Coolant Loss: Where Is My Car's Coolant Going?

Updated on January 19, 2019
Dan Ferrell profile image

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

Coolant loss can be hard to diagnose sometimes.
Coolant loss can be hard to diagnose sometimes. | Source

Coolant loss may indicate a poorly maintained cooling system, a system fault, or even a change in driving patterns. For example, a coolant leak could have any of these causes:

  1. Overfilling the cooling system
  2. A faulty radiator cap
  3. A worn-out radiator hose
  4. A leak that only occurs under certain operating conditions
  5. An undiagnosed cracked engine block or cylinder head, or a blown gasket
  6. Towing heavy loads

Whatever the cause, you need to address the problem before it turns into a more serious and expensive repair. Coolant loss can destroy your engine if a small leak suddenly turns into a larger one and the engine overheats without enough chance to cool.

Within the next sections, you'll find the most common sources of coolant loss and some tests you can do in your own garage.

Before going into full troubleshooting mode, the first few sections start you out with some simple diagnostic tests, things to check depending on your particular problem. Later come some more involved troubleshooting tests.

Overall, the tests are not difficult although some may require special tools. Just keep in mind that one or more auto parts outlets in your area may lend you these and other tools. Take advantage of this service that can help you save hundreds of dollars in diagnostics and repairs.

Table of Contents
1. Are You Overfilling the Cooling System?
2. Coolant Leaking into the Engine Oil
3. Is the Engine Overheating
4. Is a Leak the Only Way to Lose Coolant?
5. Can the EGR System Cause Loss of Coolant?
6. Inspecting the Cooling System
7. If You Can't Find the Leak, Pressure Test the System
8. Doing a Combustion Test
Make sure you're adding coolant to the correct level.
Make sure you're adding coolant to the correct level. | Source

1. Are You Overfilling the Cooling System?

Before starting to diagnose the problem, make sure you are not overfilling the cooling system.

If you recently topped up the overflow tank, check that you are adding only the required amount for your system. Consult your car owner's manual or vehicle repair manual.

The coolant recovery tank has a COLD/MIN mark indicating the level for the coolant with the engine cold. The radiator should have the coolant reaching just below the filler neck.

If you just replaced the fluid, double-check that you properly closed the radiator petcock, reinstalled the radiator hoses, and used the right sealer for the thermostat gasket if you installed a new one.

A blown head gasket can cause coolant to mix with engine oil.
A blown head gasket can cause coolant to mix with engine oil. | Source

2. Is Coolant Leaking Into the Engine Oil?

Just like a combustion leak into the cooling system, a blown head gasket, cracked head or block, or faulty oil cooler lines can cause coolant to leak into the engine oil.

Often, you can tell coolant has leaked into the lubricating system by the appearance of a milky white substance on the dipstick or under the valve covers. To check for this substance:

  • Pull out the dipstick as if you are checking the oil, and hold the end of the stick on a shop rag and examine the oil around the tip of the stick.
  • Remove the oil filler cap from the valve cover and look through the filler hole. You may need a flashlight to examine inside the valve cover or maybe remove the cover itself.

A blown head gasket, bad intake manifold gasket (V8 engines), a cracked block, or a cracked cylinder head may cause coolant to mix with engine oil.

If you suspect coolant has mixed with the engine oil, repair the problem as soon as possible. Antifreeze in the oil can damage the cylinders and pistons.

Operating the engine at higher the normal temperature may cause coolant loss.
Operating the engine at higher the normal temperature may cause coolant loss. | Source

3. Is Engine Overheating Causing Your Coolant Loss?

Coolant loss can cause overheating, but overheating can also cause coolant loss. To check this, you may need to top up the system and then see if the engine overheats and leads to coolant loss again.

The temperature gauge may be indicating a higher than normal temperature or an overheating condition. Check for the potential causes of overheating:

1. Make sure there's enough coolant in the system.

2. Check the drive belt:

  • It should be properly adjusted.
  • Look for signs of wear, tears, cuts, or splits.

