Coolant leaks are common problems that lead to overheating, dashboard warnings and reduced engine performance. These leaks often arise from defective EGR modules, corrosion on brake lines, or wear to hoses and water pumps. In some models, such as the BMW X5, coolant leaks are notorious due to specific design flaws. Do you know how to spot these problems in time?
Why your car is leaking coolant
Coolant leaks have several causes. Defective EGR modules (exhaust gas recirculation modules) cause leaks due to damaged gaskets or cracks in the module itself. This results in loss of coolant that mixes with exhaust gases. Corrosion on brake lines and other metal parts occurs due to chemical reactions between coolant and metal. Old coolant accelerates this corrosion.
Water pumps and thermostats fail due to heat load and corrosion. Electric water pumps fail because of wear to internal components. Hoses and connections become brittle due to ageing and temperature changes. In some engines, hidden leaks develop at the back of the engine. Dripping coolant evaporates there on hot components. You can only recognise this by a sweet smell or a falling level.
Questionable fuel and low-quality oil make coolant problems worse. Inferior fuel leads to incomplete combustion. This raises the engine temperature. It puts extra stress on the cooling system. Low-quality oil offers less protection against heat, causing components to wear out faster and increasing the risk of overheating.
‘ If this drops despite topping up, there is a leak. ’
Recognising coolant leaks and overheating
Dashboard warnings for a low coolant level are the first sign. These messages often appear after longer journeys when the engine is warm. A sweet smell in or around the car points to burnt coolant. White smoke from the exhaust may indicate coolant in the combustion chamber. This is often caused by a blown head gasket.
A visual check quickly gives you an idea of what is going on. Look under the car for damp patches on the undertray, near hoses and around the radiator. Take a torch and a mirror. Inspect hard-to-reach areas near the water pump and at the back of the engine. Bubbles in the expansion tank when the engine is idling indicate air or exhaust gas in the system.
You can recognise overheating from rising temperature readings on the dashboard. The engine loses power and responds less smoothly. In severe overheating, you may hear rattling noises. This happens because metal components expand as a result of heat. Check the coolant level every week. If it drops despite topping up, there is a leak.
What to do if the expansion tank has cracked
A cracked expansion tank (pressure vessel that absorbs coolant expansion due to temperature) loses coolant under pressure. You can recognise this by sudden drops in the fluid level and wet patches around the tank. Stop driving immediately when the warning light comes on and the temperature gauge rises. Continuing to drive can lead to engine damage due to overheating.
Let the engine cool down before removing the cap from the expansion tank. If you open the cap while the system is under pressure, hot fluid and steam can cause burns. Top up with approved coolant that is suitable for your car type. Never mix different types of coolant. This causes chemical reactions and blockages.
Replace the expansion tank as soon as possible. Temporary repairs with tape or sealant are not reliable. High pressure and temperature quickly cause these stopgap solutions to fail. When replacing the tank, also check the expansion tank cap. A faulty cap will not hold pressure, which leads to evaporation and air pockets in the system.
Regular maintenance prevents problems
Replace the coolant every two years. This is a general guideline that can differ per car manufacturer. Old coolant loses its protective properties against corrosion and frost. Flush the system when replacing the coolant to remove deposits and contamination. Only use fluid approved by the car manufacturer.
Clean radiators regularly from the outside. Dirt, leaves and insects block the cooling fins and reduce cooling capacity. Use compressed air or water at low pressure. Use this to clean the radiator. At the same time, check for damage to the cooling fins.
Replace the expansion tank cap preventively during major services. This cap regulates the pressure in the cooling system. Purchase costs are low. A faulty cap causes pressure loss, which lowers the boiling point of the coolant and promotes overheating. Also check all hoses for small cracks, soft spots and leaks at connections.
‘ One leak can mask another due to pressure loss. ’
Diagnosis with a pressure test
A pressure test locates hidden leaks. These cannot be found visually. Pump the cooling system up to around 1.1 bar with a pressure test kit (hand pump with pressure gauge). The pressure must not drop within fifteen minutes. If the pressure does drop, there is a leak.
During the pressure test, look for bubbles at connections, hoses and the water pump. Pay attention to the weep hole of the water pump. This is a small opening that drains fluid in the event of an internal leak. Also check the back of the engine with a mirror. Leaks behind the engine are often invisible without tools.
Always carry out a new pressure test after a repair. One leak can mask another due to pressure loss. Bleed the system thoroughly via the highest bleed screw (screw that allows air to escape from the cooling system). Fill the system until a steady flow without air bubbles comes out of the bleed screw. Air in the system causes local overheating and gives false sensor readings.
