Why My Upper Radiator Hose Is Hot But The Lower Is Cold?

Written By: Terrence Hines
Category: Engine

The upper radiator hose is hot but the lower is cold is a common problem that car owners face. This can occur due to various causes, including the thermostat, fan, or water pump failing.

To find out the issue, check the temperatures of both hoses. It should be between 140F-150F. If one hose is much hotter, then something is wrong, and you need to fix it.

Why My Upper Radiator Hose Is Hot But The Lower Is Cold? – 10 Reasons

Upper radiator hoses are an important component of the cooling system in a car; they are responsible for carrying the coolant from the radiator to the engine.

If the engine is running and the coolant is warm, the upper and lower radiator hoses should be warm to the touch.

If you notice that one of your hoses is hot while the other remains cold, it could indicate a blockage in your cooling system.

This can happen if sediment builds up inside the radiator or if a large air pocket is present as a result of trapped air or low coolant levels.

To find out what is causing this issue, it’s best to take your car to an experienced mechanic for diagnosis and repair services.

1. The Thermostat Is Defective:

If your upper radiator hose is hot but the lower is cold, this could be an indicator that your thermostat is defective and needs to be replaced.

Why My Upper Radiator Hose Is Hot But The Lower Is Cold? - 10 Reasons

A thermostat is a device that helps to regulate the temperature in a heating or cooling system. For example, it helps control the coolant level within the radiator.

If your thermostat has begun to malfunction, you may find that air is being forced past the thermostat either too slowly or too quickly.

This can lead to an imbalance in engine temperature, causing one of the radiator hoses to become hotter than the other.

To fix this issue, replacing your existing thermostat with a new one is best.

2. There Is A Leak In The Radiator:

Fixing an automotive radiator leak is quite a simple process once you identify the problem.

If you determine a leak in your car’s radiator, it’s important not to delay because it can cause severe damage to the engine and other parts of the car if left unchecked.

If you want to know if your radiator is leaking, the simplest way to tell is by feeling both the upper and lower radiator hoses.

If the top hose is hot and the bottom one is cold, your radiator might have a problem. You should act fast to prevent more damage!

Once you’re sure there’s a hole or crack in your radiator, check for any other problems and call an experienced professional to help with repair if necessary.

3. The Water Pump Has Failed:

Replacing the water pump on your vehicle can be complicated, but it’s an important part of regular car maintenance.

If you notice that the upper radiator hose is hot but the lower is cold, it is likely that your water pump has failed and needs to be replaced.

Before tackling this repair yourself, consult a certified auto professional to determine what needs to be done.

Depending on your vehicle, specialized tools or extra steps may be required to properly install a new pump.

 The Water Pump Has Failed

Once the replacement is complete, your engine will have improved reliability, and you can return to the road with peace of mind.

4. The Fan Belt Has Snapped:

Replacing a fan belt can be intimidating for those with little experience. However, the process is quite straightforward.

If your fan belt has snapped, the first step is to identify where it attaches to the motor, typically near the upper radiator hose.

Next, check that the upper radiator hose is hot. The lower one should be cold. If not, there may be a more serious issue, and repairs will be more extensive than just replacing the fan belt.

Once confirmed that only the belt needs to be swapped out, you can purchase a suitable replacement and install it in no time.

5. The Radiator Is Blocked:

If your upper radiator hose is hot but the lower is cold, it may be a sign that your radiator is blocked and needs to be flushed.

Flushing the radiator can help with the long-term performance and reliability of your car, as well as prevent serious engine damage if left untreated.

To flush the radiator, you’ll need to start by disconnecting both of the hoses.

Next, remove any deposits from the inside surface with a garden hose to clean out the blockage.

Once this is done, you can reconnect the hoses, adding a coolant mix like antifreeze into the system afterward.

By flushing the radiator regularly as part of regular maintenance on your vehicle, you should be able to keep your cooling system in top shape for years to come.

6. There Is Air Trapped In The System:

Bleeding air out of the system requires a comprehensive approach. Begin by ensuring that the upper radiator hose is hot but the lower is cold.