3. Check the serpentine belt:

  • Make sure the belt plies are not separating: look along the sides of the belt.
  • Look for glazed or frayed areas.
  • Look for missing chunks on the rib (underside) of the belt. If you see missing adjacent pieces of rubber larger than 1/2-inch, replace the belt.

4. A noisy belt may be an indication of a worn out belt.

5. Check for a blocked radiator core or grill.

6. Check for a faulty cooling fan.

7. Consider whether the radiator cap is worn out or damaged. See sections six and seven below.

8. Check for a faulty thermostat.

9. Consider whether the water pump is leaking.

10. Check for retarded ignition timing. Retarded timing can allow heat from the combustion to leak out through an open exhaust valve, raising heat in the exhaust valves and manifold.

Frequently towing heavy loads can cause coolant loss as well.
Frequently towing heavy loads can cause coolant loss as well. | Source

4. Have You Changed the Way You Drive?

Coolant loss may also come from specific driving patterns and engine operating conditions. If your driving pattern or traffic conditions have changed recently, this might be the reason for the drop in coolant in the system. Coolant loss may happen if you:

  • Frequently tow heavy loads, like a trailer.
  • Drive through steep hills.
  • Use the air conditioning while idling in high ambient temperature.
  • Frequently find yourself caught in slow-moving traffic.

Any of these conditions can lead to engine overheating, coolant loss, or both.

Some EGR cooler systems may leak.
Some EGR cooler systems may leak. | Source

5. Can an EGR System Cause Loss of Coolant?

It can happen if the EGR system (exhaust gas recirculation) system design is not as efficient as intended.

For example, an exhaust gas recirculation (EGR) cooler system was installed from the factory in some GM diesel 6.6L engines and a growing number of European models. This cooler, under certain circumstances, has been reported to leak after miles of operation to warrant an updated replacement. Apparently, this EGR cooler turned out to be vulnerable to the use of antifreeze not appropriate to specific models and exposure to exhaust gases that eventually may leak through the cooler lines after miles of operation.

A leak in the EGR system leak may cause coolant to find its way through the exhaust system, the intake, and possibly the cylinders.

Troubleshooting the EGR cooler usually requires a visit to the shop to have the system tested with special equipment.

Coolant leaks may leave rust or white colored marks.
Coolant leaks may leave rust or white colored marks. | Source

6. Perform a General Inspection of the Cooling System

If your particular case doesn't fit any of the above situations, it's time to make a general inspection of the cooling system. An inspection may turn up some clues that can lead you to the source of the problem.

When inspecting the cooling and related systems, look for discolorations: white water-like marks (dried coolant), or washed-clean or rust-colored areas. These discolorations may come from small leak(s) around the following engine or system components:

  • Radiator. Check around the seams, where the core joins the tanks and the core area. Make sure the front of the radiator is free of debris and dust that may prevent proper radiator operation.
  • Radiator shroud. Check for a broken or missing shroud that can lead to overheating and coolant loss.
  • Radiator cap. Inspect the cap for bad seals, tears or similar damage. This may lead to overheating as well.
  • Radiator filler neck. Check the neck for nicks that may prevent a good seal.
  • Overflow tank. Look for cracks in the container itself and check the cap for leaks.
  • Engine core plugs (freeze plugs).
  • Water pump. Worn out or damaged pumps may start with a small leak long before you notice the problem.
  • Upper and lower radiator hose and heater hoses. The heater hoses are connected to hard tubing, usually on the firewall, on the passenger side, inside the engine compartment. On some large models, the core may be installed towards the rear of the vehicle.
  • Heater core.

Check each system hose, fitting, and connector for:

  • deterioration,
  • cracks or worn out spots,
  • swollen, hardened or soft areas,
  • and loose or damaged clamps.

Pressure test the cooling system to diagnose hard to find leaks.
Pressure test the cooling system to diagnose hard to find leaks. | Source

7. If You Can't Find the Leak, Pressure Test the System

Coolant leaks are so small sometimes that they can escape through the system only under pressure and with a fine mist that leaves no traces behind, making them tricky to locate even through careful visual inspection. This is where a pressure test may help.

A pressure test forces coolant out through the leaking point, if it exists, so that it can be found. The test can be done at home or at the shop. The pressure tester tool is basically an air pump, and the tool may be borrowed from a local auto parts store.

