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Radiator Check: Diagnostic Tips and Troubleshooting Guide

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Failure to service the radiator leads to serious cooling system problems.

Failure to service the radiator leads to serious cooling system problems.

How to Check Radiator

As you already know, the radiator helps remove coolant heat to keep the engine working at operating temperature. Too much heat can melt pistons, crack the engine, or wrap the cylinder head. Thus, the radiator plays a key role in the efficiency and long service life of your engine.

Although it doesn't have moving components, any of the radiator's simple parts can fail and cause serious engine trouble. By far, the most common radiator issue is overheating.

However, overheating can be caused by many components and faults outside the radiator. So, this guide will show you the most common checks and some important tips you can apply to inspect the radiator at home.

Often, bringing the radiator back to good operating condition requires the service of a repair shop. But, as you'll see, many times the radiator only needs some simple maintenance that you can perform at home.

Whether the problem is overheating or coolant loss, you should pay immediate attention to the problem. Both can cause severe engine damage.

Remember though, the focus here is on the radiator as a component of the cooling system. If the diagnostic of your radiator doesn't turn out any problems with it, other components in the systems (thermostat, hoses, water pump) could be behind the problem.

Before going into the diagnostic procedure, here's an overview of the radiator operation and its basic components to make your job easier.


I. How Your Radiator Works

II. Radiator Diagnostic Tips

III. 8 Ways Your Radiator Can Cause Engine Overheating

IV. How to Check Your Radiator for Leaks

V. Do You Need a Coolant Pressure Test?

Check the radiator tanks, hose fittings, neck, and core for leaks and damage.

Check the radiator tanks, hose fittings, neck, and core for leaks and damage.

I. How Your Radiator Works

Hot engine coolant enters into a radiator tank from the top or the side of the radiator and flows through a series of thin, parallel pipes. A series of thin fins surround the pipes to remove heat away from the coolant. Air flows through the core of the radiator and transfers this heat away.

Meanwhile, coolant reaches the opposite tank at the bottom or the other side of the radiator, where the lower temperature coolant waits to enter back into the engine pulled by the water pump.

Radiator Components

Radiator tanks: Your radiator has one coolant tank at each end, either at the top and bottom (downflow type) or at each side (crossflow type). They serve as temporary storage.

Commonly, sport vehicles use the crossflow type radiator with the tanks on each side and the radiator cap located on a coolant recovery tank that connects to the radiator.

Other vehicles use the downflow radiator type with the tanks at the top and bottom of the radiator and the radiator cap on top.

Radiator core: This is the main, large block component at the center of the radiator. It's actually made up of thin pipes or tubes surrounded by thin, cooling fins. This is where the removal of heat takes place when air flows through the core as you drive, and with the help of the cooling fan.

Transmission oil cooler tank: Many automatic transmission vehicles use an oil cooler tank inside one of the radiator tanks to cool transmission oil.

Filler neck: Depending on your particular vehicle model, a radiator may have a filler neck to fill the cooling system with coolant. The neck holds the radiator cap in place and connects the radiator to a coolant reservoir tank through a thin, overflow hose.

Radiator cap: It not only seals the radiator, but also pressurizes the system, gets rid of excess pressure and allows the transfer of coolant into and from a recovery tank.

Petcock: A small fitting at the bottom of the radiator with a valve to drain the coolant out of the radiator.

Worn-out or contaminated coolant will cause the engine to overheat.

Worn-out or contaminated coolant will cause the engine to overheat.

II. Radiator Diagnostic Tips

A few tips to improve your troubleshooting strategy:

  • When stuck, check the vehicle repair manual for your particular vehicle model. The manual can help you solve difficult to diagnose problems not only in your radiator but the cooling system as a whole.
  • When diagnosing your radiator or the cooling system, think about the conditions in which the problem is occurring.

For example:

  • Are you dealing with an overheating, overcooling, or loss of coolant problem?
  • Does the problem occur when idling, on the road, or intermittently?
  • When did you replace the coolant?
  • Have you noticed signs of leaks under the vehicle?
  • Have you noticed symptoms like noises?

The answer to one or more of these questions can give you the direction you need to speed up the diagnostic process.

  • Don't forget your car computer. If the Check Engine Light (CEL) has come on, scan the computer for trouble codes that might give you a clue to the problem.
  • And, if your radiator turns out damaged, for better results install a new one instead of trying to repair it.
A damaged or worn out radiator cap leads to overheating and pressure loss.

A damaged or worn out radiator cap leads to overheating and pressure loss.

III. 8 Ways Your Radiator Can Cause Engine Overheating

Your radiator can cause engine overheating for several reasons, even unsuspected ones.

1. Fan shroud missing: Many car owners frequently miss this one. By itself, a missing fan shroud doesn't cause the engine to overheat. But it's a contributing element. The shroud helps to direct airflow through the radiator core for better efficiency. Without it, air tends to flow around the radiator, failing to remove most of the heat from the coolant inside the radiator.

2. Low coolant: Just as the previous one, you can miss this check. Verify coolant level in the radiator and coolant reservoir. Add the recommended for your vehicle as necessary. Consult your car owner's manual or repair manual.

3. Air pockets: Let's say that you replaced the coolant a couple of days ago. Now the engine is overheating. But the coolant reservoir tank indicates the correct level. How can that be?

