There's something mystical about driving through water--we know it's not the greatest idea, but it seems so darn fun, we often can't resist the lure. Unfortunately, where we want to go, the vehicle doesn't always want to follow. That's because it usually has some pretty good reasons for not wanting to dive too deep.
Four-wheel-drives are, for the most part, not intended to be driven through very deep water as delivered, so it's up to us to modify accordingly. Luckily, that's not very complicated as long as the fording depths are kept reasonable.
Our advice up front is simple--stay out of the water or keep reading.
AQUATICALLY CHALLENGED PARTS AND WHY
When it comes to water resistance, like most everything else on a vehicle, the weakest link will stop it sooner or later. With the engine, computer, and automatic transmissions, we need to make an extra effort to keep water out of them. Water in axles, manual transmissions, and transfer cases doesn't usually spell instant disaster, but that doesn't mean we should let contaminated fluids in those components destroy expensive parts just because it happens later.
The best way to waterproof a vehicle methodically would be from the bottom up, so to speak, until we reach the point where it's good enough for what we want to do. In real life, it's usually a stalled engine that makes thoughts of waterproofing surface. At this stage, however, it's too late to worry about how much water is finding its way into places it shouldn't be, and we should instead concentrate on beaching the beast. It's best to use the (electric) winch or have a friend pull you up on dry land, because if the motor takes in water, it can hydrolock when you try to restart it, which can be really nasty (expensive). Since water doesn't compress like air, even small amounts in the combustion chamber can tweak connecting rods and worse. We've seen a starter fall to the ground with a piece of the block still attached after an attempt to key-start a pickup that stalled after being driven through about a foot of water. Lesson to be learned: Take the time to pull the plugs, then turn the engine over to squirt out any water that may have come in through the intake--or exhaust if the water was really deep. If there was none, great. All you lost was time, which needed to be spent anyway to allow any water in the axles, tranny, transfer case, or oil pan to settle to the bottom. If the vehicle wasn't driven and the water consequently didn't get mixed into an emulsion with the fluids, it's now only a matter of letting it out through the drain plugs.
Just the water--not the oils.
After this little excursion, you're probably more motivated to do some actual waterproofing and figure out what made the motor stall in the first place.
RUN, BABY, RUN
While it's generally true that cats dislike water, diesels are better suited for deep-water use than are gasoline engines, because there is no distributor or other ignition components on a diesel to get wet. Yup, most likely it's the ignition that shuts down first, often helped by a mechanical fan that sprays water over the entire engine once the tips of the blades reach water level.
Flex fans can be the worst, because they also have a tendency to play propeller then flex enough to cream the radiator (making for a truly water-cooled engine). Using an electric fan and turning it off when diving in may be the best choice. Equally effective but more labor-intensive is to disable a mechanical fan temporarily by removing the belt. Of course, on some late-model four-bys, it may be easier to remove the fan than the belt.
Anything marginal in the ignition department will conk out when wet, so the first step is to not have any marginal ignition parts. Invest in good, sealed plug wires (such as Jacobs) and run them to a good, clean distributor cap atop a good rotor. Points aren't that great in water, so consider upgrading to an Ignitor if you're not electronic already. Also, ACCEL, Jacobs, MSD, and others have complete ignitions that are both upgrades and waterproof.
Should you seal the distributor cap? Yes and no. Distributor caps should be vented to some extent, so either use a temporary seal (a wide rubber band often works) or seal it up for real and drill a small hole for a vent hose in the housing. Either way, if the distributor cap gets wet,WD-40 and similar sprays work well for displacing the moisture and enabling a restart.
With the ignition capable of dealing with water, the engine should run, possibly even when submerged--that is if it can still breathe. Some modern vehicles have intake ducting that picks up the air from embarrassingly low points. If your vehicle takes in air from a level within a foot of the deepest crossing you'd ever consider, either temporarily disconnect or permanently modify the air intake accordingly. Since an air pocket tends to form at the firewall, a stock-type air cleaner with the intake end pointing toward the rear is a pretty good bet for water not deeper than the headlights. Better yet, run ducting through the firewall so the motor sucks air from inside the vehicle, using a K&N plenum or a homemade setup. The next step would be a snorkel, but if you need one of those, you probably didn't need to read this story in the first place and are already a waterproofing master.
