DIY

6 Signs of a Bad Radiator

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You know that the radiator is an essential part of your cooling system, but how can you tell when something is wrong? Spotting the signs of a bad radiator is failing isn’t something that most drivers know how to do. If you aren’t a professional mechanic, this part of your vehicle might seem foreign to you—but it doesn’t have to be.

Let’s take a look at what the radiator is, how to spot the symptoms of a bad radiator, and how to diagnose the issue.

Bad Radiator Symptoms: Common Signs that Your Radiator is Failing

When the radiator isn’t working properly, you might notice several symptoms. While this isn’t an exhaustive list, here are the most common complaints.

1. Overheating engine

A common sign that something is wrong with the radiator is when your engine starts to overheat. This can be caused by a faulty thermostat. When a defective thermostat no longer opens, the engine will overheat and cause a breakdown.

2. Leaks

You might notice coolant leaking under the vehicle. It can be a red, yellow, or green fluid. When the radiator is leaking, coolant levels become too low, and when there isn’t enough coolant to reduce the heat, the engine might become too hot.

Radiators are made of metal and plastic. And because plastic isn’t as durable as metal, it’s typical for these components to become brittle and start cracking—hence the leaks.

driver with hand on gear shift
If you have an integrated transmission cooler, you might notice trouble with shifting as the fluid becomes contaminated with coolant.

3. Shifting issues

If you have an integrated transmission cooler, you might notice trouble with shifting as the fluid becomes contaminated with coolant. This problem stems from cracks and faults in the system.

4. Fluid discoloration

Vehicle coolant is commonly bright yellow or green. Sometimes it can be pink or red. It flows freely between the radiator and the coolant passages located in your car’s engine.

If you have a bad radiator, deposits and sludge can contaminate the coolant and turn it into a rusty color—it may even look like oil.

Look in your coolant overflow tank and inspect the color. If it’s contaminated, it may also appear thicker. This causes trouble with the flow and leads to a clogged radiator.

A radiator flush might be a solution in this case.

5. Exterior fins blocked

Radiators need maximum airflow for proper cooling. The thin tubes running in the front, the ones that look like fins, carry the hot coolant away from the engine. When you drive, air passes over the fins to lower the temperature of the coolant.

If these fins become clogged by debris, leaves, bugs, or dirt, airflow can become compromised. The coolant can’t go down in temperature as it should.

With many vehicles, there is enough room to spray off the radiator with a garden hose to keep the air flowing freely.

In addition to blocking the air, it’s also possible that you have damaged or bent fins. They are delicate, which means any flying dirt or debris could lead to damage. It’s also possible that you damaged them during installation or while spraying water on them (particularly if the water pressure is too high).

If you have damaged fins, a clog can occur in the radiator, leading to an overheating engine.

6. Passenger heater not functioning

Another noticeable sign of a bad radiator is when your heater doesn’t work as it should.

Your cabin heater relies on the hot coolant that passes through the core. This produces the hot air that gets blown into your car. If your radiator is leaking or is clogged, you may not get the heat you need on that cold winter day.

Sometimes, this is just a bad thermostat too. At the dealership, we checked both when a car came in with this complaint.

car radiator
The radiator is a heat-exchanger located in the front of the car and contains two linked water tanks connected by many narrow tubes. Hot coolant from the engine is pumped through one of the tanks into the core.

What is a Radiator?

Before we move on to telling you how you can diagnose a bad radiator, we’ll explain exactly what a radiator is and what it does.

The car radiator is one aspect of the cooling system that keeps your vehicle’s engine operating at ideal temperatures. This heat-exchanger is located in the front of the car and contains two linked water tanks connected by many narrow tubes. Hot coolant comes from the engine and is pumped through one of the tanks into the core.

As the car moves, colder air is forced between all the narrow tubes. This action draws the heat from the coolant. The cooled fluid then pumps out of the radiator and back into the engine.

How to Diagnose a Bad Radiator

Checking the Cooling System

With so many components working together, you can’t just go right to the radiator, unless you already know it’s the problem. At the dealerships, we like to work through a series of checks to find the failed parts.

using temperature gun to check if the thermostat or radiator is bad
You will need an infrared temperature gun to check if your thermostat is bad or not.

