The Ultimate Guide to Diagnosing a Semi-Truck Fluid Leak To

If your rig isn’t working, neither are you. 

One of the surest signs of a problem on its way is a leak in your semi-truck. Those suspicious spots left behind aren’t just ugly and embarrassing; they’re your rig letting you know there’s a problem. The longer you wait to bring it into Kelly’s, the worse (and more expensive) the problem will get. 

What’s worse, there are over half a million semi-truck accidents in the USA every year. In 2017 alone, over 4000 people died in collisions with big rigs (and nearly a quarter of those fatalities were truck drivers). The most common cause of these accidents? Truck malfunction

You don’t keep your rig in good condition just because you’re a competent professional; you do it because it saves lives. 

With this in mind, let’s look at some of the most common fluid leaks that could spell trouble – or worse- for your rig. 

Transmission Fluid Leak

Transmission fluid lubricates all the moving parts of your truck’s transmission system. It prevents excessive wear, tear, and friction build-up, and it keeps overheating at bay.

Transmission fluid leaks can cause your truck’s transmission to “slip.” At first, a slipping transmission may feel like the vehicle changes gears even when it shouldn’t. 

Strange noises, such as clunking, humming, and grinding, also accompany transmission fluid leaks. You may notice these more often when your semi isn’t in gear. A sluggish clutch and grinding gears can also indicate a leaking transmission.

Checking for a Transmission Fluid Leak

Most semis have a transmission fluid dipstick located by or at the back of the engine. A transmission fluid leak would originate from this area.

If your truck is leaving red, oily stains on the asphalt, that’s transmission fluid. Dirty transmission flood will take on a reddish-brown fluid. If you’re looking at a reddish brown leak, you don’t just have a transmission fluid leak, you also need a fluid change, too. 

It’s not always easy to determine the color of the fluid leaking, especially on dirty asphalt. If you’re having trouble color matching your stains, try this easy “Towel Trick”: 

1) park your truck on level ground over some clean, lightly colored fabric
(old towels, an old bedsheet or a cheap plastic tablecloth works well)
2) leave the truck parked overnight.
3) In the morning, check the material for the location, size, and color of the stain. 

Getting as much information as you can about the leak will give your Kelly’s technician a better idea of what kind of leak, and the severity of it, you’re dealing with. 

Engine Oil Leak

Engine oil reduces friction by lubricating the moving parts in the engine. The additives in it also help stabilize the oil’s thickness. These additives help break down engine debris and protect your machine from corrosion, wear, and tear.

If you’re running low on oil, you’ll know. The oil pressure sensor will sense a drop in oil pressure, causing the warning light on your truck’s dashboard will light up. The truck may also “feel off” as performance declines, and the engine becomes more prone to overheating. 

You may hear a clunking, knocking, or grinding sound as the unlubricated parts start to knock against one another. A burning oil smell is also very common, as the oil leaks onto hot parts of the engine and burns. Finally, your engine will overheat and seize, causing a catastrophic failure. 

All this loss can be due to a leaky reservoir. An oil leak is a simple fix but needs to be seen to be at the first sign of trouble, as it can progress quickly. 

Checking for an Engine Oil Leak

An oil leak is easy to find on asphalt. It’s a very dark brown or black stain. When touched, it will be thick and slippery. 

If you’re using the “Towel Trick” and parking your rig over an old sheet, this leak will appear as light brown (or darker brown, if the oil is old and needs changing). 

Power Steering Fluid Leak

Power steering fluid is the hydraulic fluid that channels the power into your truck’s steering system. It creates pressure on either side of the vehicle’s rack-mounted piston, which allows you to turn your semi’s wheels.

As the power steering fluid level drops due to a leak, it won’t be quiet. You may hear your truck making a “chirping” sound, or even shrieking if the pressure is low enough. The chirping or shrieking sound will be more apparent whenever you try to park, and you may also hear a dull whine when you steer the wheel all the way to the left or right.

In more severe cases, the wheel may become harder to turn, but you’ll notice it even more when you try to park.

Checking for a Power Steering Fluid Leak

In most trucks, you’ll find the reservoir that holds the power steering fluid on the left side of the vehicle. It’s under the hood and is often near the frame-mounted steering shaft. The reservoir itself is usually white.

Since the power steering fluid is also under the hood, you can use the same trick with the towels. Like transmission fluids, power steering fluids are also usually red or reddish-brown. If you’re running on old steering fluid, the oil leak can turn out brown.

You can distinguish power steering fluid from transmission fluid through their odor. If it smells like burned marshmallows, that’s power steering fluid. If it smells like petroleum, that’s transmission fluid.

Brake Fluid Leak

Brake fluid is also a form of hydraulic fluid, and, as you’ve probably guessed, it’s the one that moves all the parts of your truck’s braking system. It delivers the force created by a pressed brake pedal to each of the semi’s brake rotors.

A truck leaking this fluid will almost always make the brake warning light on the dashboard light up. The brake pedal may have a “spongy” or “squishy” feel when pressed. You may also notice that the pedal goes too far down whenever you put pressure on it. 

If you notice any of these, it’s best to get your truck serviced right away. Driving with faulty brakes won’t only make it worse; it can also lead to deadly crashes.  Brake failures are the leading cause of accidents caused by a mechanical malfunction, so don’t delay on bringing a suspected brake fluid leak into Kelly’s

Checking for a Brake Fluid Leak

Brake lines are long and run from the engine compartment to the wheel arches. If you’re using the “Towel Trick” make sure you have lots of towels handy to catch these stains. 

Brake fluid is usually yellow, brown, or dark brown when it leaks. When touched, it’s very slippery and will have an odor resembling fish oil. 

Radiator Coolant Leak

Coolants transfer combustion heat from your truck’s engine to its radiator, which safeguards your semi’s various parts from boiling or freezing. It also protects the metal parts in your engine and cooling system from corrosion. Problems with your radiator can be a sign of a coolant leak.  

A bubbling radiator may be a sign that combustion gases got forced into a truck’s cooling system. However, this may also mean that you need replacement parts for a cracked head gasket. Sometimes, a damaged engine may also be the source of the bubbles in the radiator.

White smoke in the exhaust is also a good indication that your truck is low on coolant. Things can get from bad to worse if your engine overheats. In both cases, coolant leaks are often to blame.

Checking for a Coolant Leak

You can also employ the “Towel Trick” to check for leaks here. 

Coolants come in a rainbow of different colors, including red, blue, green, purple, and pink. In addition, coolants typically also carry a sweet aroma.

Get That Fluid Leak Checked and Fixed Now

There you have it, your ultimate guide on how to diagnose a fluid leak in your truck. As you can see, troubleshooting leaks can be quite easy, so that’s all the more reason to do it now. If the “Towel Test” reveals various leaks, then it’s time to get your truck serviced.

If you’re in or near San Leandro, CA, our truck experts at Kelly’s Truck can help. Ring us up now, and we’ll take care of all those fluids leaks now. In most cases, we’ll have your rig back on the road the next day. 

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