On a late-model four-by, there's one more important thing to keep dry--the almighty computer. If the computer gets wet, you're up the creek, and some are even mounted as low as under the seat. Venting requirements permitting, sealing it up in a box can work, as can relocating it to higher ground. If you drown the computer, let it dry out naturally before trying to start the engine (it's a good excuse to camp out an extra night, if nothing else). About the only thing that can safely be done to clean it is to spray a product such as Permatex Electronic Parts Cleaner on the circuit board and connectors, so you may want to add a can to the toolbox.
This is where waterproofing should begin, down low, because that's where the water will be, at least initially. Things to keep dry down below are primarily axles, the transmission, and the transfer case. All four are pretty well sealed or fluids would leak out, but they are also vented. Many late-model vehicles have vent hoses already, but they are not necessarily long enough. And some axles have only a hole in an axle tube, while still others have at least a vented cap of some sort. Automatics often have a very short vent pipe on the housing. Regardless, for submerging the drivetrain, it must be fitted with vent hoses that reach above the waterline. Some manuals are vented through the transfer case--and most have leakage potential at the tower, where the shifter pivots.
Using water-resistant grease,preferably synthetic, can really help keep water from entering wheel bearings, hubs, and the like. Grease can also be used to build water barriers in the wheel hubs to protect the bearings plus wherever grease is used and water can enter. Additionally, just plain lubing things according to the maintenance schedule does a lot for keeping water out.
DON'T WORRY, BE HAPPY
Certain things just can't be effectively waterproofed, including alternators, starters, brakes, and so forth. On the other hand, they tend to work under water too. The best approach to those things that will take in water no matter what is to make sure the water can also get out. Enlarge or make drain holes as needed. This goes for a lot of things including the interior and toolbox.
You'll be especially grateful for drain holes if the dunking was in muddy water, because they make cleanup with a hose that much easier. The sooner you can hose out the mud, the happier the alternator, starter, brakes, and so forth will be, and it's also much easier to remove wet mud than fighting it in dried form.
Disc brakes, by the way, are far better suited for aquatic adventures, because there's really nowhere for water and muck to get trapped, unlike drum brakes, which can wear out in less than 100 miles once packed with mud.
Don't forget also to squirt the radiator, which is a virtual mud magnet, after a bath in the goo.
SILICONE IS OUR FRIEND
How did we ever do without silicone sealants? OK, that's irrelevant now--water under the bridge--because with silicone, we can waterproof darn near anything. However, before slathering parts and pieces with silicone, remember that you may want to take some of them apart again later and silicone sealants are also a pretty good glue. A distributor cap, for example, shouldn't be permanently glued in place. By lightly oiling one of the surfaces (do the cap rather than the housing), the silicone doesn't stick to that part. It'll still seal, but it won't make the cap stick to the distributor housing.
Speaking of sealed, some things come that way. An Optima battery, for example, doesn't get offended when drowned, because it's practically sealed. A regular battery and even some of the maintenance-free kind can take in water when submerged. Many other things such as lights and gauges come both ways, so pick your accessories and replacement parts accordingly.
Some things that just can't (or shouldn't) be effectively sealed can instead be pressurized during water excursions. Don't go overboard with pressure or volume, however, or you may end up pumping the fluids you're trying to protect out of their home. And do make sure to keep the intake for the pressurization system above water.
DO WHAT YOU HAVE TO
Realistically, any four-by should have vent hoses for the drivetrain that extend to the top-of-the-firewall level, and they're a next-to-no-cost upgrade.
If you plan to go through water that gets up to floor level, using synthetic grease is highly recommended. If trips that can put the fan in water are on the agenda, it's time to go over the ignition and consider the fan situation itself--if the air intake is still above water at this stage. Headlight-deep is about as far as most of us go voluntarily, and that's also as far as we'll go in this story. Anything beyond that tends to get really involved and is mostly only enjoyed by the 'wheelers in Florida. When the vehicle and driver need a snorkel, pretty drastic measures are in order to keep water from ruining your vehicle (but do check out Skin Diver magazine for your personal needs).
Realistically and against all conventional wisdom, as long as there is water on the trail, somebody will go to great depths to put a four-by in it, thereby ensuring an endless supply of campfire stories and Rear View photos for Four Wheeler. We like it that way.