Troubleshooting a Coolant Leak
Written by Niel Stender | Reviewed for accuracy by Andy Lindman
A coolant leak is any loss of engine coolant from the sealed cooling system. A low coolant level caused by a leak can lead to engine overheating and even catastrophic damage.
A small leak might vanish before you can confirm it visually. A large leak, on the other hand, will escape outwardly and create a puddle. Other than an obvious puddle of coolant, you might notice a low coolant level in the reservoir, a warning light on the dashboard, steam coming from the engine bay, or a pungent smell.
Only check the coolant level when the engine is cold or slightly warm. Never open the radiator cap when the engine is hot and the cooling system is pressurized.
Radiator and heater hoses, clamps, and coolant tubes
Electrochemical degradation (ECD) is the most common cause of cooling hose failure. It’s caused by an electrical charge being carried by the coolant across cooling system components. ECD occurs inside hoses and creates micro-cracks, which lead to leaks that are impossible to detect with a visual inspection. Deteriorated plastic quick-connectors with worn seals can also leak. And insufficiently torqued clamps can cause joint leaks even when the hose itself is in good shape.
Look for fluid drips and bleed marks - discolored residue - on or around the components, along with swollen areas of the hose. Using your thumb and forefinger, squeeze the hose within 2 inches of the connectors. Feel for sections that are softer than the middle portion.
Non-visible pinhole leaks might only show up under a pressure test. It’s performed by connecting a pressure tester to the radiator cap or coolant reservoir. Using a hand pump or shop air, pressurize the system to normal operating pressure. If the system can’t maintain pressure for 10 minutes, it suggests an internal hose leak.
Drain the cooling system per vehicle specifications. Replace leaking hoses, worn-out clamps, and coolant tubes.
Don’t use lubricants, grease, or detergents when installing a new hose. Ensure replacement clamps aren’t fitted over the nipple. Use either constant-tension clamps or adjustable tension clamps, which require retightening after a brief run-in period. Ensure the hose clamps are designed to work with the hose, as this specification varies.
Refill the cooling system, perform a visual inspection for leaks, and immediately correct any issues.
Radiator
Modern radiators typically feature aluminum core construction with plastic end tanks. The tanks are crimped over tabbed radiator headers with rubber gaskets, providing compression. Over time, excessive thermocycles within the cooling system can cause fatigue in the tubes. This leads to cracks, leaks, and crimp tabs releasing compression on the gasket. Corrosion and road debris are also factors, but should be considered secondary to thermal cycling fatigue.
With the engine running, look for coolant leaks at the tube-to-header joints, seams on plastic header tanks, the neck of the radiator cap, and the drain petcock. Be careful not to misdiagnose a leaking reservoir attached to the radiator. Using the pressure test method, verify that the cooling system maintains its rated pressure for at least 10 minutes.
To determine if electrolysis is causing the issue, connect the negative lead of a voltmeter to the battery ground. Insert the positive lead into the coolant, either in the radiator or overflow tank. Be careful not to make contact with any metal surfaces. Switch components on and off, one at a time. If any tests show more than 0.10 volts, it suggests that component is the problem.
Replace a leaking plastic-tank radiator with a new unit. Material fatigue of the plastic and crimping tabs can’t be properly repaired.
To do so, open the drain plugs on the radiator and cylinder block and drain the cooling system. On reassembly, clean the drain petcock and install a new O-ring. Raise the front end of the car while refilling the system with coolant to avoid trapping air in the heater core. Start the engine and check for leaks from the radiator and hose connection points.
Avoid cooling system sealers. They’re only intended for leaks smaller than ½ mm in size, and can clog radiator and heater core passages.
Water pump
Water pump coolant leaks are typically the result of sealing face material breakdown, use of coolant additives, worn-out bearings, or improper use of sealants during assembly. Automotive water pump shafts are sealed with a mechanical-type seal designed to be lubricated by a small amount of coolant during normal operation. Weep holes are designed as an escape path for excess coolant. Don’t confuse this with a defect.
Check the water pump shaft rotation for roughness or excessive play to determine the condition of the bearings. A pressure test of the cooling system will confirm active water pump leaks. A UV dye and light is useful for testing in tight areas.
Dry coolant residue around the weep hole is part of normal water pump operation. Use a paper towel to determine if there are current wet spots in this area. Wet spots suggest an active leak.
Replace a leaking water pump with a new unit. Replace the timing belt at the same time if it drives the pump. Before installation of the new water pump, carefully clean the mounting surface. Ensure the gasket surface is free of residue. Follow the manufacturer’s guidance as to whether to use sealant on gaskets. After installation, bleed the system per the manufacturer’s procedure to prevent air pockets.
Thermostat housing or water outlet
Typical thermostat construction of a plastic housing bolted to an aluminum head is prone to failure at the joint. Under every heat cycle, the materials expand and contract at different rates. Plastic can become brittle and cracked, and O-rings become flattened. Under-tightened fasteners can also cause coolant leaks at the thermostat gasket.
