How to Identify a Failing Radiator in Your Car

The comprehensive list of signs and symptoms your radiator is on the fritz

Co-authored by Duston Maynes and Eric McClure

Last Updated: June 4, 2024 Fact Checked

Your car’s radiator is as important as the engine itself; without a functioning radiator, your vehicle has no way of getting rid of the heat it generates. If your radiator is failing, it needs to be addressed since excessive temperatures can seriously damage the components in your engine bay. This can be tricky, though! The radiator system is complex and isolating the problem can be a little difficult. That’s where we come in! In this article, we’ll show you how to analyze the problem to determine if your radiator is dying or needs service.

Things You Should Know

The radiator is responsible for cooling your vehicle, so any overheating is a sign something is wrong.
Look under your vehicle and inside of the engine bay for leaking coolant or bubbling around the radiator cap.
Coolant rarely needs to be refilled, so if you find your reservoir goes empty regularly, it’s a big sign there’s an issue.
Inspect the radiator visually at the front of your vehicle to look for cracks or buildup on the fins.

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1

Overheating

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Abnormally high temps are the classic sign a radiator is failing. If your temperature gauge on your dashboard is touching the yellow or red portion of the dial, your vehicle is too hot. On newer cars, you’ll see a little thermostat light pop on. Your radiator is the only thing keeping your vehicle from overheating, so if it isn’t doing its job that’s a big sign something is wrong. ^{[1]
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- If your dashboard displays exact temps, your vehicle should normally operate at 195–220 °F (91–104 °C).
- An overheating vehicle will often smoke, but that’s not always the case.
- Do not drive if your radiator is failing so badly that your car is overheating. Excessive heat can melt electrical components or permanently damage your engine.

2

Fluctuating temperature gauge

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If the temperature dial randomly moves, it’s a bad sign. The temperature dial in your vehicle rarely fails on its own, so it’s concerning if the dial moves up and down erratically. Look at it while you’re driving. If it moves back and forth rapidly, it might mean the fans attached to your radiator are starting to fail. ^{[2]
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- When it’s working correctly, your radiator should expel heat evenly and smoothly. As a result, that temperature dial should only ever move very slowly.

3

Fluid leaks

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A cracked radiator or hose will leak coolant. Look underneath your vehicle when it’s idling. If you see drips of any kind, something is leaking. The radiator expels heat, but running through the fins are hoses containing coolant, which absorb the heat from the engine. If coolant isn’t making its way to the radiator due to a cracked hose or damaged radiator, you’ll usually see it dripping.
- You should only need to add coolant every 2-5 years. If you find yourself refilling the coolant reservoir more often than that, it’s likely you have a leak.
EXPERT TIP

Hovig Manouchekian
Auto Repair & Design Specialist
Hovig Manouchekian is an Auto Repair and Design Specialist and the Manager of Funk Brothers Auto, a family-owned business operated since 1925. With over 30 years of experience in the automotive industry, Hovig specializes in the process of auto repair and maintenance. He is also very knowledgeable in common automotive issues and needs including engine repair, battery replacement, and windshield accessory and maintenance. Hovig's knowledge and hard work have contributed to Funk Brothers Auto winning Angie's List Super Service Award for five consecutive years.

Hovig Manouchekian
Auto Repair & Design Specialist

Act immediately if there is coolant in your transmission. Flush the transmission fluid immediately to remove contamination. Identify the leak source and see a mechanic for diagnosis and repair. Ignoring this can destroy your transmission.

4

Bubbles in coolant fluid

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If you see bubbles when you take the radiator cap off, there’s a leak. Your radiator system is a closed loop, so nothing should be getting in or out. Take the radiator cap off and look at and inspect the fluid. If you see bubbles coming up, it means air is getting into your radiator system. This is a sign your radiator or one of the hoses is damaged. ^{[3]
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- You may even notice thin, empty bubbles appearing around the edge of the radiator cap when it’s on.

