8 Common Symptoms of a Clogged Radiator You Must Know

Overheating is detrimental to your engine, or any component in your vehicle really. Serving to cool the engine and prevent it from overheating, the radiator thus plays a vital role in your engine performance. That said, a clogged radiator that cannot function properly can lead to severe damages. Thus, knowing the common symptoms of a clogged radiator and how to fix each problem at hand is a basic and vital maintenance skills that you must master to ensure optimal engine performance as well as saving yourself expensive and complicated repairs.

The potential damages from an overheated engine can be quite serious and include everything from blown gasket heads to damaged cylinders. If you’re not familiar with the symptoms of a clogged radiator and do not address overheating in time, you might have to deal with complicated and expensive repairs that cost up to $4,000, maybe more. Meanwhile, if the culprit is indeed a clogged radiator, the repair job would be much simpler and cheaper. Read on to find out how the radiator works, the most common clogged radiator symptoms, radiator repair and replacement costs, and a step-by-step guide on how to flush a clogged radiator.

The Role of the Radiator

Something every novice car owner knows is as an engine runs, it produces a lot of heat. Extended exposure to excessive heat is detrimental to the performance of the engine itself, as well as the proper function and lifespan of every other component located under the hood of your car. Excessive heat is never desirable for any kind of machinery. 

To ensure smooth operation, there must be an effective mechanism to keep the engine cool enough. This is the purpose of the radiator, with the help of engine coolant. 

A car’s engine uses coolant to absorb the heat and transfers it to the radiator, where the hot coolant cools down by evaporation. Once it cools, the coolant is sent back into the engine and the cycle repeats. In other words, the radiator is a form of heat exchanger.

The safe operating temperature for most cars engines is somewhere between 195 and 220 degrees Fahrenheit. It’s okay to go a little beyond that upper limit once in a while, especially in warmer months when the air conditioning is much needed. That said, excessive engine heating for an extended period is a big no no. 

Most modern engines are liquid-cooled, that is using a liquid coolant run through a heat exchanger, which is cooled by air (thus you can say they are actually partly air-cooled). Other engines are air-cooled, that is using a gaseous fluid instead of the liquid coolant. 

What Causes a Clogged Radiator?

how to flush a radiator
The radiator is located at the front, under the hood of your car. Photo Credit: AutoZone

The radiator might be clogged due to rust, internal deposit buildup over time, or debris/contaminants getting stuck in the radiator. Such blockages will prevent the proper flow of coolant between the radiator and your engine, thus impeding the radiator’s cooling capabilities.

The most common cause of a clogged radiator is old radiator coolant that has been sitting around for extended periods without being changed, or when the engine hasn’t been running for a while. Old coolant invites sediments and rust that blocks the radiator tubes and fins, impeding the flow of coolant to and from the radiator. 

Similarly, a cooling system which hasn’t been serviced and “flushed” regularly would also introduce contaminants that create blockages. You will learn the steps on how to flush the radiator to get rid of or prevent blockages at the end of this guide.

Needless to say, a bad radiator that does not function properly means big, big trouble. A clogged radiator would always lead to engine overheating. 

A Liquid-Cooled Cooling System

To intuitively understand the symptoms of a clogged radiator and remember what must be done to fix it, it is important that you grasp how the radiator works. 

In a liquid-cooled cooling system, there are interconnected coolant channels running through the engine block and cylinder head. At the top of the cylinder head, all these channels converge to a single outlet.


The hot coolant is sucked out of the engine to the radiator with the help of a pump. Coolant circulation is maintained as the pump usually sends the heated coolant up through the engine and down through the radiator, taking advantage of the fact that hot water expands, becomes less dense and lighter than cool water. Thus the natural tendency is for hot water to flow upwards and cool water downwards.

The pump is driven by a pulley and belt from the crankshaft. Heat from the hot coolant is transferred into the air, and the now cooled coolant then returns to an inlet at the bottom of the engine block and flows back into the interconnected channels again.

Radiator Fan

The radiator needs a constant flow of air to “exchange” or transfer the heat of the coolant into the airstream. When the car is moving, this happens anyway. Otherwise, a radiator fan is used to create the much needed airflow. This fan is driven by engine power. 

