Off the Road with Francois Rossouw
Beware the deep water
“Deep water” is a swearword when you’re talking 4×4 driving. Yet we regularly get brave people who take the plunge, and then they lament any resulting engine damage. And no, the water doesn’t enter through the exhaust pipe…
An interesting query from a reader, Craig Vetner, arrived on my desk, along with a newspaper clipping where a so-called expert advised readers to “keep the revs high” and “not change gears while driving through, or water may be sucked through the exhaust and into the cylinders”. Can this happen, he asks?
While it’s true that water tries its very best to get into every nook and cranny of the vehicle, it does not enter the engine through the exhaust pipe.
The design of an engine is such that the valve and camshaft timing is programmed to suck air into the engine through the air intake and force the burnt gasses out through the exhaust.
The compression ratio of a petrol engine is in the vicinity of ten to one. That is ten times more than atmospheric pressure. That means that the air is also forced out of the exhaust valves by the piston at a rate of ten to one.
Even if a vehicle is standing in deep water and water enters the exhaust, it will be forced out by the exhaust gases when the engine is started.
If there were any such danger, much more care would have been taken by manufacturers in exhaust design. Also, motorboat engines have underwater exhaust pipes to contain engine noise, as do ships, without any risk.
So, don’t worry about the exhaust – rather watch the air intake. This is where water gets sucked into the engine.
Water cannot compress like air and causes a hydraulic lock when it gets into the cylinder, which the piston then encounters during its upward stroke. The momentum of the crank then bends the connecting rod, sometimes even breaking it.
In petrol engines, I have seen blown cylinder head gaskets and broken con rods. In the case of diesel engines the damage is much greater, as a result of the much higher compression ratio.
So take care when you have to drive through deep water. The first danger is the cooling fan catching the water. As water is denser than air, it forces the blades back, towards the radiator. I have seen many radiators damaged by the fan in deep water.
It’s therefore best to keep the engine revs as low as possible in order to put as little strain as possible on the fan. Most vehicles these days have viscosity type fans that slow down when they encounter water, but are still dangerous at high revs.
Water poses more danger than merely to the engine, though. When the differentials, wheel bearings and universal joints are hot from driving and are submerged into water, they cool down quickly, as does the air inside. It then forms a vacuum, and water is sucked in. Oil seals are designed to keep oil in – not to keep water out.
The water mixes with the oil, decreasing its lubrication power and causing wear to the moving parts. Not only that, but when your 4×4 is back in your garage, waiting for the next trip, the water and oil separates, water sinks to the bottom, and then the real trouble starts.
The oxygen in the water causes the parts to pit and rust. Bearings are particularly vulnerable. When you drive the vehicle again after it’s been standing for a while, you’ll first notice a slight noise that gradually becomes louder, until it sounds really expensive.
This could ruin your budget and often your marriage, because it normally happens on the way to your family’s long-awaited holiday.
So try to stay out of deep water, or when you have no alternative, replace the oils as soon as possible afterwards and repack the wheel bearings.
And don’t forget the boat trailer. As they’re submerged often during loading and off-loading the boat, they’re prone to wheel bearing trouble.
When the wheel bearing packs up, the wheel can fall off and keep running for over two kilometres. The axle drags on the road, and is often damaged beyond repair. Repack those bearings as soon as you get home.