Look for seepage around the housing or evidence of coolant under the engine compartment. Be careful not to misdiagnose a faulty water pump due to drips leaking from the thermostat housing. A pressure test might not pinpoint the source of the leak. But raising the coolant temperature to 90° C by running the engine should provide visual confirmation due to dripping, spraying, seeping, or bubbling from the housing.
Replace only the leaking component - thermostat housing or coolant pump. If possible, upgrade from a plastic housing to an aluminum option.
During installation, ensure the sealing surface is free of debris or corrosion. Then replace the housing gasket. Use a torque wrench to provide an even clamping load across the bolts. Follow the manufacturer-specified torque value.
Vacuum-fill the cooling system with fresh coolant to avoid creating air pockets. Once complete, conduct a road test to confirm the coolant leak is corrected.
Radiator or expansion-tank pressure cap
The radiator cap maintains cooling system pressure within a specified range to increase the coolant boiling point. Some caps contain an additional valve that pulls coolant back into the system from the expansion tank as the engine cools.
When the pressure cap rubber seal swells or deteriorates, coolant seeps past the defective seal into the expansion tank. It’s then forced out of the overflow vent with the next engine operation. The coolant level drops, with no visible leak. Debris in the filling neck area can also become embedded in this rubber seal and prevent proper sealing.
Remove the radiator cap, dampen the gaskets, and connect a pressure test adapter. Operate the pump until the reading on the pressure gauge stops increasing. This is the pressure at which the cap valve is opening. It needs to be verified against the manufacturer’s specification.
The cap pressure test only verifies the valve opening pressure. So it’s critical to also inspect the seal and filler-neck seat for evidence of a leak.
If the cap fails the pressure test, replace it with a unit that matches the OEM pressure rating specification. Note that on vehicles with a history of overheating, the cap might be unusable due to a deteriorated seal, even if it passes the pressure test. Clean the filler neck seat of any contaminants and verify it has a flat sealing surface.
Coolant reservoir or expansion tank
The coolant reservoir on a modern cooling system is a pressurized plastic component, with seams. It experiences material degradation from heat cycling and a chemical reaction with coolant. Repetitive temperature and pressure fluctuations can cause tank seams to leak with no visible crack. An overfilled reservoir can also leak. With no available headspace, the hot engine coolant expands and escapes through the overflow vent.
Verifying a leaking coolant reservoir requires unbolting and removing it for a visual inspection of the seams. Note that other coolant leak culprits - like a faulty cap or combustion gas from a failing head gasket - might show up as wet spots on the tank.
Drain the cooling system and replace the leaking coolant reservoir with a new unit. Before installation, inspect the mounting points for cracks on hard mounts or rubber isolators in need of replacement. Refill the reservoir with fresh coolant, keeping it between the MIN and MAX marks.
Heater core
The heater core is a small radiator within the dashboard that uses engine coolant to warm the cabin. It typically leaks from the inside. Electrolysis is a major cause of failure due to quickly forming pinhole leaks. In electrolysis, an electrical current passes through the coolant. Degraded coolant can corrode heater cores with aluminum construction. Flowing coolant might erode the metal as well. Physical damage and punctures can also create a coolant leak.
The heater core is the only cooling system component that might present visual evidence inside the cabin. Look for damp carpet, a pungent odor, or a mist exiting the air vents. When the defrost function is activated, the mist leaves a greasy film on the inside of the windshield.
If you can see pinholes in the core and no other signs of damage, check for electrolysis using the test described in the radiator section. A reading above 0.3 volts would explain the pinholes.
It’s important to be thorough with this repair to avoid repeating the process, due to the amount of labor required to access the heater core deep behind the dashboard.
Once accessed, look for leaks on the inlet and outlet tubes. These are caused by a high coolant flow rate. If the vehicle has a flow restrictor in the heater hose, replace it with a new one.
Don’t use a stop-leak product. It can clog up the narrow passages of the heater core. Thoroughly flush the cooling system to eliminate electrolysis. Flush the heater core to clear any sediment buildup. Replace the heater core with a new unit if it can’t be properly serviced.
Intake manifold gasket
On certain 1990-2005 GM V6 and V8 engine families, the original intake manifold gasket is prone to failure. It features a nylon-filled plastic-carrier design. After 60,000 to 70,000 miles, the gasket’s raised sealing beads around the coolant passages tend to soften, deform, and leak.
If coolant is leaking externally, it can be seen with the naked eye or with dye. Internal leaks that end up in the crankcase can be identified by foaming or contaminated engine oil. You can also pull the spark plugs and look for chalky white deposits on the electrodes. For further evaluation, run a cooling system pressure test and inspect the gasket, engine oil, and spark plugs for signs of a leak.
