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Top Radiator Hose Supposed To Be Hot? (Beginners guide)

It’s not fun sitting in traffic watching the temperature needle climb towards the red, especially if you suspect the thermostat isn’t opening. Yea, we’ve all been there. You’re in the right place, all your questions are covered here.

The top radiator hose is usually but not always the location of the thermostat. Either way, the top hose gets hot after the thermostat has opened. The time the thermostat takes to open depends upon factors such as:

In this post, you’ll learn how the coolant system works when the top hose should get hot and what you should do if it doesn’t.

Coolant System Overview

The coolant system after the oil system is the next most critical system. A relatively simple and inexpensive issue can develop into something far more serious and expensive. So you are absolutely right to check out your suspicions. Here’s a quick overview.

The coolant system isn’t complex. Having an understanding of what things are and what they do will help shorten your diagnosis. The thermostat is often simply referred to as “The Stat”.

Engine coolant system

Components of the system:

  • Coolant – Specially formulated to raise boiling point (226 deg F) and lower the freezing point of the coolant (-34 deg F). It also contains a lubricant and anti-corrosive properties. It’s good for about 3 years after which it becomes harmful to your engine as it becomes acidic, which eats seals, gasket and metal.
  • Radiator – Uses airflow over large surface area to help reduce coolant temperature.
  • Fan – Electrically powered and often with it’s own module (PWM Pulse Width Modulated) controlled ultimately by the PCM. Usually kicks in at slow speed at about 203 deg F and full tilt 221 deg F.
  • Water-pump – Used to move the coolant throughout the system. Driven by aux belt or timing belt. The water-pumps typical life span is about 100k miles.
  • Engine coolant passages – Passage ways through the engine block and head that carry coolant and heat away.
  • Coolant temperature sensor – Sensor readings used by the PCM to change fuel trims and to operate the cooling fan. Often more than one sensor used.
  • Cabin heater core – The Core is not unlike a radiator, it’s used to transfer engine heat to the cabin. The heater fan blows air across the core to demist the windshield and/or warn occupants.
  • Thermostat – Used to regulate coolant flow around the engine. The stat remains closed until about 180 deg F, this helps the engine warm up quickly. Stats are typically a simple thermo mechanical assembly but some vehicles use an electronically controlled stat.
  • Hoses – Rubber hoses or silicone used to carry coolant from rad to engine and engine to heater core.
Coolant system info graphic

How Does A Stat Work

Coolant is as you know designed to cool your engine, it moves around the system in a circuit. The thermostat has a special function within the circuit, its job is to restrict coolant flow back to the radiator when the coolant temperature is below 180 deg F (83 deg C).

Above this temperature, a wax-filled cylinder within the stat melts and expands causing the valve to open proportionally until fully open at about 199 deg F (93 deg C). Coolant is then free to move from the engine to the rad and back to the engine again.

As the coolant cools the wax contracts and a tensioned spring closes the valve once again.

This is a continuous cycle and is important, a car’s engine isn’t efficient until it reaches and maintains operating temperature. Modern cars as you know use computers to control fuel and emissions, but those systems don’t work until the engine is warm.

The stat blocks flow so as to get the engine up to operating temperature ASAP. And as the heater core is plumbed on the hot side of the stat, your toes get warm too.

Testing The Thermostat

I use an inferred gun in the workshop but there’s a couple of other ways to test the stat. Note while stats are usually fitted to the top hose they can in some models be fitted to the bottom hose.

Testing stat in place

Coolant hose

Start the car from cold, run the engine for 10 minutes or so, and check that one hose is warm and the other cold. That indicates the stat is closed as it should be, so far so good. Now drive the vehicle until normal operating temperature is reached. Shut off the engine and check both hoses, they should now both be warm.

If they are not, your stat is not opening, it will need to be addressed immediately as you’ll run a serious risk of engine damage.

If the on the other hand your stat is stuck open (both hoses warm from the get-go) you can continue driving but the faulty stat will cause your engine to run poorly until warm and will eat gas.

Removing stat to test

Thermostat testing

Using a pot of warm water, dunk the stat into the water and heat it slowly, a kitchen thermometer is useful to measure the opening but visual works too. Bringing the pot off the heat will cause the stat to slowly close. This is a normal function, but the temperature it opens at is important. Stats aren’t expensive, if in any doubt change it out. Stat should be changed every 3 years.

Replacing The Thermostat


Before attempting this project you should know, the coolant system will need to be drained refilled and air bled. Failing to bleed the system will cause an overheating condition which could cause engine damage.

Replacing the thermostat is a job you can do at home, but it does cause a mess on the driveway, you’ll need to invest in a coolant catch tray. The coolant is sweet and colorful, it’s attractive to children and animals, every precaution should be taken to catch and dispose of it carefully.

To replace the stat, first

  • Remove the splash tray and push the coolant catch tray in.
  • Remove the thermostat cap, usually on the top hose at the engine. Note the stat’s orientation, it is possible in most cases to fit it back-ways, which causes an overheating condition.
  • Clean the stat seat on the engine and cap.
  • Lube the new stat seal with fresh coolant and fit it, check the orientation (Jiggle valve up).
  • Tighten down the cap bolts and torque to approx 10Nm (89in lbs).
engine coolant system

Most coolant systems use a rad and reservoir tank. Now the system needs to be drained completely and bled. You may find parking on a slight incline (engine uphill) helps trapped air escape. A no-spill funnel makes bleeding the coolant system a ton easier, you can check it out here on the Coolant system tools page.

  • Reposition the catch tray below the rad bottom hose
  • Remove the bottom hose (or drain valve) and drain
  • Refit the hose
  • Remove coolant system bleed screw (if fitted)
  • Fill the coolant reservoir with premixed coolant until full
  • Fit the bleed screw (If fitted)
  • Fit reservoir cap
  • Start the engine and turn on cabin heat and fan to full
  • Check coolant level, if it drops significantly, shut off engine and top up
  • Check temperature gauge and cabin heat
  • Bring engine up to 2000 rpm and hold for 5 minutes while checking cabin heat and checking gauge
  • Check cabin heat, check coolant level, shut off engine and allow cool if top up is necessary
  • Test drive vehicle but only if cabin heater is hot
  • After test drive, check for leaks
  • Check top and bottom hose are warm
  • Allow the engine idle until the cooling fan operates
  • Allow engine cool before adjusting the coolant reservoir once again, if needed

Note some vehicles can be difficult to bleed, especially minivans with rear heating systems. If it’s proving difficult to get heat from the cabin, jack the front of the vehicle so that the coolant reservoir is the highest point in the system.

Faulty Thermostat Symptoms

A stat can fail in one of two ways, stuck open or stuck closed, closed being more common and serious. The symptoms differ for each. I’ll bet you already know some of these symptoms, but here’s a full list.

Stuck closed:

  • Engine check light on
  • Engine overheating
  • Steam
  • Constant topping up of coolant

Stuck open:

  • Hard on gas
  • Takes forever to warm up
  • Cabin is always cold
  • No heat from the heater

Related Questions

How can you tell if the thermostat is stuck open? If the top and bottom radiator hose are equally warm after a cold start. The thermostat is likely stuck in the open position.