There are few things more alarming than coolant pouring from your car’s coolant reservoir like a hot and steaming volcano of excess heat and liquid. Panic sets in, and a flood of questions comes to mind;
Why is this going on? Is this the end of my engine? Can I make it back home? Should I immediately top off the coolant level?
Knowing the reasons why the coolant just unexpectedly overflowed could mean the difference between a minor inconvenience and a Major repair.
Common Causes of Coolant Reservoir Overflow
There are several reasons why a vehicle’s coolant may overflow from its overflow/surge tank, some of which may be easier to pinpoint than others. For the layman crying for help, those who have been through the process a few times before can usually achieve a rapid diagnosis.
Here are a few of the most common reasons coolant may overflow from the engine cooling system.
#1. Failed Thermostat.
One of the most common sources of over-pressurized, or overflowing, coolant is a thermostat that is stuck in the closed position. This allows heated coolant to overflow and bypass the radiator, resulting in extremely high engine temperatures.
#2. Water Cap Failure.
Another common reason for a failed cooling reservoir is a faulty pressure cap. If the cap has begun to fail over time, it sadly can allow boilover at lower than rated system temperature or pressure.
In these instances, overflow from the cooling reservoir can occur with nothing but a moderate drive for extended periods on the highway, or from a prolonged stop at a red light on a warm day.
#3. Overfilled Coolant.
Yet another common cause of cooling system overflow could simply be that it has been overfilled. When there is too much coolant in a cooling system, not much room exists for thermal expansion. Over a defined range of temperatures, system pressures could rise.
#4. Failed Water Pump.
It is less common, but a water pump can ultimately fail under normal operating conditions on occasion, which may suffice to induce engine overheating.
Water pumps malfunction when impellers corrode and degrade into an ineffective component and disrupting coolant circulation.
#5. Head/Head Gasket Failure.
If a vehicle’s engine starts to overflow coolant from the surge tank multiple times with no valid reason, we may be staring down the barrel of a defective, or cracked head and/or blown head gasket.
Lubricating oil and compression can find their way into the cooling system in these situations, reducing the effective cooling and pressurization capacity of the engine’s cooling system.
Other Rare Causes
Here are some more possible causes of coolant system overheating and excessive pressure. While these are less common, they are still important to consider.
- Broken serpentine belt
- Dirty or blocked radiator
- Clogs in a cooling system
- Failed cooling fan
- Air is locked in the cooling system
Watch Out For Fake Online Information
Be careful with any online advice about how to top off your cooling system, or how you can get home when/if you experience overheating.
Most of these articles never explain that if an engine is at operating temperature, you should not fill a cooling system with cold water.
Doing so could lead to cracking the cylinder heads or block of the engine. So, you should let it cool down closer to room temperature before topping off the cooling system.
All that said, ignore everything you might read occasionally, concerning trying to drive a vehicle home from repeated overheating. You should ignore anything that says it is okay to top off an engine’s cooling system on multiple occasions just to get it home.
Doing so after it overheats repeatedly can lead to failure of the head gasket, cylinder heads, or block. These cases are better served by calling for a tow truck.
Why You Need to Address the Problem
Overheating, or any signs of engine overheating, such as your cooling system has over-pressurization, should never be ignored.
If you neglect situations like these, you risk an internal engine disaster that will not be easy to rectify without costly, invasive repairs.
If there are repeat overheating events, or even just one really bad one, you may have head gasket failure or maybe even cylinder head failure.
If you have a head gasket failure/malfunction or a cylinder head failure/malfunction, the only thing left to do is replace the defective head gasket or the compromised head.
The repairs that typically need replacement can be very expensive and easily total over $1,000-$1,500.
Regardless of what is going on, you should always make it your priority to try to diagnose and repair the root cause of your vehicle’s pressurization problem as soon as possible.
You will literally save thousands of dollars by tackling this issue, if you ignore it you may need to spend thousands on subsequent repairs.
How to Diagnose the Coolant Overflow Process
The following process will help to identify the root cause of the overflow of coolant, and you should consult your vehicle’s model-specific service literature for diagnostic help that may pertain to your particular case.
Step 1: Check Coolant Level
Prior to starting this diagnostic process, ensure that you have not overfilled your engine cooling system. Pay particular attention to the capacity marks on the surge tank.
Step 2: Test Reservoir Cap
Using a pressure tester for your cooling system with the reservoir cap adapter attached, make sure that the reservoir cap does not open until the applicable pressure is reached. If it opens below this pressure, it must be replaced.
Step 3: Perform Running Tests
Make sure your vehicle’s serpentine belt is attached and encompasses the water pump pulley, then start the engine. Allow it to warm until it is operating temperature, and do a quick verification that your top radiator hose is warm/pressurized.
When doing this verification, look at all cooling fans to visually see if they are engaged and operational.
Step 4: Check for Combustion Gases
If everything is a normal operating condition and you are not experiencing overheating, visually check your coolant overflow reservoir for bubbling. If you see no bubbling, turn off the engine and allow it to cool down and reach ambient air temperature.
When at ambient temperature, use a compression gas tester as it is designed to be used with the vehicle at idle and running, use the tester only in the overflow reservoir.
If you detect any changes in the testing medium, you can suspect head gasket defects, head o-ring defects, or defects of any nature related to surface sealing problems at any time internal to the engine.
Step 5: Removethe Water Pump for Inspection
If no evidence of combustion gases is apparent, it is time to inspect the impeller on the water pump. You will need to remove the water pump from the affected vehicle, and if you see evidence that the impeller has degraded, you will have a need to replace it.
Step 6: Temp Gun for Assessment
If you have not identified any issues with the above areas, it’s prudent at this point to use an infrared temperature gun to isolate any potential blockages that may exist within the cooling system.