Why is My Lower Radiator Hose Cold?

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lower radiator hose cold

I was driving to work the other day when I noticed my upper hose was hot, but the bottom one was cold. It’s been really hot lately, so I didn’t think much of it and continued on my way. But then it started to bother me, so I decided to look into it.

I did a quick Google search and found out that this is a common issue with cars. The heat from the engine can cause the top radiator hose to get hotter than the bottom one. So why is this?

The answer has to do with how radiators work. So, in this article, we will cover reasons for the cold lower radiator hose & the relation of this problem with the working of the entire cooling system.

Cooling System Overview

Some car owners might neglect their cooling systems, especially if they do not really understand the significance of the coolant system. It will not be a bad idea for you to familiarize yourself with how it works so that you can take good care of your vehicle before something bad happens.

The coolant system of the car functions by providing the engine with a constant coolant flow, which ensures that heat dissipates from different areas of the engine block. This is to prevent the temperature from rising too high and causing damage to other components inside your engine.

If this happens, the engine will most likely fail. The fluid used in a car’s coolant system is usually made up of water and ethylene glycol.

The function of your vehicle’s radiator is to provide heat for the coolant contained inside its heater core. It does this by sucking in air from outside and letting it flow through the tubes on the inside of its core.

Once this air has absorbed enough heat from your engine, it will exit your radiator and go back into the atmosphere. This process is repeated over and over again until the coolant in your car’s coolant system has reached a desirable temperature range.

Another component involved in keeping your engine stable and at the right temperature is the thermostat. This component is responsible for controlling how much heat your engine can release once it receives a supply of coolant.

When the outside air is cold, the thermostat will allow more hot coolant to pass through its core. Once the engine has warmed up, it will start restricting the flow of coolant by closing its valves. The coolant is directed to the radiator and back into your engine block instead when this happens.

Purpose of Top and Bottom Hose

Both Radiator Hoses are known to be part of the engine coolant system, but there seems to be confusion between what each hose does. For clarification, here is a quick summary below:

Lower Radiator Hose

The bottom hose runs from the lower radiator fitting on your vehicle’s radiator to an area just below your vehicle’s engine. This ‘lower’ area of the engine is usually where your transmission fluid (e.g., gear oil) is held in reserve for cooling or lubricating the gears inside your transmission. Depending on the year, make and model of your car, you may also find some coolant here as well – but continue reading to understand more.

Lower Radiator Hose Job & Purpose:

The lower hose is responsible for transporting the liquid coolant from the engine, through the lower hose, and into your transmission. This ‘transmission cooler’ effect helps to maintain a better operating temperature of your engine by taking heat away from your engine and into your transmission. This means that by having a bottom hose burst or split open, your engine is more likely to overheat if you do not have a replacement immediately.

Top Radiator Hose

The top hose runs from the upper radiator fitting on your vehicle’s radiator. It leads directly into an area often referred to as ‘radiator’ or ‘header tank’ on the top of your engine. This top hose part is responsible for transporting the liquid coolant from the radiator through to the engine and back into the lower radiator hose. In some cases, if you have a coolant system leak, you will find that this hose has been supplying your engine with additional coolant – very bad news if this is the case.

Top Hose Job & Purpose:

The upper hose will often have a ‘header tank’ on top of your engine. Depending on its size and shape, this header tank is only there to supplement or assist your vehicle’s coolant system in keeping its ideal operating temperature under normal conditions. This means that if your upper radiator hose bursts or splits open, you are not likely to have the engine overheat due to a lack of liquid coolant entering into the engine.

Lower Radiator Hose Cold: Possible Causes

In a situation where the lower radiator hose is cold but the upper and both heater hoses are hot, there could be several problems with your vehicle’s coolant system.

  1. Faulty Radiator Pressure Cap
  2. Defective Water Pump
  3. Blocked Radiator Core or Condenser
  4. Low Coolant Level
  5. Air Pockets
  6. Bad Thermostat
  7. Loose or Slipping Water Pump Belt

Let us see these problems in a bit of detail:

1. Faulty Radiator Pressure Cap

The first thing to do when you are experiencing symptoms like this is to check the condition of your pressure cap. Your radiator cap should have a pressure range printed on it, usually between 13 psi and 16 psi. If you are outside these parameters, it might be faulty or no longer offer enough pressure to keep the coolant under enough pressure.

To test this, you can try removing the pressure cap and replacing it with a new one. Once you have your new (or old if possible) pressure cap in place, start up your engine and watch closely for any changes.

2. Defective Water Pump

Another cause of this problem could be a defective water pump. If you remove your pressure cap and the upper radiator hose is hotter than usual, it may indicate air is trapped in the coolant system due to an inadequate water pump or circulation issue.

The temperature gauge on your dashboard will enter the red region due to this issue. Remove the upper radiator hose from the thermostat housing to test for this. If there isn’t enough pressure to force coolant out of the hose, it is an indication that your water pump may not be circulating adequately.

Proper coolant system circulation relies on adequate water pump function and sufficient power steering fluid pressure (which pushes coolant from the water pump into the radiator).

3. Blocked Radiator Core or Condenser

If you have a vehicle sitting for any length of time, it is possible that insects could have built nests around the heater core or condenser. This mostly happens with older vehicles with exposed heater core or condenser.

If the heater core or condenser becomes blocked, it can prevent proper coolant system function and cause coolant pressures to rise inside the engine.

In this situation, you should remove your bottom radiator hose and use a screwdriver (or something similar in shape) to carefully scrape away any debris blocking the radiator core/condenser in your engine.

4. Low Coolant Level

If you have recently added coolant to your engine and the bottom radiator hose is still cold, it could indicate that you have a low coolant level in your car’s coolant reservoir.

