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How to Check Your Cooling System

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Radiator shroud, upper radiator hose, and coolant reservoir (left) are typical cooling system components.

Radiator shroud, upper radiator hose, and coolant reservoir (left) are typical cooling system components.

Usually, the first sign of cooling system problems will come from the temperature gauge indicating an engine running consistently warmer or cooler than normal.

Common causes of overheating include:

  • low coolant level
  • low engine oil level
  • coolant leaks
  • loose drive belts
  • faulty water pump
  • failed radiator cap
  • stuck-close thermostat
  • clogged reservoir return line
  • restricted radiator
  • incorrect ignition timing (on adjustable models)
  • blocked passages in cylinder head or block

Common causes of overcooling include:

  • stuck-open thermostat
  • locked fan clutch (continuously running fan)
  • shorted fan switch (continuously running fan)

Often, you won't get a visual clue to the source of the potential problem. For example, you may not realize from a quick inspection that your water pump is failing, the radiator is clogging, or that your thermostat is unable to close or open appropriately.

Under these conditions, you need a more systematic approach to checking the most common sources of trouble in the cooling system. The following sections deal with the main components of the cooling system and the key points you should be checking or inspecting for each. This can help you speed up your diagnostic.

Checking Coolant Level

Before you start, though, begin your diagnostic by checking the coolant level. Often, topping off the cooling system reservoir to the appropriate mark will solve your problem.

Make sure to use the correct antifreeze for your particular model. If necessary, consult your car owner's manual.

If you don't have a repair manual yet, you can buy a relatively cheap copy through Amazon. Haynes manuals include photographs, images, descriptions for the different systems in your vehicle, step-by-step instructions for troubleshooting, parts installation and maintenance. You'll also know the appropriate torque to use when installing components. So you'll recoup your small investment in a short period of time.

Better yet, even if you don't know much about car repairs, the manual will help you prevent unnecessary breakdowns by following the maintenance schedule listed in the manual.

Beware of Coolant Contamination

Failing to properly service the cooling system makes contamination a common issue.

Here are some points to follow:

  • Regardless of the type of coolant color your car uses, it should be translucent.
  • If the coolant turns rusty, flush the system and add new coolant.
  • If the coolant is opaque, milky or murky, it's contaminated and you should find the source of the problem, probably a leaking gasket, intake manifold gasket or transmission cooler.

Before You Start

Before you start the diagnostics below, check your coolant level. Often, topping off the cooling system reservoir to the appropriate mark will solve your problem.

Index

1. Is Your Engine Actually Overheating or Overcooling?

2. Retrieving Trouble Codes

3. Checking the Thermostat

4. Checking the Radiator

5. Checking the Cooling System Hoses

6. Checking the Cooling System Fan

7. Checking the Drive Belt

8. Checking the Water Pump

9. Checking for Cooling System Leaks

10. Performing a Cooling System Pressure Test For Internal Engine Leaks

11. How to Prevent Cooling System Problems

Usually, the first sign that something is wrong with your cooling system is the temperature gauge.

Usually, the first sign that something is wrong with your cooling system is the temperature gauge.

1. Is Your Engine Actually Overheating or Overcooling?

The most common problem you'll encounter with your cooling system is overheating. Usually, you'll see the needle on the temperature gauge rise to the higher end of the red zone.

If the engine is not reaching operating temperature, you'll see the needle in the temperature gauge staying in the lower end of the gauge, never reaching the usual middle point.

Whatever the case, you need to confirm that your temperature gauge is reading correctly.

The most practical tool for this task is an infrared thermometer, or a digital multimeter (DMM) that has a temperature probe. If you don't have either of these tools, you still can use a simple kitchen thermometer.

  1. Engage the emergency brake. Start the engine and allow it to idle. If your engine is overheating, run it until just before the temperature on the gauge rises to the red line; if your engine is overcooling, run it for 15 minutes.
  2. Turn off the engine.
  3. Take a temperature reading of the engine block near the thermostat housing, and jot down your reading. (The thermostat housing connects to the upper radiator hose, on the engine side.)
  4. Take a temperature reading of the inlet radiator tank. This is the tank that connects to the upper radiator hose.
  5. Take a temperature reading of the outlet radiator tank. This is the tank that connects to the lower radiator hose, which connects to the water pump.

Most engines have an operating temperature range of 195° to 220° F (90° to 104° C).

If there's a great difference in your first two readings, with an engine block temperature higher than normal operating range, this may indicate that coolant is not circulating. Possible causes include:

  • stuck-close thermostat
  • failed water pump
  • restricted or clogged radiator

If you don't know what part of the system might be causing the problem, go over the sections below.

