The coolant reservoir, or the overflow tank, keeps your car’s coolant reserved. It keeps hot coolant until your car’s engine is chilled. But, overflowing the tank during the engine operation is something alarming.
And your question is, why is my car coolant coming out of an overflow tank? It can happen due to the following reasons.
- Broken overflow tank cap
- Too much coolant in the cooling system
- Damaged thermostat
- Clogged or damaged radiator
- Damaged radiator cap
- Blown head gasket
In this problem-solving guide, I will discover the reason behind the coolant coming off the overflow tank in detail. I will also find out the potential solution as well. Read on, and let’s dig deeper.
Coolant Coming Out of Overflow Tank | Overview of All Reasons
There are several components related to the overflow tank. They are the radiator, water pump, thermostat, etc. So, most of the problems are likely to happen related to these components.
Let’s see the reasons in detail that make the engine coolant coming out of the overflow tank.
1. Too Much Coolant in the Cooling System
One of the most common reasons for coolant coming out from the tank is too much coolant in the overflow tank. In your car’s coolant reservoir, there are two marks labeled F(Full) and L(Low), or Maximum and Minimum.
The coolant level can go up to the maximum or F level. But, if you add coolant that exceeds the maximum level, it will create an issue during the engine operation.
While the engine is running, it heats up and expands the coolant. As a result, the coolant comes out of the overflow tank.
If this keeps happening, this can cause leaks in the system, damage to the engine, damage to the radiator caps, and many more.
Diagnosis and Solution
- Check for the coolant level if it stays in between the Minimum and Maximum marks.
- If the coolant exceeds the maximum mark, please remove the excess amount. Also, don’t fill up the reservoir above the Maximum mark. Make sure there is no dirt in coolant reservoir.
2. Broken Overflow Tank Cap
The overflow tank cap helps to keep the coolant inside the tank. However, the cap can be damaged due to age and too much pressure from the coolant. Also, too frequent overheating of the engine can cause the cap to be faulty.
As a result, the faulty cap can’t manage the coolant stability, and the coolant gets hot gradually. Thus, at a time, the cap gets entirely damaged, and coolant comes out of the tank.
Diagnosis and Solution
- Check for the overflow tank cap for any instability or physical wear and tear.
- If you find the coolant is coming out due to the damaged cap, replace it.
- Changing the cap is too easy. Just turn the cap anti-clockwise and remove it. Put the new cap and turn it clockwise.
Here’s a video showing how to do this.
3. Damaged Thermostat
A thermostat keeps the minimal engine temperature to operate the engine. If the thermostat is okay, the coolant will go through the radiator to the engine to keep it within the rated temperature.
But, a faulty thermostat can’t control the coolant, and the engine overheats. At the same time, if such happens, the coolant also overheats, gets thinned, and gets more pressure than usual.
As a result, coolant can come out of the overflow tank. You may also see the coolant creates a bubble inside the overflow tank.
Diagnosis and Solution
Along with coolant overflow through the tank, check for unusual noise and erratic temperature changes in the car. If so, the thermostat is faulty. You need to replace the faulty thermostat to get rid of the current situation. Follow the steps to replace it:
Step 1
Put a pan under the thermostat housing to hold the drips of coolant.
Step 2
Remove the thermostat housing under the car to remove the thermostat.
Step 3
After removing the thermostat, place the thermostat in the thermostat housing and reattach the thermostat housing.
Watch the following video on how to replace the thermostat.
4. Clogged Radiator
The radiator passes the coolant to the engine. When the radiator is clogged due to dirt, debris, and grime, it can’t pass the coolant to the engine. As a result, the coolant can get hot and push it backward.
So, in such a case, you will see the coolant coming out of the overflow tank.
Diagnosis and Solution
- If the radiator is clogged, the engine will overheat, and the coolant will be discolored and thicker.
- If so, you must flush the coolant through the radiator.
- After flushing the radiator, you need to replace the thermostat if required. Replacing the thermostat during the coolant flash is called super flash.
- Now, refill the coolant using Valvoline’s Multi-Vehicle 50/50 Prediluted Ready-to-Use Antifreeze/Coolant.
This video shows how you can properly flush the coolant system.
5. Damaged Radiator Cap
The coolant mixture goes through the radiator to the engine. A damaged radiator cap can’t keep the coolant sealed, and the coolant comes out of radiator with cap off during the engine operation.
Also, the cap may be loose, and the coolant can come out.
Diagnosis and Solution
Finding out the broken radiator cap is as simple as the broken coolant reservoir tank cap I have done previously.
- Check the radiator cap. If it is loose, tighten it by twisting and turning clockwise.
- If the cap is damaged, replace the broken cap with a new one.
Here’s how you can replace it in a simple way.
6. Blown Head Gasket
When the engine overheats due to any internal fault, the engine head gasket can be worn out.
After the head gasket is worn out, internal engine pressure creates high temperatures and throws back the combustion gasses to the coolant system.
In this case, the coolant is boiled and creates a bubble through the coolant system. As a consequence, you will see the coolant coming out of the overflow tank and radiator.
Diagnosis and Solution
- If the engine gets an issue and the head gasket blows out, you will see white smoke through the tailpipe due to engine overheating. You will also see the “check engine light” is illuminated.
- This is a serious issue, and I would suggest you consult a professional mechanic. The mechanic will diagnose the engine overheating and gasket blown-out issue and will resolve the issue accordingly.
- This may cost you anywhere from $200 to $4,000.
Wrapping Up
Coolant coming out of the overflow tank can happen for many reasons, as I discussed in this guide. I hope you can solve the issue if it happens since most of them are easy to fix.
However, if the engine head gasket is blown out due to the engine’s internal issue, consulting a professional mechanic is highly recommended. This may need an engine repair, as well as gasket replacement.
