If you have a problem with your coolant temperature and realize that the lower radiator hose is cold instead of hot, you could be experiencing problems with the coolant system that needs to get fixed.
You will need to look at the various parts that influence the radiator temperature to properly diagnose a lower radiator hose cold problem.
So, what causes a lower radiator hose to stay cold?
The most common reasons why your lower radiator hose is cold are a faulty thermostat, bad water pump, or low coolant level. It can also be caused by airlocks or a clogged coolant system.
Here is a more detailed list of the 6 most common causes of a cold lower radiator hose:
6 Causes of a Cold Lower Radiator Hose
1. Faulty thermostat
This is the most common problem when it comes to a cold lower coolant hose. Engines have a valve-based thermostat that determines and regulates coolant inflow so that the engine operates at an optimal temperature and does not overheat.
If your car is overheating and at the same time won’t heat the lower coolant hose – You most likely have a problem with your thermostat, which needs to get replaced.
The thermostat should be closed when the engine is cold and not running. Regulation of water by the thermostat prevents engine hot spots. Once the optimal temperature has been reached, the coolant water will be let into the radiator for energy dissipation.
If the thermostat is not opening at this time, it may cause the lower radiator hose to stay cold, and at the same time, overheat the engine.
2. Faulty Water Pump
The water pump‘s job is to pump coolant from the engine block towards the radiator to cool the coolant down.
If the water pump is not functioning properly, the water will stand still in the engine, and no water will get pumped to the radiator.
The lower coolant hose is most likely the coolant output side of your engine, which means that the hose will stay cold.
3. Low Coolant Level
A low coolant level can create a lot of problems in the coolant system. A low coolant level may create air pockets in the coolant system and may be causing the water pump to stay in an air pocket.
The coolant level is super easy to check, so open your hood and read your car manual to find the coolant reservoir. Remember never to open it when the engine is hot, though, as it is boiling and will splash everywhere if you open it.
4. Air in the Coolant system
Air pockets in the coolant system are something you really do not want to have there. Air pockets can create overheating and may stop the circulation from working properly.
It may also create an air pocket around the water pump, making it can’t pump water anywhere.
The coolant system’s air can come from a faulty head gasket or after a coolant fluid change. If you know that you recently changed coolant – you need to bleed the system properly.
Air in the coolant can also have been caused by a too low coolant level.
5. Clogged Coolant System
In some rare cases, the coolant system can get clogged. This is not very common, though, because the main coolant pipes are pretty big. However, if someone before ran your car without coolant and just water, it can have been created a lot of rust inside the engine.
This, together with a leaking head gasket, can create a lot of mess inside the coolant system. Try to flush the coolant system if you tried everything else.
6. No problem at all
In many cases, when it comes to a cold lower radiator hose, there is actually no problem at all! The engine needs to be really hot and almost overheat before the thermostat lets the coolant pass by.
If your car is not overheating, but the lower coolant hose is cold when you feel it – it may just mean that it is not hot enough, and therefore there is nothing wrong with your engine.