If your three readings are close to each other on the lower end, it's possible your thermostat is stuck open and the coolant is continuously circulating. Again, if you don't know what part of the system is causing the problem, go over the sections below.

If your reading seems normal, first two readings on the higher end and the third reading in the lower end, possibly the temperature gauge isn't working correctly. Check the temperature gauge electrical circuit.

A generic scan tool.

A generic scan tool.

2. Retrieving Trouble Codes

On modern vehicles, the computer is capable of detecting some cooling system problems. Even if your check engine light is not on, scan the computer memory for codes, or have your local auto parts download the codes for you.

If there are any codes stored, they can guide you in the right direction to fix a cooling system problem.

3. Checking the Thermostat

You can do a quick check of the cooling system thermostat operation using your thermometer.

  1. Start the engine and let it idle for about 15 minutes, so the engine reaches operating temperature; or just before the temperature gauge reaches the red zone, if you think the engine is overheating.
  2. Take a reading of the upper radiator hose, at a point close to the radiator.
  3. Take a reading of the engine block, close to the thermostat housing.
  4. Shut off the engine.

Compare your two readings:

  1. If the hose reading is significantly lower than the engine reading, the thermostat is stuck closed or not opening correctly.
  2. If both readings are similar but lower than operating temperature, the thermostat is stuck open.
  3. If both readings are similar but around operating temperature, then the thermostat is working properly.

Also, touch the lower radiator hose, after the engine has reached operating temperature. If the lower hose is hot to the touch, coolant is circulating. If the lower hose is not hot, it's possible the radiator is restricted.

Just be aware that a thermostat may open initially and then fail to close properly. If you want a more comprehensive test, this other post will help you troubleshoot the thermostat.

A faulty radiator neck can cause engine overheating.

A faulty radiator neck can cause engine overheating.

4. Checking the Radiator

Even when there are no obvious signs of external leaks or damage, you can still inspect the radiator for other problems that are pretty easy to discover, especially external and internal restrictions that can lead to overheating issues.

Locating External Restrictions:

  1. Check the front and back of the radiator. Remove bugs, dirt, road salt and debris that can affect proper air flow. You can remove dirt, bugs and debris from the radiator front and back panel using a garden hose. If you need to repair flattened fins, use a radiator fin straightener.
  2. Look for wet, white or greenish spots that might indicate coolant leaks.
  3. Check that cooling fins are firmly held in place. Corrosion from road salt can destroy the solder that holds the fins to the radiator tubes. Loose fins make it harder to transfer heat out of the radiator.
  4. Check the radiator cap and coolant return hose, which connects to the reservoir, for restrictions.

Locating Internal Restrictions:

Internal restrictions are harder to detect. But your thermometer can help you locate cold spots. These cold spots indicate flow restrictions that will result in overheating.

  1. Start and idle the engine for about 15 minutes, or until the temperature gauge rises close to the red line.
  2. Then turn off the engine.
  3. Take temperature readings around the top, middle and lower sections of the radiator.

Cold spots indicate restrictions, usually produced by lime, corrosion or scale buildup.

If you suspect the radiator is clogging, you may need to remove the radiator for a deep flush, or have it serviced at a shop. A regular cooling system flush might not do the trick. Sometimes, replacing the radiator is necessary.

Radiator hoses are a common source of coolant leaks.

Radiator hoses are a common source of coolant leaks.

5. Checking the Cooling System Hoses

Cooling system hoses also wear out and deteriorate over time. You need to trace and squeeze each hose at different spots with your hand to feel for rough spots hidden from sight.

Look for:

  • chafing
  • swelling
  • soft spots
  • hard spots
  • wet spots
  • cracks
  • sticky spots (oil contamination)
  • rust spots around the clamps

Rust spots around the clamps may indicate leaks caused by an overtight clamp.

The lower radiator hose should feel firm. Lower hoses usually come with a reinforcement spring to prevent the hose from collapsing as the water pump sucks (vacuums) coolant through the hose. If this hose feels soft, or you can squeeze it without much effort, replace it.

Replace any coolant hose that shows signs of wear or damage.

6. Checking the Cooling System Fan

An engine that gets hot when driving within the city but cools down when driving on the highway, it's a first sign of a failing fan, especially an electric fan.

Besides a possible bad electric motor, there are other factors that can interfere with cooling fan operation, including a faulty engine coolant temperature (ECT) sensor.

If you suspect a failing cooling fan, start with some simple checks around the fan, fan shroud, and (depending on your particular model) fan clutch.

