Top Radiator Hose Hot and Bottom Cold: Causes and Sure Fixes

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When the top radiator hose hot and bottom cold in your car, then you should worry because the engine of your vehicle could soon suffer overheating. This is a common occurrence with several causes, making it difficult for drivers to pinpoint what is wrong with the radiator hose. Top Radiator Hose Hot and Bottom Cold In this article, you’ll discover a list of typical causes of the problem and how to solve each of them. We’ll also provide ways to avoid this problem and answer a few common questions on the topic.

What Typically Causes Top Radiator Hose Hot and Bottom Cold?

The typical causes of the top radiator hose becoming hot and the bottom radiator hose cold are a faulty open or closed thermostat, a restricted cooling system, and failed water pump. Other reasons include a dripping head gasket, collapsed hoses, and a clogged radiator.

– A Faulty Thermostat Failing To Regulate Engine Temperature

The thermostat is a device that regulates the temperature of the engine by opening and closing to allow the coolant to circulate. When the engine is getting too hot, the thermostat valve opens to allow the coolant to counteract the excessive heat. However, when the engine is becoming cold, the thermostat valve closes to reduce the flow of the coolant. Therefore, if the thermostat is stuck open, it’ll allow excess coolant to flow through it, depriving the engine of reaching its optimal temperature for operation. This leaves the top hoses hot but the down ones cool as the coolant is unable to flow through the radiator properly. If the thermostat is stuck close, then it won’t allow enough coolant to go through the radiator rendering the top hose hot. The engine’s optimal temperature for operation is around 105 degrees Fahrenheit, thus any figure beyond that will impact the engine’s performance. Fixing a bad thermostat depends on the position it was stuck in (opened or closed), which we will soon discuss.

– Malfunctioning Components of the Engine Cooling System

If you check the thermostat and it works perfectly, then the next place you should consider is the entire cooling system. The system is responsible for keeping the engine’s temperature from soaring. However, if the system is blocked, the coolant is unable to flow properly, leaving some parts of the radiator hot including the top hose. One part of the system that should be on your list is the water pump. The pump’s duty is the circulate the fluid (coolant) through the system but a faulty one won’t be able to do that. This keeps the coolant stagnant, making the top hose of the radiator hot. Unfortunately, one of the car parts that fail frequently is the pump, therefore ensure you inspect it when your engine is overheating.

– A Damaged Head Gasket Allowing the Coolant to Leak

Another typical cause of a hot upper radiator hose and a cold bottom hose is a leaking gasket. A gasket seals the engine and prevents the coolant from leaking into it. However, if the gasket is broken, the coolant will start dripping into other parts of the vehicle.
This will reduce the quantity of the required coolant to keep the engine running smoothly. Thus, the radiator won’t have enough coolant running through it, rendering the top hose hot.

– Collapsed Hoses or Pockets of Air

Coolants rely on unrestricted flow through their hoses and around the engine to keep it running at optimal temperatures. Any obstacle that obstructs the free flow of the coolant will undoubtedly cause the top radiator to heat up. Some of these obstacles are pockets of air and collapsed hoses. Air pockets can also cause cavitation; the forming and collapsing of air bubbles in a liquid which is detrimental to the engine. Collapsed Car Hoses   Air pockets usually occur when the one who bled the radiator did the work poorly. Also, a damaged gasket can allow air into the system, paving the way for the formation of air pockets. A collapsed hose may restrict the flow of the coolant and increase the reading on the coolant temperature sensor. This could prove detrimental to the engine’s health as it might cause overheating. Sometimes, a vacuum problem as a result of a damaged radiator cap can be blamed for the collapse of the radiator hoses.

– A Congested Radiator Restricted by Rust or Dirt

A blocked radiator might occur when you use only water in the cooling system or the coolant may have been contaminated with some sort of debris. This restricts the flow of the coolant, making the upper radiator hose extremely hot. Reasons of Top Radiator Hose Hot and Bottom Cold Another common cause of congestion in the radiator is rust, which can build up over time as water comes into contact with metal. If you also reside in a dusty environment, then you may suffer constant radiator clogging. Traveling on sandy or graveled roads can also lead to the blockage of your radiator. A congested radiator could hurt the engine’s health as it can cause overheating or low coolant levels. The symptoms of a congested radiator include a change in coolant color, a leaking coolant, bent radiator fins, damaged radiator hoses and engine overheating. Though some of the signs seem harmless, they could damage the radiator and the engine as well.

What Are Practical Ways To Fix a Top Radiator Hose Hot Bottom Cold?

The practical ways to fix a top radiator hose that is hot and bottom cold include fixing the defective thermostat, repairing the restricted cooling system, fixing the dripping gasket, and cleaning the radiator if it is restricted by dust or rust. There are several methods we can utilize to fix this issue depending on the cause. Some of the methods are labor-intensive and costly while other methods are easy and straightforward. Below are the various ways you can fix some of the problems discussed in the earlier paragraphs.