Check #1 – Failed thermostat

You will need an infrared temperature gun. Then, follow these simple steps:

  1. Warm up your car engine. Give it ample time.
  2. Look for the radiator hoses. There is an upper and lower radiator hose on your car.
  3. Scan the radiator hose temperatures with the infrared gun. Wait for your engine to overheat and measure both hoses.
  4. If both hoses stay cold, or just one becomes hot, then you have a bad thermostat. Don’t be a macho man and attempt to measure this by touching the hoses. Obviously, they can get very hot and you could burn your hand.
using garden hose to clean radiator external clog
You should be able to clean an external clog with a garden hose or compressed air.

Check #2 – Clogged radiator

If your radiator has an internal clog, the coolant flow is restricted. If you have an external clog, it might cause the airflow to become restricted and lead to overheating. Here’s what you need to do to check:

  1. Your engine must be cool. Leave it parked and off until it’s not warm at all.
  2. Look inside the radiator. You need to remove the cap and inspect for debris. If you have an internal clog, you’ll need to replace the radiator.
  3. Look for external clogs. Examine the front of your radiator for debris that could have clogged it. You should be able to clean an external clog with a garden hose or compressed air.
overheating engine is a sign of clogged radiator
If you have an external clog in your radiator, it might cause the airflow to become restricted and lead to overheating.

Check #3 – Cooling system leak

If there’s a leak in the cooling system, your engine will overheat. You must repair this immediately if you want to prevent further damage. To perform this check, you’ll need a cooling system pressure tester and coolant dye kit.

  1. Cool the engine completely.
  2. Take off the pressure cap and set it somewhere safe.
  3. Follow the manufacturer’s instructions and apply pressure with your tester. Don’t exceed what’s written on your radiator cap.
  4. Examine the system for a leak.
  5. If you need to, add some coolant dye after removing your tester.
  6. Put your radiator cap back on and start your engine.
  7. Watch for a leak by keeping an eye open for the dye. Sometimes, it takes days of regular driving until you see the dye—this indicates a slow leak.

Check #4 – Defective cooling system pressure cap

Can a bad radiator cap cause overheating? Yes. If your cap isn’t holding pressure, the coolant starts to boil and leads to an overheating engine. Keep your system pressure tester handy for this check as well.

  1. Cool the engine completely.
  2. Unscrew the pressure cap and remove it.
  3. Use your pressure tester on the cap. Make sure it holds the recommended amount, as per the cap. If not, replace it.

Check #5 – Malfunctioning water pump

When a water pump breaks, it doesn’t circulate the right amount of coolant. This also leads to an overheating engine. You don’t want to check this until you’ve done the thermostat inspection first.

  1. Cool the engine completely.
  2. Take off the pressure cap.
  3. Start the engine and watch the coolant to see if it begins circulating.
  4. If it doesn’t move, you could need a water pump.
  5. Look at the water pump. It might have signs of a leak. Look for wet spots or dry green or white residue.
When a water pump breaks, it doesn’t circulate the right amount of coolant. This also leads to an overheating engine.

Check #6 – Faulty radiator cooling fan

When the cooling fan fails, your engine overheats.

  1. Park the car.
  2. Look for the cooling fan. It might be mechanical or electric, but the engine drives it.
  3. Start the car and let the engine warm up.
  4. When the engine temperature rises higher than normal, inspect the cooling fan. If it doesn’t turn on or seems to be running slow, this might be your problem. For a mechanical fan, you must replace the clutch. With an electric fan, you want to diagnose the circuit first.

Check #7 – Defective head gasket

The issue no one wants to have is an internal problem with the engine, but it happens often. The way to prevent this is to repair cooling system defects immediately. That means, don’t put water in your radiator and ignore the leaks.

To perform this check, you’ll need a block test kit.

  1. Cool the engine.
  2. Remove the radiator cap.
  3. Hook up your block tester. Make sure you follow manufacturer specs.
  4. Watch for an indication of combustion gases in your cooling system. If it shows evidence of gases, you have to disassemble the engine for further troubleshooting.
No one wants to have an issue with the head gasket because it is an internal problem with the engine–but it happens often. The way to prevent this is to repair cooling system defects immediately.

As you can see, finding out if you have a bad radiator is more complicated than just looking at the part. That’s because the cooling system contains multiple components, and when one aspect isn’t running right, it leads to the same symptoms. You can’t automatically tell that a thermostat or radiator is the culprit just because the engine overheats.