Check the fan shroud for:

  • looseness
  • cracks
  • missing pieces

If the shroud is damaged, replace it.

Check the cooling fan for:

  • damage
  • distortion
  • wobbling

A faulty engine-driven fan can wear out a water pump's bearing and seal.

Check the fan clutch for:

  • oil leaks
  • oil deterioration (fan slipping at low speeds)
  • a fan that easily spins manually with the engine off
  • missing or damaged clutch fins
  • a clutch that wobbles when the fan is moved in and out with the engine off

If you need to replace the clutch because of age, it's recommended to replace the water pump as well.

Check an electric cooling fan for:

  • proper mounting
  • damaged fan blades
  • operation when AC is on
  • operation after engine reaches operating temperature

If the fan doesn't come on when it should, check the motor first. You can connect battery power directly to the fan using a couple of jumper wires. If the fan doesn't run, the motor is faulty. If it runs, check the fan control circuit. If necessary, consult your vehicle repair manual for your particular vehicle model.

On belt-driven water pumps, like this timing belt, replace the belt at the schedule recommended by the manufacturer.

On belt-driven water pumps, like this timing belt, replace the belt at the schedule recommended by the manufacturer.

7. Checking the Drive Belt

Checking the belt is important, especially on belt-driven water pumps. The number one enemy of drive belts is heat. Over time, heat causes the belt to crack, harden, and underperform. Heat not only comes from engine operation. A loose or oil-contaminated belt will produce its own heat when the belt rotates around the pulleys and slips.

Check drive belts for:

  • proper tension
  • cracks
  • missing spots
  • tears
  • glazing
  • fraying spots
  • wear

Modern drive belts won't show external signs of wear. Use a belt wear gauge for this. Also, check the wear indicators on the belt tensioner that may point to the need for a new belt.

A worn belt can slip, become noisy, and produce excessive heat which can damage pulley bearings.

A bad water pump impeller can stop circulating coolant and cause overheating.

A bad water pump impeller can stop circulating coolant and cause overheating.

8. Checking the Water Pump

The water pump's job is to push coolant through the engine. So when a water pump isn't working properly, the engine will overheat.

In general, a water pump doesn't suddenly stop working. A faulty pump will start leaking coolant or make a screechy sound.

  • A water pump may start leaking because of a worn seal or faulty bearing. You'll see coolant seeping through the pump's weep hole or outer seal.
  • A water pump may become noisy because of a faulty bearing or shaft.
  • Also, the blade assembly may wear out, corrode or become loose. Often, it is necessary to remove the pump to check on the condition of the internal assembly.
  • If you have an electrically operated pump, the pump may stop because of a circuit failure.

On engine-operated fans, the problem may start with a cracked or broken fan, or a worn fan clutch.

If you know the thermostat is working properly, you can apply the following water pump tests.

A quick water pump check:

  1. Set the transmission to Neutral (manual) or Park (automatic).
  2. Start the engine, engage the emergency brake, and let the engine warm up so the thermostat opens.
  3. Using a shop rag, squeeze the upper radiator hose while an assistant depresses the accelerator.
  4. You should feel coolant surging through the hose. If not, either the water pump isn't working or the radiator is restricted.

Another way to check water pump operation:

  1. Allow the engine to cool.
  2. Remove the radiator cap.
  3. Start the engine and allow it to reach operating temperature.
  4. As the thermostat opens, you'll see the coolant circulating through the radiator opening; otherwise, the water pump isn't working or there could be a radiator restriction.

Air leaking into the cooling system can also prevent a water pump from working properly.

A quick way to check for air leak in the cooling system:

  1. Submerge the end of a small hose in the coolant reservoir tank or radiator overflow tank.
  2. Submerge the other end into a jar with water.
  3. Set the transmission to Neutral (manual) or Park (automatic).
  4. Engage the emergency brake, start the engine and let it idle.
  5. Once the engine warms up, depress the accelerator to raise engine speed.
  6. If you see a stream of bubbles in the jar, air is leaking into the system.

If you suspect a blown head gasket, do a compression test or combustion leak test.

Poor cooling system maintenance can cause corrosion and leaks like this one from an engine core plug.

Poor cooling system maintenance can cause corrosion and leaks like this one from an engine core plug.

9. Checking for Cooling System Leaks

The coolant reservoir may need a bit of coolant every three months, because of the coolant that naturally evaporates. However, when the coolant reservoir level frequently goes down or your engine keeps running too hot, it's usually from a coolant leak.

A coolant leak can start anywhere in the system. Sometimes, a leak becomes obvious once you pop the hood open and look around the cooling system. But leaks, external or internal, are not too evident all the time.