For this issue, try adding more fresh coolant as soon as possible.  You should also bring your vehicle to a qualified mechanic for further inspection and diagnosis of the root cause of the issue.

If the problem is only related to the coolant level, it should not be too serious of an issue.  However, if this happens after adding coolant and there is no visible evidence of a leak (or external leaks are found), you should bring your car in for further inspection.

5. Air Pockets

As stated earlier, air pockets in the coolant system can cause problems such as making the lower radiator hose cold.  They can also result in a lack of power and overheating.

The most common cause of air pockets is trace amounts of air introduced into the engine either through the intake or by way of an improperly installed component (related to the cooling/exhaust system).  This air will cause the coolant system to lose its prime and not function properly.

Remove your lower radiator hose and start up your engine to check for this.  If there is no visible coolant in the hose, it could indicate that you have an issue with the air pockets.

One possible solution that can help remove air pockets is using a coolant system pressure tester.  You will need some sort of pressure gauge (which can be purchased at most auto parts stores) and the special adapter(s) needed for this process.

Using the correct adapter, you can pressurize your coolant system through the radiator cap.

6. Bad Thermostat

If the problem with your vehicle is only happening when you are stopped, it could indicate a bad thermostat.  In this case, you should have some type of diagnostic test performed on your engine to determine if there is a faulty component in play.

A faulty thermostat can contribute to overheating and an inability for the cooling system to function.  The important thing to note is that this should not cause your cooling fan to come on even when you are stopped because the coolant system has not reached its normal operating temperature at that point in time.

If you have ruled out all other possible causes, it would be best to bring your car to a professional to have it checked.

7. Loose or Slipping Water Pump Belt

Finally, you should check your vehicle’s water pump belt and make sure that it is in good condition and not loose.  A loose belt can limit the amount of power steering fluid being pushed from the system to the radiator.

In some cases, this can also cause a humming noise when driving at certain speeds.  If this is the case, you should replace your belt as soon as possible.

A slipping or losing water pump belt on your engine can cause many problems, such as it can make the lower radiator hose cold.  It will prevent proper heat transfer and decrease the overall performance of your coolant system.  It may also contribute to overheating of your engine.

How Do I Fix a Cold Lower Radiator Hose?

If you notice a wet patch or steam spewing from the car’s front, it’s time to inspect the lower radiator hose.

  1. Before you even begin working on your vehicle, turn off its engine and allow it to cool completely.
  2. Use gloves and protective eyewear when checking out the area where you suspect the hose to be.
  3. Look for debris or loose screws around your engine bay, and remove them with a magnetic pick-up tool.
  4. After you’ve done this, look at the radiator hoses carefully. If the lower hose’s rubber appears soft and spongy, there’s a chance it has burst or sprung a leak somewhere along its length.
  5. If you can’t find any clear signs of a problem, look at the upper radiator hose and then check out the coolant levels in your vehicle’s reservoir.
  6. If there is air in the cooling system, you’ll need to add some water-based coolant until it reaches its maximum level or is about one inch below the filler neck.


The cooling system in a car can be considered the engine’s heart. It is responsible for maintaining a stable temperature inside the engine block, which is essential to ensure that your engine runs smoothly and does not break down easily.

The importance of this system cannot be stressed enough. In fact, it will only take an average of 30 minutes for the engine to overheat and become useless once this system is no longer functional. Your cooling system’s lower hose should be checked regularly for signs of wear and tear.

There are a number of different reasons why they’re in need of replacement, but if you spot any damage or see your vehicle’s fluid levels dropping rapidly, get the issue checked out quickly. If your car’s manufacturer recommends replacing a lower radiator hose every two years or so, this is usually the case.

It’s also important to inspect your vehicle’s upper radiator hose and coolant reservoir on a regular basis too. If both of your Radiator Hoses burst or split, it is very important that you get them replaced as soon as possible in order to prevent your engine from overheating.

If you only have a Radiator Hose burst on the Upper Radiator Hose, it is more important that you get this replaced as soon as possible to avoid over-supplying the engine with liquid coolant.

Frequently Asked Questions

What are the signs of a clogged radiator hose?

You may detect a problem with your radiator hose by its smell. You might notice the strong scent of antifreeze, often mixed with a sweet fragrance if it is leaking from an un-pressurized hose–such as one leading to the heater core.

Leaking from a pressurized hose, such as those connected to or leading from the engine, can emit a watery or wet-dog smell as the coolant is forced out and the under-hood heat causes vaporization.

However, the most common sign of a clogged radiator hose is simply reduced performance. You may not be able to notice antifreeze seeping from the hoses, but you will probably notice if your car takes longer to warm up, loses power more quickly, and doesn’t idle as smoothly. The reduced radiator hose size or restricted coolant flow can also cause overheating.

Why is the lower radiator hose larger than the upper hose?

The lower radiator hose is larger than the upper radiator hose because it carries more fluid. Engine coolant flows through the engine and then into the radiator to be cooled. As it passes through the radiator’s cooling fins, heat is transferred from the coolant to the air passing over the fins.

The hot coolant (now less dense and more buoyant) rises to the top of the radiator, where it is drawn into the upper hose by the water pump. The suction created in the pump pulls more and more air into the pump until a full coolant column is formed (from bottom to top of the radiator).

This creates suction at the pump’s outlet port, and if the outlet hose is the same size as the inlet, it will tend to draw air into the system. Air bubbles in the coolant are bad because they cannot transfer heat efficiently and take your engine longer to warm up.

So it’s important not to restrict flow through your hoses by using hoses that are too small for their job.

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