That said, the fan is not always needed while the car is moving. Thus, to minimize the energy used in running the fan, most cars these days cars have an electric fan that switches on and off when the coolant temperature reaches a set point, which is detected by a temperature sensor.  

Radiator fans in other cars have a thermal fan clutch for optimal cycling. They come with a temperature-sensitive coil spring on the front that expands when the air coming through the radiator is hot enough and reaches a set point. This expansion opens an internal valve that reduces clutch slippage, making the fan spin faster for increased cooling effect.

Without the fan, it is impossible to remove almost half of the waste heat of the engine. The radiator fan is especially important in hot weather, slow driving or continuous stop-and-go driving. In hot weather, when the air conditioning is on more often, an electric fan is turned on more often and is more prone to failure.


Hoses, Tubes & Fins

The hoses, tubes and fins of the radiator are parts which are prone to contamination and wear and tear. First, rubber hoses link the radiator to the engine. They route coolant to the radiator to be cooled and then return it back into the engine.

The radiator has a top tank that collects incoming coolant and a bottom tank connected by a core, which consists of many rows of fine vertical tubes. Older vehicles would have vertical tubes, while modern, low-fronted cars have crossflow radiators with tubes that run horizontally.

Sandwiched between the rows of tubes are thin sheet-metal fins. The sheet fins means the core has a very large surface area, thus when the coolant passes from the top tank through the tubes to the bottom tank, the fins can quickly conduct the heat away from the coolant and dissipate this heat into the cooler air passing through them. 

Blocked radiator tubes means the water pump cannot properly circulate coolant inside the engine block and back to the radiator. Blocked tubes also means the hot coolant cannot easily flow through them, thus heat from the coolant cannot be effectively conducted away and dissipated into the air surrounding the tubes and fins.

Blocked tubes sound unimpressive, but will have a severe impact on the engine’s performance as well as causing other components of the cooling system to start deteriorating prematurely.


Radiator Cap

At the normal working temperature of the engine, the coolant is just below the boiling point for water. To minimize the risk of boiling, the boiling point must be raised, which is achieved by raising the pressure in the system, or pressurizing the coolant.

This pressurization is controlled by the radiator cap that comes equipped with a pressure valve. It is designed to hold the coolant in the radiator under a predetermined optimal level of pressure. When the pressure is too high, the valve opens, allowing coolant to flow out through an overflow pipe. 

When the radiator cap does not seal properly, air could enter the cooling system. Air pockets can get inside the heater core, thermostat, and radiator hoses. As a result, the engine will not be able to sustain the consistent and optimal temperature and will start to overheat.

When the engine runs very hot, a cooling system of older vehicles that uses the radiator cap for pressure control would experience a negligible but continual loss of coolant, so it would require topping up of coolant regularly. 

More modern vehicles would most likely have a sealed cooling system that prevents such coolant loss. When the engine runs hot, any overflow goes into an expansion tank, then the coolant in the expansion tank will flow back into the engine when the remaining coolant cools down.

While we’re on raising the boiling point, note that the coolant’s freezing point also needs to be lowered to a safe level by adding an antifreeze, usually ethylene glycol. This is because water expands when it freezes, and if the coolant freezes, it can burst the block or radiator.


All liquid-cooled cooling system has a thermostat that sits between the engine and the radiator, usually located above the pump. Its job is to block the flow of coolant to the radiator until the engine has warmed up to its normal operating temperature of about 200 degrees F or 95 degrees C. The radiator is closed off by the thermostat until the engine has warmed up enough, or in other words, the thermostat allows the engine to warm up as quickly as possible. This would reduce engine wear, deposits and emissions. 

The thermostat has a small cylinder located on the engine-side. This cylinder is filled with a wax that begins to melt at a set temperature, most commonly around 180 degrees F. When the engine warms up, the wax melts, expands and pushes the valve open, allowing coolant to flow through the radiator. When the engine stops and cools down, the wax cools and shrinks, closing the valve again.

8 Symptoms of a Clogged Radiator

Of course, prevention is always better than messing up and fixing afterwards. That said, you must be well familiar with the most common symptoms of a clogged radiator below to detect problems early before expensive engine damage occurs.

Overheating/ High Temperature Gauge Reading

The most obvious symptoms of a clogged radiator is engine overheating. The temperature gauge on your dashboard will tell you when this happens. Some newer cars will display the temperature digitally and warn you when the engine temperature gets higher than the safe level.