Having said that, there are certain places where leaks occur most frequently, especially at the end of coolant hoses and the radiator. Perform a visual inspection around these common areas:

  • radiator
  • radiator hoses
  • heater core hoses
  • water pump
  • thermostat housing
  • heater core
  • cylinder head gasket
  • transmission oil cooler

Look for spots that look:

  • wet
  • darkened
  • discolored
  • rust colored

Also, visually inspect the ground where you park your car. A small, greenish-looking puddle under the car can point you in the direction of the leaking component.

If you can't find any signs of leaks, you may be dealing with a small leak or an internal engine leak. Go to the next section.

A blown head gasket may leak coolant into the combustion chambers.

A blown head gasket may leak coolant into the combustion chambers.

10. Performing a Cooling System Pressure Test For Internal Engine Leaks

You can do a simple leak testing using a radiator pressure tester. This is a hand-operated air pump. Most car owners don't own this tool, and it probably won't be practical to buy one because you wouldn't be using it that often. Still, you can borrow this tool from most auto parts stores.

To perform a pressure test:

  1. Engage the emergency brake and allow the engine to cool.
  2. Remove the radiator cap and add coolant to bring it up to the correct level, if necessary (about 1/2 an inch below the bottom of the radiator neck).
  3. Connect the tester to the radiator filler neck.
  4. Pump the tester to apply 14 or 15 psi of pressure to the system, depending on the specifications for your system. Check the markings on your radiator cap or your car owner's manual, if necessary.
  5. The cooling system should hold the pressure for 15 minutes or more; otherwise there is a leak in the system. If so, continue to the next step.
  6. Pump the tester to bring pressure back to system specifications.
  7. Look for the leak around hoses, radiator, water pump and other common spots. Also, check under the car for a wet spot.

If you can't find it, the leak might be internal. If you suspect a blown head gasket, perform the same test, following the next steps:

  1. Set the transmission to Neutral (manual) or Park (automatic).
  2. Relieve the system pressure you applied with the pressure tester.
  3. With the pressure tester connected to the radiator neck, start the engine and let it idle.
  4. Watch the gauge on the tester.
  5. If the gauge starts to register building pressure, combustion gases are leaking into the cooling system because of a blown head gasket.

If you need a more accurate test of the head gasket, do a compression test or combustion leak test.

Before testing the radiator cap, visually inspect the cap for:

  • Rust. Heavy rust deposits might indicate the need for a deep system flush.
  • A hard, brittle or damaged O-ring or gasket.
  • A damaged valve.
  • Worn or damaged radiator neck sealing surfaces.

Replace the cap as necessary.

Testing the radiator cap with a pressure tester:

  1. Connect the correct adapter to the tester to test your radiator cap.
  2. Install the cap to the pressure tester.
  3. Apply pressure to the cap equal to the marking on the cap.
  4. The cap should hold the pressure for at least one minute; otherwise, replace it.
  5. Reapply pressure to the cap. This time, exceed the cap rating pressure.
  6. The cap should vent the excess pressure; otherwise, replace the cap.

The following video shows the cooling system pressure test, including a test for possible head gasket failure.

11. How to Prevent Cooling System Problems

The most effective way to prevent most common cooling system problems is to:

  • Service the system as recommended by your car manufacturer. You'll find this information in your vehicle repair manual.
  • Visually check system components from time to time, especially on newer models with extended maintenance intervals.
  • Check coolant level frequently.
  • Test for acid content (pH) using coolant test strips.
  • Check coolant-to-water ratio using a hydrometer.
  • Replace old coolant as needed.
  • Visually check the drive belt or serpentine belt and tension at regular intervals

Replacing the coolant at the appropriate schedule helps the engine operate at the appropriate temperature. And checking the coolant level and condition of system components will often help you spot problems at the onset.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: If I have a slow leak, do I need a new radiator? My 2002 Mazda Protege has 167k and dented both sides. Firestone wants $600 to install a new radiator. They say the neck of the radiator is deteriorating. I need time to decide if I should buy another cheap old car. In the meantime, isn't there a way to fix it or seal it? The reservoir was empty when it overheated. I filled it and it's OK. But it was 1/2 full the next day!

Answer: If it's leaking through the neck, check for buildup, you may be able to clean it with a wire brush. If it looks good, try a new radiator cap. The cap seals deteriorate overtime and may cause all kinds of trouble. If the leak is through one of the tanks, you may be able to find a product in one of your local auto parts stores that might help, if you are dealing with a small, pinhole size leak. Other than that, you may need to replace the radiator.

© 2019 Dan Ferrell

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