5 Reasons Your Coolant Reservoir Is Overflowing (Step-by-Step Diagnosis)
Few things are as alarming as seeing coolant gush from your car’s coolant reservoir like a steaming volcano of excess heat and liquid. Panic sets in and questions flood your mind.
Why is this happening? Is this the end of your engine? Can you make it back home? Should you immediately top off the coolant level?
Understanding the reasons for this unexpected overflow of coolant can be the difference between a minor hiccup and a major repair.
Common Causes of Coolant Reservoir Overflow
A vehicle’s coolant can overflow from its respective overflow or surge tank for a number of reasons, some of which tend to be more difficult to isolate than others. Luckily, those with a little preexisting knowledge on the matter can often still reach a timely diagnosis.
The following are several of the most common causes of overflowing engine coolant.
#1 – Failed Thermostat
One of the most common causes of overflowing or overpressurized coolant is a thermostat that is stuck in the closed position. When this occurs, coolant is unable to circulate through a vehicle’s radiator, leading to rapid overheating.
#2 – Faulty Pressure Cap
It is also not uncommon for a coolant reservoir’s cap to fail with time, thereby allowing boil-over to occur at a lower than maximum system temperature/pressure.
When this happens, overflow of the coolant reservoir can occur after nothing more than a spirited period of interstate driving, or a lengthy stop at a traffic light on a warm day.
Related: Symptoms of Radiator Cap Failure
#3 – Overfilled Coolant
Yet another common cause of cooling system overflow is the overfilling of this system itself. Whenever too much coolant is introduced into the cooling system, room for thermal expansion is drastically reduced.
As a result, overall system pressure can rise dramatically, leading to overpressurization.
#4 – Water Pump Failure
Though somewhat less common, an engine’s water pump can actually fail under normal operating conditions, thereby leading to rapid overheating. This occurs most commonly when the impeller of an aging water pump corrodes and degrades to the point of failing to provide adequate coolant circulation.
#5 – Head/Head Gasket Failure
If a vehicle’s engine has overflowed coolant from its surge tank on more than one occasion, with little apparent reason, a cracked head or blown head gasket can be to blame.
Under these circumstances, lube oil and compression are often allowed to infiltrate the cooling system, thereby leading to decreased cooling capacity and overpressurization.
Other Less Common Causes
The following are additional potential causes of coolant system overheating and overpressurization. Though somewhat rarer, these possibilities are all worth keeping in mind when contending with such issues.
- Broken serpentine belt
- Dirty or clogged radiator
- Blockages within cooling system
- Failed cooling fan
- Air pockets in cooling system
Beware of False Online Information
Be cautious of online advice about topping off your cooling system, or the best ways to get your vehicle home after an instance of overheating.
Many of these articles fail to specify that an engine’s cooling system should not be filled with cold water if at operating temperature. Attempting to do so can lead to the cracking of an engine’s block or cylinder heads. Ideally, an engine should be allowed to cool to near ambient temperatures before attempting to refill the cooling system.
Despite what is occasionally read, you should avoid attempting to drive a vehicle home that repeatedly overheats. Disregard information that describes topping an engine’s cooling system off on multiple occasions just to make it home.
Attempting to do so, despite repeated overheating, can lead to head gasket, cylinder head, or engine block failure. In these situations, it’s best to call for a tow truck.
Read Also: Can You Use Water Instead of Coolant In an Emergency?
Why You Shouldn’t Ignore the Issue
Overheating, or any potential signs of engine overheating, such as coolant system overpressurization, should never be overlooked. Ignoring issues of the like can lead to significant internal engine damage that is not easily resolved without invasive, and often costly repairs.
Repeated overheating, or even a single severe case of overheating, can lead to head gasket failure, or possibly cylinder head failure. When this occurs, the only viable option is to replace the defective head gasket or the compromised cylinder head. These types of repairs can easily exceed $1,000-$1,500 in overall cost.
No matter the circumstances, you should always make an attempt to diagnose and repair the root cause of your vehicle’s coolant overpressurization as soon as possible. Doing so can literally save you thousands of dollars in additional repairs that can prove necessary, should the issue itself be ignored.
Diagnosing Coolant Overflow
The following steps can be used to assist in diagnosing the root cause of coolant overflow. It is also advisable to consult model-specific service literature for your vehicle, to assist in situation-based diagnostics.
Step 1: Check Coolant Level
Before jumping into the diagnostic process, check to verify that you have not been overfilling your engine’s cooling system. Pay close attention to the capacity marks on your engine’s surge tank.
Step 2: Test Reservoir Cap
With the use of a cooling system pressure tester with a cap adapter, check to ensure that your reservoir’s cap does not open until the pressure is specified. Replacement will be necessary if the cap opens below this pressure.
Step 3: Perform Running Tests
Check to verify that your vehicle’s serpentine belt is in place, encompassing the water pump pulley, then start your vehicle’s engine. Allow the engine to warm to operating temperatures, before verifying that the top radiator hose is warm/pressurized.
Next, visually verify the engagement of all equipped cooling fans.
See Also: 7 Reasons Why Your Lower Radiator Hose is Cold
Step 4: Check for Combustion Gases
If all appears to be operating as intended, and no overheating is evident, visually inspect the coolant overflow reservoir for signs of bubbling. If none are present, shut off the vehicle’s engine, and let it cool completely to ambient air temperature.
Next, use a compression gas tester as designed, to check for detectable gasses within the reservoir with the vehicle started, and at idle. If changes to the testing medium indicate contamination, head gasket failure, or other internal sealing surface failure is to be suspected.
Step 5: Remove Water Pump for Inspection
If no evidence of combustion gasses are evident, remove the affected vehicle’s water pump for inspection of its impeller. Replacement will be necessitated if any signs of degradation are evident.
Step 6: Use Temp Gun for Further Assessment
If no issues have been found to this point, an infrared temperature gun should be used to isolate any potential blockages that might exist within the cooling system.
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