Other Parts You May Want to Check if You Have a Bad Radiator

When the radiator fails, you may see other parts break as well. The three main components to inspect are the heater core, water pump, and thermostat.

Heater core

If you’ve ever seen the heater core, you know it looks like the radiator had a baby. It’s just a tinier version. Inside, it even works the same. The main job of a heater core is to produce warm air for your cabin. When debris gets stuck in the miniature heater core tubes from the faulty radiator, you can’t get heat.

In addition, when the engine begins overheating, the heater core breaks under intense temperatures and pressure. The car windows start to fog and your floorboards may remain wet from a leak.

Water pump

You need an operational water pump to move coolant into the passageways and hoses of your cooling system. It’s often constructed of plastic parts. If debris begins to break away in the radiator, the water pump receives damage or abrasions.

As the flow is disrupted, the water pump may fail completely.

Thermostat

thermostat getting stuck because of malfunctioning radiator
You can find the thermostat at the end of the top radiator hose.

For proper control over your engine temperature, the thermostat needs to do its job. You can find this part at the end of your top radiator hose. It regulates coolant flow through the cooling system. As temperatures hit the desired point, it opens to allow cooling system fluid to flow freely.

When your radiator quits functioning right, there’s additional pressure on your thermostat. This leads to a malfunction.

The valve commonly gets stuck closed or open. When it doesn’t open, the engine overheats. Touching the radiator hoses lets you know if the thermostat is stuck. The top hose should feel hot while the bottom one will be cold.

If the thermostat gets stuck open, your engine won’t ever reach the right temperature. This causes you to use more fuel.

That’s why it’s imperative you get a faulty radiator fixed right away. Otherwise, you might damage your engine and these other cooling system components. Why end up with a higher repair bill than you need.

And, do us all a favor and don’t put water in your radiatorno matter what your grandpa taught you.

Can You Drive with a Radiator Problem?

An underperforming radiator has profound effects on the vehicle. Overheating leads to serious damage to an engine and can cause the head gasket to fail. Continuing to drive with a faulty radiator leads to additional damage and costly repairs.

If you notice that your engine is overheating, it’s best to pull over and let the vehicle cool down. If you aren’t able to operate it at a normal temperature, it should be towed to a local shop.

I can’t tell you how many times people finally had their cars towed in after doing major damage due to neglect. It’s not okay to put water in your radiator and keep driving it. In fact, it’s a huge pet peeve of many mechanics. All you do is rust out the cooling system components and create a bigger mess.

Frequently Asked Questions

How can you tell if you have a bad radiator cap?

There are two common methods you can use to determine whether the radiator cap is bad: performing a visual inspection (to check for damage or wear) or using a cooling system pressure tester.

1. Perform a visual inspection: If the cap’s seals are worn or damaged (or the cap is compromised in any way), replace the cap right away.

Also, obviously, a cap that’s leaking coolant (even when the system is not under extreme pressure) is faulty. Caps are inexpensive, so go ahead and replace yours if you have any doubts.

2. Use a cooling system pressure tester: A professional may test a suspect radiator cap using a cooling system pressure tester. The tool is attached to the cap using a special adapter, then pumped up to the cap’s pressure rating. A good cap will hold the pressure for a short time, then allow the pressure to decrease slightly.

Because a pressure tester typically costs more than a replacement radiator cap, using such a tool is impractical for most DIYers. That’s why, to prevent overheating and costly engine damage, it’s a good idea to simply replace a questionable radiator cap.

How much does it cost to replace a radiator?

The average cost of replacing a radiator is around $800. But that price can vary a great deal, depending on the year, make, and model of the vehicle.

How long do radiators last?

A radiator is designed to last the life of the vehicle—but that doesn’t always happen. It can sometimes fail and require replacement long before that. But if properly maintained, your radiator should last somewhere between 8 to 10 years.

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Author

Brian Jones

ASE Certified Master Tech and Parts Specialist

Brian Jones spent more than 20 years working as an ASE Certified Master Tech and Parts Specialist at multiple dealerships. Currently, he lives with his wife and children in a suburb of Dallas, TX. He spends his time dreaming about pickup trucks, muscle cars, Jeeps and anything related to motorsports. He works with numerous dealerships around the country as a consultant. In his spare time, Brian likes to visit new places and hopes to travel the world.

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