The temperature gauge is located somewhere in your fuel gauge or your odometer and features a needle that moves between C and H on two ends. If it starts heading towards the red zone on the H, that means your engine is overheating.

Another warning is when the needle of the temperature gauge is sitting noticeably higher than usual. A normal operating temperature for most cars is somewhere between 195 and 220 degrees F. While your engine should typically be operating at around the same temperature with only slight deviation, especially in summer months when the air conditioner is constantly running, noticeable temperature increase should receive due attention.

If you notice a sizable temperature increase for an extended period, a clogged radiator may very well be the culprit. A good flush will usually solve a clogged radiator. At the end of this article, you will learn the proper steps to flush the radiator. 

A common question is whether a clogged radiator is the only reason for engine overheating. The answer is: not always. Engine overheating could also be caused when the coolant level is low or a malfunctioning radiator fan or a cap that does not seal properly. 

Coolant Leakages 

When the radiator is clogged due to severe corrosion caused by contaminated coolant, the cooling fins of the radiator may develop tiny holes or cracks on them, and might lead to coolant leaks. You might see visible coolant leaks on your garage floor or driveway and/or the low coolant level light may illuminate in your dashboard. This is the second most noticeable and common symptoms of a clogged radiator.

Radiator corrosion is often due to a low quality coolant or when you regularly add tap water instead of distilled to the coolant mix, as tap water has a lot of contaminants. If coolant leak is indeed caused by rust, you’ll need to get it repaired quickly because these leakages will get worse with time and can even permanently damage the radiator. 

With even a minimal coolant leak, your vehicle will be forced to operate with an insufficient amount of coolant, which means suboptimal performance. If prolonged, your engine may overheat and suffer serious damages that call for expensive repairs.

As always, check the colour of the leaking fluid to make sure that it’s coolant rather than another type of fluid from your car. Most coolant also has a distinct odour that’s rather “sweet”. If it is coolant, try to clean it up quickly because coolant is toxic, and will attract animals to drink it. Diagnose and fix the leak as soon as possible or at the very least keep the coolant level topped off to keep your engine running for the time being. 

Do note that if your radiator is leaking because of severe rust, a thorough radiator flush will not be able to repair this problem. The radiator might be failing altogether and you may have to replace it. 

However, having said that, if a radiator flush is not regularly done, the system will more likely be prone to rust and blockages. 

Change in Coolant Colour, Viscosity & Smell

symptoms of a clogged radiator
Contaminated coolant has a rusty colour and is opaque. Photo Credit: Libertyville Auto Lab

Another symptoms of a clogged radiator is changes in the characteristics of coolant, which most likely means it is contaminated. During your regular maintenance, you should make a point to inspect the coolant, see if it needs refill and check its colour, viscosity and smell.

Coolant should be a bright green or yellow color. There are other colors of coolant out there including pink, blue, and purple, just make sure you’re aware of the colour of your coolant. Coolant is typically of a certain thickness that would allow it to flow freely inside the cooling system. 

Over time, internal deposits, sludge or external contaminants (like from tap water) can contaminate the coolant, typically turning it into more of a rusty color. Because it’s contaminated it’s going to be much thicker than normal, which will cause it to flow more slowly and create blockages.

To check the colour and viscosity of the coolant, the easiest way is to check the coolant overflow tank (you should have a coolant overflow tank that you can easily access and quickly inspect). If you see a drastic change in colour and/or thickness, you’re going to need to thoroughly flush the system to prevent clogging the radiator. 

In addition to the change in colour and viscosity, while coolant should have a distinct sweet smell, contaminated coolant would smell a bit like rubber.

A side note: some vehicles have a transmission cooler located within the radiator. If a leak occurs in the barrier that separates the two, the coolant and transmission fluid would mix and cause double the trouble.

Exterior Radiator Fins Blocked

Radiator fins can block the airflow due to various reasons, one of which is clogging due to foreign material building up on these fins.

You should be able to see the radiator fins at the front of your radiator. Because they are outside and exposed to the air, these fins can easily become clogged with dirt, road dust, and even small branches or leaves and insects. 

If you see white crusty deposits corroding these fins, they are clogged. These clogged fins are blocking the airflow and not allowing the coolant to cool down effectively.  