Consequences of long-term coolant leaks
Long-term coolant leaks lead to serious engine damage. Overheating causes the cylinder head to warp. It also damages the head gasket. Repairing this often costs thousands of euros. The engine can completely seize when critical temperatures are reached.
Corrosion accelerates when coolant levels drop. Components that are not fully submerged rust faster. This applies to radiators, heater cores and metal pipes. Corrosion causes new leaks and blockages, allowing the problem to spread.
Reduced engine performance is a direct consequence of poor cooling. The engine runs less efficiently at high temperatures. It uses more fuel. Sensors produce error messages due to abnormal temperatures. This leads to malfunctions in engine management. Weekly topping up masks the problem but does not solve it. Timely repair saves considerable costs.
Practical tips from owners
Carry out a pressure test regularly, especially after repairs. This prevents you from overlooking a masked leak. Check the coolant level weekly, preferably with a cold engine. Note down when you top up to identify patterns in leakage.
When fitting a new thermostat, also replace the housing and gaskets. These parts cost little but prevent future leaks. When topping up, always use the correct coolant for your engine. The wrong fluid can attack aluminium components.
Monitor the temperature gauge while driving. Unusual rises point to problems before warning lights come on. If you are unsure about noises, smells or changes in performance, have the cooling system checked. Early detection prevents costly engine damage and keeps your car reliable.
‘ On the Carnews website you will find extensive information about engine problems, maintenance tips and model-specific issues. ’
More information about car problems
Coolant and thermostat problems are just one aspect of car maintenance. On the Carnews website you will find extensive information about engine problems, maintenance tips and model-specific issues. Discover more articles about diagnosis, repair and preventive maintenance to keep your car in top condition.
Frequently asked questions
You can recognise them in time by regularly checking the coolant level and watching for warning lights for coolant or engine temperature on the dashboard. Look out for symptoms such as a rising engine temperature, a sweet smell around the car, damp or splashing traces under the engine and white smoke from the exhaust. You often notice a faulty thermostat by an engine that either does not reach operating temperature (low temperature gauge, little heat from the heater) or overheats quickly. If in doubt, a pressure test of the cooling system and reading out fault codes is useful to detect leaks and thermostat faults at an early stage.
The most common causes of coolant leaks are aged or cracked hoses, leaking water pumps, porous gaskets (such as the valley pan) and defective connections or sensors, often made worse by heat and old coolant. Thermostat failures usually arise from wear of the mechanism or electrical control, corrosion and prolonged driving with too high an engine temperature. Prevention consists of regularly replacing coolant with the correct specification, replacing hoses, clamps and the water pump in good time, and periodical pressure testing of the cooling system for small leaks. In addition, checking engine temperature and coolant level more often and having diagnosis carried out immediately in case of fault codes or temperature fluctuations helps to prevent problems.
Coolant and thermostat problems can cause the engine to run too hot or too cold, leading to reduced performance, higher fuel consumption and possibly limp-home mode of the engine management system. In the event of overheating, components such as the head gasket, cylinder head and water pump are damaged, which can cause serious and expensive engine damage. Under heavy load (motorway, mountainous terrain, towing a trailer), a poorly functioning cooling system makes temperature spikes worse, greatly increasing the risk of sudden failure. In the long term, this reduces the reliability of the engine and increases the likelihood of breakdowns or costly overhauls.
You can regularly check the coolant level yourself and, if necessary, top up with the prescribed coolant, visually inspect hoses, radiator and expansion tank for traces of moisture or corrosion, and pay attention to smells, smoke and temperature warning lights while driving. Simple maintenance also includes replacing coolant in good time according to the manufacturer’s instructions and checking for air bubbles in the expansion tank after topping up. Professional help is necessary in case of repeated coolant loss, visible leaks that do not come from a hose clamp or cap, overheating, bubbles or exhaust gas smell in the expansion tank, or fault codes indicating water pump or thermostat problems. In the event of suspected head gasket damage or unclear, “invisible” leaks, diagnosis with a pressure test or CO₂ test by a garage is also required.
For performance cars and motorsport use, high-grade, ready-mixed coolants (for example with higher boiling point and corrosion protection, often based on manufacturer or motorsport specifications such as BMW/MB/VW standards or specific race coolants) are usually recommended, in combination with a correctly set-up cooling system. A “cooler” or performance thermostat can be useful for heavily loaded engines with a lot of extra heat generation (turbo, tuning, track use), but must match the engine management software and the entire cooling system (radiator capacity, fan control). The most important considerations are: the engine’s operating range (cold versus operating temperature), reliability under high load, compatibility with materials (aluminium block, gaskets) and manufacturer’s warranty. For road use with occasional sporty driving, an OEM-approved coolant and original thermostat is usually the safest choice; true racing products are mainly worthwhile for prolonged track or competition use.