5

Low coolant levels

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Your coolant tank’s level should remain constant. Look on your coolant tank for the hash mark that indicates the proper level. If your coolant is low, fill it up to that hash mark. The next few days, check the tank again. If the levels are dropping (even a little bit!) there’s something going on with your radiator system. ^{[4]
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- The type of coolant you need depends on your engine and model—the specific coolant mixture is listed on the radiator cap.

6

Overflowing coolant reservoir

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If coolant spills out from the radiator cap, the cap is bad. Your radiator cap should be 100% airtight. Coolant expands when it gets hot, so if the cap is damaged, the coolant will boil over the reservoir. You may notice weird antifreeze smells after you drive, and you’ll find yourself refilling the reservoir more often than you normally would. ^{[5]
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- If you ever see white smoke coming from your hood on a hot day while you’re driving, it’s the coolant spilling over and coming into contact with a hot component in your engine bay.

7

Fluid discoloration

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Brown coolant is a sign that it’s time to change your fluids. Coolant should be some bright, fluorescent color—not brown. If your coolant is contaminated, the hoses are dirty, or the transmissions heat exchanger is damaged, your coolant will turn a sludgy brown color. Your entire system will need to be flushed so that a mechanic can identify the problem and re-seal the system. ^{[6]
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- Most of the time, coolant is bright green. However, it may be yellow, pink, red, or blue depending on the type of antifreeze used in the mixture.

8

Trouble shifting gears

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A sticky gear shift is a key sign the radiator is damaged. In most modern vehicles, the transmission is integrated with your vehicle’s radiator system. So, if the radiator is messed up, the transmission—which is responsible for shifting gears—will also start to fail. In an automatic car, your vehicle may jump when the gears change, or you’ll notice engine stuttering when changing gears. ^{[7]
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- You’re probably going to notice your vehicle overheating before you notice any gear shifting issues if it’s hot outside.

9

Sludge on your radiator

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If your radiator is filthy it’s a signal the coolant might be leaking. This can be a sign of a failing radiator if the hose is leaking coolant fluid on the fins, but it might just be a buildup of dirt and mud. Try hosing down your radiator to see if the temperature issues resolve themselves.
- The radiator is always just behind the grill on your car, so just keep the hood down and hose the front of your vehicle.

10

Damage on the radiator fins

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Take a look at the radiator and inspect it for imperfections. Grab a flashlight and look at the radiator behind your grill (you can see it between the seams). If you see any bent fins or there are gouges missing from your radiator, it needs to be replaced.
- Once a radiator is damaged enough to the point that it impacts your vehicle performance, you can’t physically repair it. The entire set of fins needs to be swapped out.
- This can be caused by rocks kicking up into your grill, accidents, or general wear and tear over the years.
EXPERT TIP

Hovig Manouchekian
Auto Repair & Design Specialist
Hovig Manouchekian is an Auto Repair and Design Specialist and the Manager of Funk Brothers Auto, a family-owned business operated since 1925. With over 30 years of experience in the automotive industry, Hovig specializes in the process of auto repair and maintenance. He is also very knowledgeable in common automotive issues and needs including engine repair, battery replacement, and windshield accessory and maintenance. Hovig's knowledge and hard work have contributed to Funk Brothers Auto winning Angie's List Super Service Award for five consecutive years.

Hovig Manouchekian
Auto Repair & Design Specialist

Be sure to accurately measure your current radiator before buying a replacement. Use a tape measure to determine the radiator core's length, width, and depth. Also, note the number of core rows (horizontal channels) in the radiator. Aim to find a replacement with the same or more core rows to ensure optimal cooling efficiency.

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You can try to fix the radiator yourself by replacing lines, sealing leaks, or swapping the radiator fins out if you’re a gearhead. However, this is normally a job left best to mechanics.

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How to Identify a Failing Radiator in Your Car

Things You Should Know

Steps

Get Car Support Solve Any Problem

Overheating

Fluctuating temperature gauge

Fluid leaks

Bubbles in coolant fluid

Low coolant levels

Overflowing coolant reservoir

Fluid discoloration

Trouble shifting gears

Sludge on your radiator

Damage on the radiator fins

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