Fortunately, if it’s the fins that are clogged with debris and contaminants, you’re likely able to spray them clean with a garden hose. On most cars, it should be quite straightforward to remove a plastic cover or two to get access to these fins located on the front of the radiator.

Bent or Damaged Fins on the Radiator

Radiator fins are extremely thin and delicate. They can get bent from physical contact with foreign material while the car is in motion. When enough fins get bent or damaged, they will flatten against the radiator. This will restrict airflow, thus impeding your radiator’s ability to keep things cool and cause engine overheating. 

A negligible physical contact, like a piece of tiny gravel hitting the fins while driving, or even when spraying water to clean off the fins using a strong pressure washer, or anything else banging into the fins can cause damage. Damage can also occur during radiator removal and replacement. 

Leaking or Damaged Head Gasket 

A clogged radiator will most definitely result in engine overheating and if prolonged cause  serious engine damages such as a leaking or blown head gasket or cracked cylinder head. These damages are expensive and troublesome to repair. 

In addition to rapid engine overheating, a damaged head gasket or cylinder head will manifest as rough running of the engine, white smoke coming from the exhaust, and excessive use of coolant.

Heater for Passenger Area Not Working

The cabin heater of your car relies on passing the hot coolant through the heater core and then using a blower fan to blow the resulting hot air into the passenger area. A clogged radiator means not enough hot coolant makes its way to the heater core for warming up the cabin. 

Although improper working of the heater is among the common symptoms of a clogged radiator, do note that a bad thermostat is more likely to be the culprit.

Water Pump Malfunction 

As above, the most common cause of a clogged radiator is old engine coolant that gets contaminated. Contaminated coolant loses its lubrication and anti-corrosive properties. This can result in damages to the engine’s water pump, which leads to improper circulation of engine coolant.

Radiator Repair Cost

Radiator Flushing

Radiator hoses, and hose connections are prone to corrosion and sediment buildup over time, which can result in cracks or holes in the radiator. Replacing the radiator hose could cost you only around $30 to $60 and this is a comparatively easy job that you can actually do yourself. 

If you’re lucky, a thorough radiator flush might be all you need to unclog the system. If you can do it yourself, this DIY project might cost you as little as $40 for a few simple tools. Or you may want to go to a mechanic, or if you want to save a bit of money, opt for a tire and lube place instead to get the job done. These places are generally cheaper than mechanics, but do check with the businesses in your area to see what kind of services they offer and whether there is any deals to lower the cost.

Radiator Flushing: DIY 

The good news about a radiator flush is that you can also do it yourself with some widely available kits. You will need a bottle of radiator flush and cleaner that only costs about $6; you can get them online, at your local supermarket chain or an auto shop. Chip in another $8-$10 for a full flush and fill kit that you can clean up the entire cooling system with. 

These kits typically don’t come with a hose for spraying down the radiator. So if you already have a garden hose at home, you can flush your radiator in the driveway for only around $15.

Radiator Replacement 

If your radiator suffers serious damage, like severe corrosion, and fails altogether, a simple flush might not be able to restore it to functioning condition. It will need to be replaced. The cost of a new radiator can vary greatly depending on the make, model, and year of your vehicle, plus whether the radiator is OEM or aftermarket. 

In general, you can expect to pay somewhere along the lines of $100 to $900 to replace a radiator with aluminum core and plastic tanks. Something like a Ford or Toyota radiator is going to be cheaper than that of a luxury car or a truck with a large, powerful engine.

For most vehicles on the road these days, the average cost in most cases would be around $400 to $500. That’s just for the part itself, not including labour cost. 

Labour costs also vary, since the amount of time it takes to replace a radiator also varies depending on your vehicle. For some cars, it’s easy to access the radiator and thus replacement would only take at most  one hour. Meanwhile, some cars have more setups, most commonly luxury cars like an Audi or Porsche, so radiator removal and replacement can take up to 3 hours. So you can expect to pay anything between $100 to $300 for labor.

In addition, while you’re having your radiator replaced, other radiator parts may also need replacement, including the hoses, hose clamps, radiator cap, and the thermostat. New coolant is usually recommended in most cases. So on top of the cost of the radiator and labour cost, add in another $15 to $100 for replacement of miscellaneous radiator parts.