However, if you didn't purge the air from the system or didn't do it properly, you probably left air pockets that prevent coolant to reach properly every corner of the system, causing the engine to overheat.

On modern vehicles, this type of problem can trigger the Check Engine Light (CEL). This happens because of a high-temperature signal from the Engine Temperature Sensor or a sensor malfunction.

4. Blocked radiator core: A blocked radiator core can also lead to an overheated engine.

Over time, dirt, leaves, bugs and other junk gets stuck over the rear of the radiator, blocking airflow and, hence, leaving the radiator unable to efficiently remove coolant heat. Visually inspect the outside of the radiator for debris.

If the radiator grille is dirty, clean it with a water hose or a special radiator cleaner tool.

5. Coolant leak: Use a flashlight to closely inspect the radiator tanks, neck, petcock, coolant reservoir tank and hose fittings for signs of leaks. Even a small coolant leak that doesn't leave a puddle on the floor is bound to give you problems.

6. Old or contaminated coolant: Check the coolant color. If your coolant has lost its original color, it may indicate contamination (combustion gases, oil leak), or worn out coolant. If necessary, check the coolant with coolant testing strips to check for strength or acid buildup.

7. Coolant flow obstruction: Check that the coolant can circulate. On models with a radiator cap, remove the cap, start the engine, and let it warm up. When the engine reaches operating temperature, through the opening you'll see the coolant beginning to flow. Otherwise, you are dealing with an obstructed radiator core, failed water pump or thermostat.

Without proper service, rust and scale can accumulate throughout the radiator core and prevent proper flow or entirely block coolant flow, causing the engine to overheat.

However, keep in mind that a bad water pump or thermostat can also prevent coolant flow.

8. Failed radiator cap: This is another one of those frequently missed engine overheating sources.

The cap on your radiator does more than seal the radiator. It helps maintain the necessary pressure to keep the engine working at operating temperature, and also functions as an escape valve when coolant temperature rises over the operating range.

When the engine is cool, remove the radiator cap and closely examine the cap seal. If the seal seems worn out or damaged, that might be the cause of your engine overheating.

Replace the cap with another one rated for your particular cooling system. Consult your car owner's manual or repair manual, if necessary.

You can use a coolant system pressure tester to test the condition of the radiator cap. Check the "Do You Need a Coolant Pressure Test?" section and the video below.

Also, with the radiator cap removed, visually inspect the radiator neck where the cap seals. If the neck has dents, even small ones, system pressure could leak and not allow the radiator cap to seal properly.

Check for coolant leaks around the coolant reservoir and radiator.

Check for coolant leaks around the coolant reservoir and radiator.

IV. How to Check Your Radiator for Leaks

Almost any part or fitting in the cooling system can be the source of a leak, but the radiator and cooling system hoses are common suspects.

  • Using a flashlight, look for wet, darkened areas around the radiator tanks.
  • Closely check the radiator core for signs of leakage.
  • Also, check the radiator upper and lower hose clamps. Make sure they're tight.
  • Examine the oil dipstick and under the valve cover. Your coolant may be going into the oil (which turns the oil into a milky substance) through a damaged component.
  • Combustion gases may be leaking into the coolant system. For this, you need to use a combustion leak tester or an exhaust gas analyzer in a car shop.
Learn to check your radiator for potential problems.

Learn to check your radiator for potential problems.

V. Do You Need a Coolant Pressure Test?

Conduct a coolant pressure system test, if you've noticed frequent coolant loss. You may be dealing with a small radiator leak or an internal system leak due to a cracked engine block or cylinder head or blown head gasket.

The pressure test can help you locate or confirm hard-to-find leaks. The test uses low pressure to force coolant to come through the exterior leaking spot or prove that the system can't hold pressure if you're dealing with an internal leak or a damaged radiator neck.

You can conduct this test at home if you have the pressure tester, or you can take your car to the shop.

When using the pressure tester at home, never go over the pressure marked on the radiator cap (usually 14 to 16psi) or you will cause system damage.

With the system pressurized, carefully inspect the radiator with a flashlight and, if necessary, the rest of the cooling system.

Also, pressure test the radiator cap. If the cap doesn't hold pressure during the first minute, most likely that's your leaking point.

A Simple Task That Goes a Long Way

In general, checking the radiator is a simple task that can usually be done with some simple tools. Sometimes, though, you may need a special diagnostic tool, but you should check with your local auto parts store before buying. To prevent radiator and cool system problems, follow the maintenance schedule in your repair manual for a longer cooling system service life.

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: My car overheats. Why would engine coolant leak on the passenger side?

Answer: Check the heater core and hose connections. This is the most likely cause when coolant leaks into the passenger side.

Question: Why does my heater only blow hot air when you press the accelerator, and not when idling?

Answer: Make sure the coolant level is correct. If not, check for internal or external leaks. You may need to bleed the cooling system of air pockets. Another possibility is a worn water pump (faulty impeller); a partially restricted heater core may show the same symptoms; bad heater control valve, or its vacuum hose. Hope this helps.

Question: Can a radiator leak when there is no sign of a coolant leak?

Answer: Check the radiator cap. If it's bad and allowing pressure to escape this may cause a coolant leak. It could be a small leak.