Replace the leaking gasket with the redesigned OEM GM component (p/n 89017279) or an aftermarket solution like the Fel-Pro intake manifold gasket. The Fel-Pro gasket has an aluminized steel carrier that resists coolant damage and contains torque limiters to avoid bolt overtightening. Before installation, clean both sealing surfaces of any debris or fluid contamination. Use the revised, GM-specified manifold attaching bolts with a pre-applied thread-locking compound.
Failed head gasket or cracked cylinder head
With a failed head gasket or cracked cylinder head, combustion pressure can leak past the seal and pressurize the cooling system. This pushes coolant out of the system and causes the coolant level to drop. On the intake stroke, an engine in this condition can also pull coolant into the combustion chamber, where it’s burned off.
The gasket failure is typically caused by dissimilar metal expansion (aluminum and iron) wearing out the gasket fire ring, an overheating engine, or the cylinder head lifting away from the engine block due to high pressure.
Visible indicators include white steam exiting the tailpipes, milky engine oil, and bubbling in the radiator or coolant tank. However, the presence of bubbling alone could also be due to a leaking radiator cap seal.
To determine if engine exhaust is present in the coolant, use a chemical combustion leakage kit with a container of reactive leak-detector fluid. Draw warmed coolant into the container and look for a fluid color change according to the kit’s instructions.
Similarly, a dye kit can be used to test air from the top of the radiator tank for the presence of exhaust gas. Note that the air test method will only confirm the leak when the faulty cylinder is firing.
Perform a cylinder compression or leak-down test to evaluate for loss of compression. To perform a pressure test, remove all spark plugs and pressurize the cooling system overnight. The next day, inspect for signs of coolant in the cylinders using a borescope.
Remove the cylinder head(s) and inspect for damage and warpage using a straightedge and feeler blade. Also check the block surface for irregularity. Resurface a head with more than 0.003 inches of warpage. If the head has experienced extreme heat damage, replace it with a new or rebuilt unit.
Cracks in the head or block, except in the main bore or web area, might be repairable. Costs can be high, so it’s important to weigh the cost of repairing cracks against a new unit.
When replacing the head gasket, also replace the intake and exhaust manifold gaskets. Use replacement cylinder head bolts on late model engines with non-reusable torque-to-yield bolts.
Freeze plugs (core plugs)
Freeze plugs are used during engine manufacturing to seal casting core holes and close the cooling system. They aren’t intended as a freeze-protection device and shouldn’t be relied on to do so. Over time, deteriorated coolant and electrolysis can corrode these plugs from the inside, causing leaks.
Examine the side or back of the engine block for freeze plug coolant leaks. But be aware that visible coolant here is often due to the water pump weep hole. Given the inherently difficult access, add UV dye to the cooling system, run the engine, and use a UV light to pinpoint the leak source.
A cooling system pressure test can also be performed by connecting a pressure tester to the radiator cap or coolant reservoir. Using a hand pump or shop air, pressurize the system to normal operating pressure. If the system can’t maintain pressure for 10 minutes, it suggests an internal leak.
Apply grease to the existing plug(s) to prevent metal shavings from entering the cooling system. Drill a pilot hole in the plug. Remove it using a slide hammer with a sheet metal screw. Clean the holes with isopropyl alcohol to remove any sealant. Then fit new units. Replacement steel or brass freeze plug kits are available from Melling.
Note that replacing rear plugs typically requires pulling the transmission for access. Thoroughly flush the cooling system. The coolant itself is the underlying problem.
Engine or transmission oil cooler
An engine or transmission oil cooler is a small heat exchanger that keeps the oil temperature in a safe operating range. They’re often located inside the radiator tank. They operate by running two fluids on either side of one wall. Oil cooler pressures run higher than cooling system pressure. So when the wall is breached, oil will leak into the coolant tank.
Look inside the radiator tank for an oily sheen on top of the coolant (early leak) or a brown milky sludge within the coolant (long-term leak). Coolant that is pink and foamy points to a leak from the transmission cooler in the radiator tank. Use oil dye in the crankcase lubricant and a leak detection lamp to pinpoint the source of the leak.
Replace the engine or transmission oil cooler with a new unit, along with any gaskets. Flush and refill the cooling system. Note that if the transmission is contaminated with engine coolant, all friction materials, all internal seals, all plastic thrust washers, and the torque converter should be replaced.
Niel Stender is an automotive journalist and mechanical engineer, and his work has appeared in SlashGear, CarBuzz, HotCars, BGR, and CarsForSale. His first job out of college was at Ford, working on the S197 GT500 with SVT. He also runs The Knocking Rod, an independent automotive publication. He's a member of the New England Motor Press Association and is based in New Hampshire.
Andy Lindman is an ASE Master Technician with more than 30 years in the automotive industry. He started as a dealership technician in 1993, advanced to shop foreman, and then became a service manager leading a department of 26 people. As an automotive educator, he has taught at Chattahoochee Technical College and spent 12 years as Automotive Program Director at Gwinnett Technical College, where he built educational partnerships with Mopar CAP, Mercedes-Benz, Deutz diesel, and Daimler Trucks North America.