This is the pressure test general procedure:

  1. With the engine cool, remove the radiator cap and attach the tester hose to the radiator neck or the coolant recovery tank, depending on your particular vehicle model.
  2. Start pumping air into the system until the tester gauge reads the system pressure as indicated on the radiator cap. System pressure is usually 14 or 15 psi. Check your vehicle repair manual if necessary.
  3. Once the system is pressurized, start making a visual inspection of the system looking for a potential leak: around the radiator, heater core, hoses, water pump, engine core plugs, hose fittings, and engine gaskets.

Also, pressure test the radiator cap:

  • Attach the cap to the pressure tester.
  • Pump the tool to a pressure exceeding the rated pressure capacity of the cap. The radiator cap should release pressure when it goes past the cap's rated capacity. Otherwise, the cap relief valve isn't working.
  • Then, pump the tester until the rated pressure is reached and stop. The cap should hold pressure for at least 60 seconds. If the cap can't hold pressure, the expansion valve is stuck open.

Watch the next video for some quick tips to check the cooling system, especially around the radiator.

A cracked engine block will require a replacement.
A cracked engine block will require a replacement. | Source

8. Perform a Combustion Test

If no leak showed up with the pressure test, it's time to do a combustion leak test. Like a pressure test, a combustion test can be done at home or at the shop.

The combustion test can help reveal serious problems like a blown head gasket, a cracked block or cylinder head. Often, this type of problem appears with symptoms like bubbles in the coolant, engine overheating, or even a rise in coolant level when you start the engine.

The test is simple and can be done with a "block tester" or combustion leak tester, a relatively inexpensive and easy-to-use tool.

The test only takes a few minutes:

  1. With the engine cool, remove the radiator cap and install the block tester on the radiator neck.
  2. Then start the engine.
  3. Now, squeeze the bulb on the tester tool. This will pull cooling system air through the liquid inside the tool.
  4. Normally, the liquid in the tool is blue but will turn yellow if there are combustion gases present in the cooling system, meaning there is a leak from the cylinders into the cooling system.

Follow the instructions in the tools package.

If the test turned out positive, you can find out which cylinder has the problem.

  1. Repeat the test, but this time unplug a spark plug wire.
  2. Short out the wire to ground with a jumper wire. Insert a long screwdriver into the wire boot in place of the spark plug and use a small jumper wire to short out the screwdriver to ground, by connecting the other end of the jumper wire to the engine. A bracket or screw will do.
  3. Then, start the engine and squeeze the tester pump to pull air through the liquid.
  4. Repeat the test for each spark plug. The one plug that doesn't cause a change of color in the fluid is the one where the tear in the gasket, crack in the block or head cylinder is located.

Whenever you are faced with coolant loss, try to find the source of the leak as soon as possible. Even a small leak now can become a serious and expensive repair job in a short period. A sudden engine overheat can catch you in the middle of the road and cause serious damage. The diagnostic strategy outlined here will help you find that tricky spot sooner than you may expect, and you may be able to fix it yourself at home.

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

  • Ok so the coolant is running perfectly through the upper hose. I replaced everything literally but when I stop at a stop sign for a minute, the temperature rises. But when I go down the road running about 50 mph, it cools down. Please help. And we've done a pressure test and it was good. It was for the head gasket. Please help me. It's leaking really bad. Please help. What could be the problem?

    You need to find the source of the leak. If the engine keeps overheating at idle and low speeds, this other post may help you too:

  • I had the water pump and serpentine belt replaced about a month and a half ago in my vehicle. I find the car now losing coolant. Would it take that long to show up if there was a problem with the installation?

    If the water pump is leaking coolant because of a faulty gasket or faulty pump, you should be able to see the coolant leaking. Depending on your particular model, though, the water pump may be out of sight under a cover. On some models, it's not difficult to remove the cover. Check for another potential, external leaking points, if necessary. Check the oil for signs of coolant leaks as well. If you can't find the leak, it's possibly an internal leak. You may need a system pressure test or a combustion test to find the leak.

© 2017 Dan Ferrell


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