This implies that the thermostat works as it should, allowing coolant to flow through the upper radiator hose and back into the engine block.

If cold coolant flowed through both hoses, bleeding air out of the system would be impossible (the coolant wouldn’t be hot enough to produce steam).

Start your engine and slowly turn one of the bleeder screws while watching your reservoir level.

You will know that you have successfully bled most of the air from your car’s cooling system when you see bubbles passing through with each stroke of the bleeder screw.

7. The Coolant Mix Is Incorrect Or Insufficient:

When the coolant mix is incorrect or insufficient, you might experience some serious engine problems. If such an issue occurs, the upper radiator hose will be hot, but the lower one will be cold.

Why My Upper Radiator Hose Is Hot But The Lower Is Cold?

This happens due to the incorrect mixture of elements; it is unable to disperse heat in the radiator’s rear correctly and fails to spread out warmth evenly.

Adequately mixed coolants can fill this role by providing maximum cooling, which is needed to continuously regulate hot temperatures in the engine space.

You should regularly check and flush your radiators as part of your vehicle maintenance routine. This will prevent a major problem if the coolant mixture is incorrect or insufficient.

8. The Radiator Cap Is Not Sealing Properly:

A vast majority of modern vehicles run on liquid coolant, which is circulated by a pump through a series of hoses, radiators, and other components.

When the radiator cap is not sealing properly, there will be an imbalance in the pressure of the coolant system.

As a result, the temperature of the upper radiator hose will become dangerously hot. In contrast, the lower radiator hose will remain cold since there isn’t much coolant moving through it.

Installing a new radiator cap ensures that your heater maintains the right pressure and coolant circulation, allowing for an optimal driving experience.

With this simple switch, you will have peace of mind on the roads and enjoy efficient engine performance over many miles to come!

9. The Engine Is Running Too Hot:

When the engine is running too hot, this indicates a mechanical issue.

If you feel the upper radiator hose, it should be warm to hot. Meanwhile, the lower radiator hose should stay consistently cold.

This temperature difference powers your vehicle and keeps it running adequately for long distances.

When this balance of temperatures shifts towards the upper radiator hose being hotter and the lower radiator hose colder, it’s time to get your car checked immediately.

An unforeseen breakdown while on the road won’t just be expensive and inconvenient – in some cases, it can be dangerous if not quickly resolved.

10. The Coolant Level Is Low:

The coolant level in your vehicle can greatly affect the overall temperature of your engine.

The Coolant Level Is Low

If the coolant level is too low, unequal heat distribution between the upper and lower radiator hose results.

In this situation, you will usually notice that the upper radiator hose is hot to the touch while the lower one remains cold or barely warm.

This could be a sign that there isn’t enough coolant to circulate properly throughout your engine and cause problems such as overheating or other mechanical damage.

It is essential to check your coolant level regularly to ensure it’s optimal, so your engine doesn’t suffer from overworking or perfuming.

Tips For Avoiding This Problem In The Future

An upper radiator hose is hot but the lower is cold can be an alarming symptom, but it’s quite simple to avoid.

Firstly, you should always check both the upper and lower hoses regularly to ensure they’re running at normal temperatures.

Secondly, it’s important to remember that coolant should be flushed once every two years to prevent any scale buildup or rust inside the system.

Finally, if something is wrong with your car, it’s best to diagnose the issue with a professional as soon as possible before this problem can present itself.

Following these three tips will help you immensely in avoiding this particular issue in the future.


The upper radiator hose being hot when the lower one is cold could signify a wide range of issues on your vehicle.

The engine might be overheating because the cooling system is blocked. This causes the coolant to go through the upper part of the radiator more. This makes it hotter than normal.

Another likely explanation is that some part of the radiator, such as a fan motor, thermostat, or water pump, has failed and this, too, disrupts the even flow of coolant around the system.

It may be wise to enlist the services of an experienced mechanic who can perform diagnostic tests on your car and recommend further steps if needed.

In any case, taking note of this anomaly can help you stay ahead before it becomes something more serious for your vehicle and wallet.

Terrence Hines

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