fabricate these items. If you make them too wide you may crack the manifold when installing it. This advice also applies to civilian G.M.C. 228, 236 and 248 engines.

If you suspect a leaky intake manifold gasket, and you've already tried tightening the mounts, a simple way to check is to squirt engine oil around the gasket with the engine idling. If the idle picks up a bit or smoothes out, then the gasket is leaking air.

Lastly, we come to the components of the Positive Crankcase Ventilation System, which may be as simple as a PCV valve and a single short section of tubing (as on most WWII Jeeps) or a complicated maze of small tubes, hoses, connections, fittings and valves such as used on a waterproof M-series vehicle like an M37 or M35. For a WWII jeep, about all you need to do is occasionally check to make sure the connections are tight, and remove, disassemble and clean the PCV valve about once a year. For M-series vehicles with waterproof engines, you will just have to read your manual and painstakingly trace and inspect every little hose, tube, and connection all the way from the distributor clear down to the transfer case and fuel tank, and tighten or replace things as needed to seal up your air intake system.

Hopefully, we have covered just about everything you ever wanted to know about air cleaners. At the very least you should now know where that "extra oil" comes from.

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