Bad Radiator Symptoms – Signs of A Bad Radiator & How To Diagnose

Your car’s radiator is one of its most important components, but it’s also one you may take for granted. The job of a radiator is to transfer heat from the engine and coolant fluid through the vehicle’s plumbing and into the air that flows around it. It helps to keep your engine at an optimal temperature.

If your radiator isn’t working properly, though, it can cause all kinds of problems. In this post, I’ll be taking you through the different symptoms you’ll notice once your radiator goes bad.

bad radiator symptoms

Common Symptoms of a Bad Radiator

1. Overheating

If your vehicle is overheating, there are a number of signs to look for. The most obvious symptom is that your car temperature gauge goes up and down while driving. Sometimes, it just stays up.

You can also check the coolant level in your radiator by looking at the dipstick (which will be located in a tube under the hood). If it’s low or nearly empty, that’s another indication that you may need to top up or replace your coolant.

If you’re not sure whether or not you have enough coolant in there already, ask someone who knows about cars to take a look at it for you, or ask them where exactly under the hood would be the best place to put some more coolant if need be.

Another thing to keep an eye out for when diagnosing overheating problems: check all of the hoses going into and out from around your engine compartment.

Look especially closely at any rubber hoses. These tend to become brittle over time and then crack open during use which could cause leaks into other parts of the system causing overheating issues due to loss of pressure inside the cooling system.

2. Leaking Coolant

If you notice that your vehicle is leaking coolant, this could be a problem. With a leaky radiator, many different issues can lead to the problem.

The most common reason for coolant leaks is corrosion of the radiator’s metal parts or damaged seals in the water pump or thermostat housing.

Other potential causes include holes in the radiator itself, seals on internal valves or gaskets within the engine block or head, and cracks in engine components such as cylinder walls or valve seats.

Once you’ve determined where exactly your car’s coolant is escaping from (there are specific tests for each location), you’ll need to find out what caused it before trying any repairs yourself.

The best way to do this is by having an expert inspect your vehicle so they can diagnose exactly where and why there was damage done to them and then recommend how best to fix it with minimal cost involved. You can check out this post on how to fix a radiator leak with black pepper.

3. Sludge Builds Up in The Radiator

Sludge is a thick, sticky substance that can build up in your radiator. Sludge is made of minerals, acids, and water. It builds up over time as coolant seeps through the cracks in your radiator and mixes with sediment inside it.

As this mixture hardens into sludge, it blocks airflow through the radiator which prevents it from cooling properly. Overheating causes further damage to components and can result in leaks or even complete failure of your engine.

4. Shifting Issues

If your car is shifting slowly, or not at all, first check your oil level and make sure it’s full. If it’s fine, you may have a problem with the transmission. If this is the case, shift into neutral and move forward slowly until you reach a safe place to pull over.

Once parked, turn off your engine and call a tow truck to get you home safely while they diagnose the problem (and hopefully fix it).

How to Diagnose a Bad Radiator

1. Warm Up Your Car Engine

To properly diagnose a bad radiator, you’ll need to warm up the engine. This is the first step in diagnosing most problems with your car since it will allow you to see if there are any issues at all. You should allow ample time for your car’s engine to warm up. The exact amount of time required will vary based on the weather.

But, generally speaking, if it takes longer than 20 minutes for your vehicle to reach normal operating temperature. Then there may be something wrong with its cooling system or radiator.

Once the engine is fully warmed up and still running smoothly (it shouldn’t be sputtering or making odd noises). 

2. Look For The Radiator Hoses

Radiator hoses are a pretty easy thing to spot. They run from the radiator to the engine and are generally black, but they can also be white or gray depending on your car model.

Radiator hoses should be flexible, not cracked or split in any way, and they should not leak coolant as that would signal a serious problem with your car’s cooling system. If you notice any of these symptoms it may be time for some new hoses.

3. Scan The Radiator Hose Temperatures with The Infrared Gun

If your car is suffering from a weak radiator, it’s important to identify the problem quickly so you can get it repaired. One of the easiest ways to test whether or not your radiator is leaking is by using an infrared temperature gun.

When you turn on your engine, allow it to idle for 10 minutes, and then point the gun at any part of your radiator hose (make sure they are not hot.). The highest temperature reading that shows up on the gun indicates that there’s a leak in one of these hoses.

4. If Both Hoses Stay Cold, or Just One Becomes Hot, Then You Have a Bad Radiator

Wait for your engine to overheat and measure both hoses. If both hoses stay cold, or just one becomes hot, then you have a bad radiator. The engine is overheating because the coolant isn’t circulating through it properly. Often, when this happens the problem is that the thermostat is stuck open or closed.

If your vehicle overheats and only one hose heats up then most likely it’s leaking into another part of your cooling system such as an expansion tank or radiator cap. If this happens: Drive with caution.

How to Diagnose a Clogged Radiator

If you have an internal clog, you’ll see debris inside of the radiator. This can happen if sediment has built up over time or if something got stuck in there during installation.

On the other hand, an external clog will always be visible as there will be debris on the outside of your radiator that needs to be cleaned off before it causes problems with your cooling system. Here are steps to diagnose a clogged radiator:

1. Your Engine Must Be Cool

The first thing you need to do is turn your engine off. You should wait until it has cooled down and stopped running before turning it back on again, which may take a few minutes or even longer. This will help prevent damage from overheating or from steam escaping from your radiator as you drive.

Furthermore, park your car, turn off your engine, and let it cool. Don’t go for a drive or turn on any accessories like the heater or defroster (if you have one), as these things can bring temperatures up rapidly.

When it comes time to drive again, start your car when it’s not warm outside and keep an eye on the temperature gauge. If it starts to climb too high, turn off the engine immediately and let it sit until cool before restarting again.

2. Look Inside The Radiator

It’s important to note that you’ll want to remove the radiator cap before checking inside the radiator. The cap should be located on top of your vehicle’s engine and can often be removed with a wrench or pliers.

Once it’s removed, take a look at your radiator. If you see any debris, gently remove it using an old toothbrush or rag. If there are no clogs present in your car’s cooling system and you’re still experiencing problems with overheating, look for external sources of blockage (such as leaves) that might be obstructing airflow through the radiator fins.


One of the most common symptoms of a bad radiator is overheating. If your car overheats, it can damage the engine and other parts of the car. Many people don’t realize when their cars are overheating because they don’t have a gauge to tell them how hot it’s getting inside their engine. The best way to avoid this problem is by regularly checking your coolant level so that you know when something needs attention before it becomes an emergency.

Another common symptom is leaks or drips coming from under your hood or around certain parts of the radiator area. Sometimes these leaks are caused by cracks in hoses which may be difficult to detect without proper tools or knowledge about where they could be coming from.

If you have an older vehicle with worn-out seals around fittings that leak fluid into other areas like underneath floorboards, then replacing those parts will help solve this problem as well.  Also, you may notice sludge building up in your radiator or shifting issues. They are both part of the symptoms of a bad radiator.

Lastly, you can diagnose a bad radiator through the following processes; warm up your engine, look for the radiator hoses, and Scan the radiator hose temperatures with the infrared gun. Note that, if both hoses stay cold, or just one becomes hot, then you have a bad radiator.