Top Radiator Hose is Hot and Bottom One Cold: Is It Normal?

Check the cooling system of your car. You’ll notice a temperature difference in the hoses because of their regular operations.

The lower radiator hose needs to be at a lower temperature, but it’ll not be completely cold.

If it is, you are probably dealing with a clogged thermostat, insufficient coolant levels, a bad water pump, or a clogged cooling system.

Read on to find what’s normal and when you should worry.

The Purposes of Lower and Upper Radiator Hoses

Everyone knows that the radiator is responsible for keeping the overall temperature of your engine in check.

Your car will not perform optimally if it’s overheated or is below the operating temperature, so it’s quite important to keep things under control.

If we take a quick look at the operations of a cooling system, you can divide it into 3 separate components.

  1. The top hose: Responsible for transferring hot coolant from the engine to the radiator
  2. The radiator: Conducts heat exchange to cool down the incoming hot coolant
  3. The lower hose: Responsible for transferring the cold liquid back to the engine for heat transfer

In the entire cycle, a water pump circulates the coolant throughout the cooling system and ensures proper operations of the vehicle.

There are other additional components like the fan and thermostat that facilitate the cooling process and maintain the operating conditions of your vehicle.

Normal Temperature of the Lower Hose

As stated before, a hot top radiator hose and a relatively cold lower hose indicates normal operations of the radiator. The coolant enters the engine for heat transfer through the lower hose as your thermostat switch opens up.

However, you must remember that the lower hose shouldn’t remain completely cold.

Normally, a functional cooling system will have a lower hose temperature around 150F, which will seem warm to you. Anything lower than that shows that something is wrong.

Common Causes of Cold Lower Radiator Hose

Now that you know how hot or cold your top and bottom radiator hoses need to be, let’s look at some of the possible reasons why your lower hose might be getting too cold.

This is usually due to a small issue but may lead to further problems down the road if you don’t address it immediately.

Here are some of the most common reasons behind a cold lower radiator hose.

1. Bad Thermostat

This is the most common reason behind most faults in a vehicle’s cooling system.

The thermostat is a simple device but plays a key role in maintaining the temperature of your engine, so you should regularly check its operations.

During regular operations, the thermostat switch works by opening and closing which regulates the coolant flow and keeps the engine temperature in check.

A faulty one won’t do that. It’ll instead stay stuck at a single point and obstruct the flow, causing your engine to overheat.

A colder lower radiator hose is one of the giveaway signs that your thermostat is not operating. During regular operations, the radiator takes off most of the heat from the coolant, but it still remains warm.

Because of that, your radiator hose will have an optimum temperature of around 150F.

Anything colder than that means there is no flow and that is probably because of the thermostat.

In case of a faulty thermostat, your best bet is to replace it immediately.

It is quite small and doesn’t even cost much. However, that doesn’t mean that it’s not important.

Delaying the repair can cause your car to overheat, which can lead to further problems that are costly and time-consuming to repair.

2. Faulty Water Pump

In case your thermostat is operating properly and you still feel that your bottom radiator hose is cold. Then the next probable cause is a bad water pump.

As stated before, the water pump is responsible for keeping your cooling system functional and circulating the fluid.

A bad pump will not do that and keep the fluid stagnant, thereby overheating the car.

Unfortunately, water pumps do fail regularly, so you need to keep an eye on them.

One of the best ways to ensure the maximum life of your water pump is by ensuring that your coolant levels are always sufficient.

The pump in your coolant system is a regular centrifugal pump, which means that it is designed to operate with a fluid present. Lack of fluid will cause it to overheat and lose its primary function.

Repairing your water pump might be a little difficult if it’s too far gone. Your mechanic will be the best guide in this situation but you should be prepared to buy a new one in such cases.   

3. Insufficient Coolant

Think of the coolant like its blood in your body. Will you function properly in the absence of it? Similarly, your vehicle’s cooling system will not work if your coolant levels are low.

Fortunately, checking your coolant level and maintaining it is super easy. You can open your radiator cap directly or check the coolant reservoir.

Top off if required but do check for any leakages if you find yourself doing it quite regularly.

One important thing to remember is that you should always check the coolant levels on a cold engine.

Your coolant will be above its boiling point in regular pressure when the engine is hot. Opening the cap at that time will splash the coolant everywhere and cause problems for you.  

4. Air Pockets or Collapsed Hoses

One of the primary requirements for a vehicle’s cooling system is an uninterrupted coolant flow.

Air pockets and collapsed hoses prevent that and may cause the car to overheat and sustain damage.

Not only that, but air pockets also cause cavitation and other issues in your cooling system, which can cause a series of other issues in your engine.

One of the most common causes of air pockets in your engine is a shoddy job at bleeding a radiator. If you recently had your coolant changed, then make sure that you bleed the system properly.

Another serious problem that can cause air pockets in your engine’s cooling system is a damaged head gasket.

In that case, you must immediately contact a mechanic because a faulty gasket can be an indicator and cause further issues in your engine.

5. Clogged Radiator

If you are not careful about the coolant you use or may have used only water in the cooling system, then there is a chance that your radiator is clogged.

A clogged radiator will naturally keep your bottom radiator hose cold because there won’t be enough coolant flow.

The best way to counter this issue is to flush the cooling system. Doing so would take away all the rust ad any other obstruction in the cooling system to give you a better overall flow.

Frequently Asked Questions

Should both radiator pipes be hot         

No, both radiator pipes are not supposed to be of the same temperature. One of them (the lower one) acts as an inlet to the engine and the other is an outlet.

If the lower one is already hot, then the heat transfer from the engine won’t be as effective and may cause your engine to overheat.

Which radiator hose should be hotter?

The top radiator hose needs to be hot because its function is to take away the hot fluid from the engine to the radiator.

The radiator then takes the heat away from the coolant and sends it back to the engine to continue the cycle.

My lower radiator hose is hot. What to do?

A hot lower radiator hose means ineffective heat transfer at the radiator or a bad thermostat. Both are easy to repair but ignoring them may lead to further complications.

The best course for you is to get it checked asap and ensure that the cooling system of your radiator is operating at its best.

The upper radiator hose is cold. What to do?

The top radiator hose is not supposed to be cold and its job is to take away the hot coolant from the engine to the radiator.

If it’s cold, then it means that there might be an issue with your water pump or the thermostat.

Final Thoughts

To sum up, a hot top radiator hose and a warm bottom one indicates normal operations. If your car is not overheating and all systems are working properly, then you have nothing to worry about.

The problem arises when the bottom radiator hose is cold, not warm.

This means that you either have a faulty thermostat, water pump, or may even be dealing with leakages, blockages, or a simple issue of insufficient coolant.

Nevertheless, it’s something that you should get checked as soon as possible because a functional cooling system is essential for the optimal operations of your vehicle.


Dean Alvarez, TireForge Head Author

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