– Fixing a Defective Thermostat

As already discussed, the thermostat can get stuck in two ways — either it is stuck open or stuck close. However, these two different issues involve the replacement of the thermostat and you’ll require a screwdriver, a socket wrench, pliers and a microfiber cleaning cloth. Fixes of Top Radiator Hose Hot and Bottom Cold To begin, locate the device which is near the radiator on the side of the engine. Next, use the screwdriver to remove the fasteners holding the thermostat in place and then remove the thermostat from its housing with a socket wrench. Replace the old thermostat with the new one and put it back in its original position while holding it in place with the fasteners. Now, turn on the engine and allow it to run for a few minutes to check whether the problem still persists. If it does, the issue is certainly not with the thermostat, thus you have to look elsewhere. In cases where the thermostat is sticking or binding, just remove the device and clean it and it’ll work alright.

– Repairing a Restricted Cooling System

The first step to fixing a restricted or clogged cooling system is to determine what is causing the clogging. Usually, the blockage is caused by debris or a build-up of sludge, in which case you’ll need to flush the cooling system. You can use warm water to flush the cooling system and it’ll get rid of all the accumulated debris or sludge. However, if you determine that the fault is from the radiator (probably too small for the engine), then you’ll need to replace it. If the restriction is caused by a faulty water pump, then it must be replaced to get the cooling system working again. Ensure you use the right water pump designed for your vehicle because the wrong one may not work well. Remember to bleed the cooling system, which is the act of removing air bubbles to allow the system to work optimally. You can also check the hoses for cracks or leaks and then replace them.

– Fixing a Dripping Gasket

When a gasket is broken, the only solution is to replace it, which is a tricky thing to do. First, carefully remove the old gasket from the engine block with a plier. Next, wipe the surface of the block clean with sandpaper and place the new gasket on the engine block. Now, replace the cylinder heads to their former positions and pour coolant and engine oil into the engine. That should do it for repairing a worn-out gasket.

– Cleaning a Radiator Restricted by Dust or Rust

Allowing the radiator to sit for a long time without being used or using it for a long period without changing its oil makes it prone to rust. To clean this rust, pour a large quantity of radiator flush into the said component and allow the engine to run for 15 minutes. This will ensure the liquid travels through the radiator. After the 15 minutes have elapsed, turn off the vehicle engine and allow the radiator to cool before going on to the next step. Now, place a bucket under the petcock/valve underneath the radiator and drain the liquid. Next, pour water into the radiator and watch it as it comes out through the petcock under the radiator. If the water is clear, then it means the radiator is clear of any filth.

Frequently Asked Question

– What Happens When Both Radiator Hoses Hot?

When both radiator hoses are hot, it is a sign that the coolant in the engine is hot, thus it needs replacement. Also, the top radiator hose being cold while the bottom radiator hose is hot is a sign of a hot coolant as well. Top Radiator Hose Hot and Bottom Cold Details

What Are Some Possible Solutions for a Car Overheating When Idling?

Car overheating issues while idling can have various causes. To tackle this problem, first ensure that your coolant levels are sufficient and the radiator is in good condition. Consider cleaning or replacing the radiator if it’s dirty or damaged. Additionally, check the cooling fan and thermostat for proper functioning. Regular maintenance, like changing the coolant and flushing the system, can help prevent overheating while idling.

Can a Constantly Running Radiator Fan Cause the Top Radiator Hose to be Hot and the Bottom Cold?

Should a constantly running car radiator fan concern you? Yes, it should. If you notice that the top radiator hose is hot while the bottom one remains cold, it could indicate a problem with the cooling system. A constantly running radiator fan may be causing inadequate circulation of coolant, leading to inefficient cooling. It’s essential to have this issue checked by a professional to prevent potential engine damage.


When the top radiator hose gets hot and the one at the bottom is cold, it is an indication that the radiator and the engine are overheating. Here is a summary of the causes and how to fix them:
  • A broken thermostat could be restricting the flow of the coolant and the only way to solve this issue is by replacing it with a new one.
  • The same applies to a leaking gasket which allows the coolant to leak, thus reducing the amount of coolant required to normalize the engine’s temperature.
  • If the cooling system is restricted, you’ll have to check it for the exact cause and then determine the best way to fix it.
  • However, if the radiator is clogged, then a large quantity of flush liquid will get rid of all the rust and debris accumulated in the radiator.
  • If all else fails, you can call on a professional mechanic to help you diagnose the problem and fix it.
Remember to bleed the system whenever you change a water pump to get rid of all pockets of air that may have gathered in the system.
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