How To Flush A Clogged Radiator

Flushing your radiator at home is not the most complicated maintenance job, and if you can do basic maintenance like oil change or tire replacement, you’d most likely find this a manageable task. And if you’re not 100% confident, you can check a number of videos on the internet which can walk you through the process step by step. 

Although getting this job done at a mechanic isn’t the most expensive job in the world, you can save money doing it yourself given that you have the time, as you can do this for under $20 only.

Follow the steps below to unclog your radiator:

Drain and Remove The Radiator

  • Park your car on a level platform. Turn off the central heating unit and let it cool completely. Wait for your engine to sufficiently cool down (you should be able to touch the radiator without being burned).  
  • Place an empty bucket under the radiator drain petcock, typically located on the bottom of the radiator toward the driver’s side. 
  • As things may get messy, wrap absorbent towels beneath and around the pipes that serve the radiator.  
  • With an adjustable wrench, start with the thermostatic radiator valve and proceed to turn off the valves. 
  • Use an adjustable wrench to turn off the lockshield valve. Remember the number of clockwise turns required to close this valve because you must repeat the same turns counterclockwise when you replace the radiator.
  • While keeping that bucket in place beneath the thermostatic radiator valve, now use the radiator key to loosen the bleed valve. This will release all the water in the unit. 
  • To drain the radiator completely, use the radiator key to loosen the lockshield valve and tilt the radiator.
  • Close the bleed valve with the radiator key and remove the radiator from the wall. You can now take the dirty radiator outs and proceed to manually flush out dirt and residue as follows.

Flush The Radiator

  • Get a bottle of heavy duty radiator flush. Remove the radiator fill cap and pour in the radiator flush, then fill the remainder using fresh water. Replace the cap back.
  • Let the engine run for at least about 15 minutes to let the radiator flush flow throughout the whole cooling system. You may need to run the engine longer if you’re dealing with a severe case of contaminants and accumulation in the cooling system. Then switch the engine off and always allow it to cool before jumping to the next step.
  • Remove the radiator fill cap and open the drain petcock. Allow the radiator fluid to pass into the bucket that you prepared.
  • Put a water hose and inside the radiator fill cap to allow water inside the vehicle’s cooling system. Wait until the escaping water is clear and drains smoothly. This indicates that your radiator clog has been cleared.
  • Shut off the water and allow it to drain completely. 
  • Now you can fill the system with the right coolant and then add water until the system is full. Make sure the radiator is full and the overflow tank is at least half-full. 
  • Now replace the radiator cap and you’re done. All you need to do is to start the engine and check for leaks.


Important Note:

It is extremely important that you must NOT attempt to drive your car just yet. Before driving the car, you need to make sure all air is removed from the system or else the engine may overheat, as aforementioned. To do this, let the engine idle for 15 to 30 minutes. 

During this time, a valve in the thermostat opens. When the engine reaches a certain temperature, usually about 180°C, the coolant and air circulates and air pockets are removed from the system. 

You can double check that the thermostat has opened by feeling the upper radiator hose while the engine idles. The hose will feel hot if the thermostat has opened. 

The coolant level in the radiator will drop significantly during this short period, so you need to top it off right away. This step requires you to be very careful, since you’ll need to remove the hot radiator cap. The best measure is to wait until the engine cools down again.

Then re-fill the system until it is full, and replace the cap.

Or if you don’t want to wait that long, you can use a towel to grab the cap. Still, be careful because the coolant will be just below boiling point, so make sure you stand out of the way in case the cap comes off. 

Other Radiator Maintenance

Check the Coolant Often

The most common cause of a clogged radiator is old, contaminated coolant. In addition, inadequate coolant level also causes engine overheating, while the water level of the coolant decreases over time. So for the best prevention, you should change the coolant every two years. This would lower the risk of a clogged radiator caused by severely contaminated coolant, and save you the trouble of having to flush the system.

Check the Fan Belt

The radiator cooling fan is driven by a rubber fan belt. When it is damaged to the extent that the rubber part melts and slips, this will jeopardize the turn cycle of the fan and results in engine overheating. The belt then should be replaced. 

You should regularly check the tension of the fan belt by hand. Push the middle of the fan belt using your thumb, and if you can push it down by between 13mm to 20mm, all is well. 

Check the Radiator Hose

You should also regularly inspect the radiator hose for any cracks, distortions or damages as they can cause water leakage.