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How a Cold Air Intake Bypass Valve Works to Prevent Engine Hydrolock

Here’s a cold air intake kit on a Nissan Sentra. The actual air filter is moved down behind the bumper cover in front of the wheel, where it can get clean, cold air. The bypass valve is the foam donut looking thing circled in red. (Image/OnAllCylinders – Paul Sakalas)

A cold air intake kit (CAI) is the gateway mod for many first-time gearheads. And it’s easy to see why—they’re relatively cheap, easy to install, and offer some serious bang-for-buck performance.

Yet there’s some stuff you should know before bolting one on.

Before we get to that though, let’s quickly explain how CAIs work: A cold air intake draws colder air from outside the engine bay into the combustion chamber. The cold, dense air improves combustion. Better combustion equals better performance. (That was an overly-simplified explanation of course; read our Air Intake 101 article for an in-depth look.)

OK, so here’s the catch. In order to find that colder air, many cold air intakes run tubing outside of the engine bay. This may put the air filter element lower to the ground, where there’s a greater chance it could suck-in water from, say, a really deep puddle, ditch, or pothole.

It’s important to realize that not all air intake kits are cold air intake kits. You can still get significant performance improvements by installing an intake kit that lives entirely in your engine bay, mitigating some of the risk of hydrolock outlined below.

Understanding Hydrolock

Sucking a lot of water into an engine is a big, big problem. That’s because water doesn’t compress as easily as air, so when your pistons try to squeeze water into a tight combustion chamber it’s likely to severely damage your valves and connecting rods. That’s called hydrolock. And it’s killed many innocent engines.

Here’s what happens to a connecting rod when an engine hydrolocks—they’re not normally supposed to be so squiggly. (Image/MichaelXXLF, Creative Commons)

Note that we’re talking about a lot of water here, not ordinary ambient rain or even a heavy splash or two. For catastrophic damage like this, the filter would have to be at least partially dunked.

It’s also worth pointing out that hydrolock isn’t an exclusive problem to cold air intake kits either. A stock vehicle airbox can suck in water just as easily as a CAI if it’s submerged. The difference is that OE engineers usually place the stock airbox much higher in the vehicle, so you’ll likely be waist-deep in water before the engine hydrolocks—meaning you’ve probably got a host of bigger problems to deal with at that point.

Here’s a closer look at that earlier Nissan’s air intake tube routing, as it passes through some sheetmetal down behind the bumper. While the filter cone is near the road, the bypass valve is much higher at the top of the engine, meaning any water would have to be alarmingly deep already before the bypass was in any peril. (Image/OnAllCylinders – Paul Sakalas)

How a Cold Air Intake Bypass Valve Works

Thankfully, there’s a really simple solution to the problem. It’s called an Air Intake Bypass Valve. It’s essentially a smaller filter placed upstream of the main filter. If the main filter becomes submerged in water, then air is drawn through the bypass valve instead. But here’s an easier explanation:

Imagine sucking air through a straw.

Start by putting a small hole in the middle of the straw. When you suck air through the top of the straw, it’ll be drawn from the bottom like normal. But when you put the straw into a drink, submerging the bottom, you’ll immediately begin sucking air out of the little hole you added in the middle. Make sense?

That, in a nutshell, is how an air bypass valve works.

Super simple. Super-er important.

Air intake bypass valves are really basic, consisting of a foam air filter element in a plastic frame, attached by rubber couplings and hose clamps. (Image/Summit Racing)

Where to Buy Cold Air Intake Bypass Valves

Many aftermarket cold air intake kits include the bypass valve already. But they’re available separately if you’re building your own setup (or replacing one that’s worn-out). In fact, AEM Induction has several air bypass valve options for a range of tubing sizes.

These bypass valves tend to last a few years, but the continuous underhood heat cycles mean that the plastic supporting the bypass filter can become brittle and the filter element material will gradually decay, so it’s worth inspecting yours every couple of months.

In short, consider cold air intake bypass valves cheap insurance against catastrophic engine hydrolock damage—especially if you live in an area that gets a lot of rain or is prone to flooding. And considering how easy they are to install, it’s a smart move to retrofit a bypass valve if your intake’s air filter lives low in the vehicle (and isn’t already equipped with one).

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One Comment

  1. Good article. Hydrolock also can occur with a regular air filter that sits on top of the carb. My hood-less ’34 coupe was once parked overnight at a hotel when it rained heavily. The water ran down the center bolt of the air cleaner and filled a couple of cylinders (it rained that hard). Unbeknownst to me, when I tried to crank it in the morning, it wouldn’t turn over. Fortunately, my buddy was with me and knew about hydrolock and stopped me from trying to start it again. We pulled all of the plugs and compressed water shot over the fenders onto the ground. we checked the oil and there was no water in the oil (new engine build), so we then spun it a few more times with the plugs out and purged the rest of the water from the cylinders/intake, dried the plugs and put them back in. It fired right up and all was good. I was VERY lucky my buddy was there, otherwise I would have had a set of S-shaped rods like the one in your picture. I now put a trash bag over the intake when it is parked at an event and it rains. Not classy, but cheap insurance against another hydrolock!

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