Reasons & Preventions: What Causes a Freeze Plug to Pop Out?

You may call it core or expansion plugs. Well, whatever the name is, a freeze plug is a small, cylindrical device. It protects automotive engines from cold-related damage. And you can find it more in water-cooled internal combustion engines. But do you know that this plug can come out? I welcome you to find out what causes a freeze plug to pop out.

In short, it may happen due to several reasons. The primary reason is corrosion that breaks down the seal, and then the plug leaks. Again, low coolant levels can cause engine overheating and increased pressure. This heat and pressure can force the plug out. Sometimes, exposure to salt water, high humidity, or extreme temperatures can also cause freeze plug failure.

Then, how are you going to prevent the issues? I guess you will learn it soon by reading the whole article. Read it completely and maintain the overall health and performance of your vehicle’s engine.

Freeze Plug: Definition, Material, Design, & Function in an Engine

Now, you may ask why you should know about its whereabouts. Indeed, knowing about the freeze plug may give you primary ideas as to why it can pop out.

A freeze plug, as I said, is a small device used to allow for the insertion and removal of sand during the engine casting process. In this process, molten metal is injected into a sand casting to form the engine block (1,2). Automobile companies use freeze plugs to prevent coolant leaks by filling the empty passages (3).

Material & Design

You can find freeze plugs made from metals like brass, rubber (expanding), steel, or stainless steel. Brass plugs are more popular due to their resistance to corrosion. But if your vehicle engine has steel plugs, they may rust soon (4).

However, these plugs are designed to fit snugly into the engine block’s passages. This design allows the plug to protect the engine from coolant leakage. Besides, the thickness and diameter of a freeze plug can also impact its performance.


When the engine coolant freezes, it may cause the block to crack. Water expands as it freezes, which is absolutely natural (and scientifically appropriate). This expansion creates pressure that eventually causes cracks to form. And this is where the freeze plug comes. They prevent the crack-forming by allowing the frozen water to expand out the plugs’ passages.

Unfortunately, a low-quality plug made from poor materials can pop out soon. As well, it may not prevent the cracks from appearing in the engine.

The Process: How Does a Freeze Plug Pop Out?

That’s when I found the other front freeze plug was just lying in place. Neither plug appeared to be tampered with. How could this happen.”- Someone on a discussion asked about what causes a freeze plug to pop out once he faced this issue (5). Another query on a different discussion was similar to this, “I’m looking for opinions on what else to check for the reason why I blew a freeze plug out of my block.”

So, you can see that knowing the reasons for a plug to pop out is a hot topic. Now, without spending more time, let me explain the possible causes of a popped-out freeze plug.

In general, freeze plugs are designed to pop out to prevent engine blocks from cracking. They allow the frozen coolant to expand out of the engine. Beside this reason, there are some more that can be:


Earlier, I said about it, right? Well, most freeze plugs are made of metal or galvanized steel. It makes them vulnerable to rust and corrosion. And corrosion is the most common reason for core plugs to come out.

It can occur due to environmental factors like exposure to salt water, high humidity or temperatures, and acidic contamination. If you do not change the coolant according to the recommended schedule, corrosion will happen (1, 6).

Low Coolant Level

Did you check the level of coolant in your engine? If it is low, the engine can overheat. But it will create a lot of pressure inside the engine. Then, this pressure will force the plug to get out (7).


No, I am not talking about the long duration of the engine that can cause an overheated engine. It may also happen due to a leakage or problem in the coolant. When coolant leaks, the engine temperature rises faster and makes the freeze plug pop out. So, you must be sure that overheating is not causing the issue.

Aged Freeze Plug

With its age, a plug can wear and tear over time. The degradation can cause freeze plug leaks and vibration. Gradually, it will pop out due to the vibrations and damage.

Improper Installation and Manufacturing Defects

This reason may not be common, yet it exists. If you have installed the freeze plugs incorrectly, they may fail prematurely. Additionally, manufacturing defects, though rare, can also cause freeze plug problems.

Symptoms and Early Detection of Freeze Plug Failure

You can identify if a freeze plug pops out through some signs and symptoms. They can be:

Coolant Level

Suppose you have detected a low coolant level in the reservoir but found no visible leaks. Then, it can refer to the freeze plug that has ruptured. If the level is fine, then there must be another reason.

Heated Coolant Odor

Do you get any strong smell of heated coolant? It may indicate that the plug has blown and the coolant is leaking from the engine.

Unusual Noises and Engine Performance Issues

If you hear any odd noises from the engine area, then there is a chance of a popped-out freeze plug. If the freeze plugs are failing, they may create a pathway for coolant to enter the combustion chamber. As a result, it will cause performance issues like misfires or rough idling.

White Exhaust Smoke and Bubble Formation

These smoke and bubbles can be another symptom of a blown freeze plug. You must check the coolant reservoir for assurance.

Engine Overheating

When you get your engine overheated consistently, it is possibly a sign of coolant leaking outside the engine. As I have already said, it causes a freeze plug to pop out.

Prevention: How to Avoid a Freeze Plug from Popping Out

Freeze plugs are constructed to protect the engine from cold-related damage. Like other mechanical things, they are also not immune to failure (3). Still, you can prevent them from popping out following these tips:

Regular Inspection and Maintenance

Expansion plugs can fail over time, even though they often last for hundreds of thousands of miles. So, you must check their condition regularly. If you drive primarily under cold temperatures or extreme conditions, the engine may need more frequent maintenance (8).

Maintain the Coolant Level

I suggest you do not forget to keep your coolant level between high and medium. This is because a low coolant level can cause the engine to overheat. In this way, you can prevent excessive pressure as well as resist the force to pop out the plug.

Avoid Overheating

Overheating can occur due to a leakage or problem in the coolant. If your engine is overheating, I suggest you find the main problem. Also, check for any leakages and repair them if necessary.

Replace Aged Freeze Plugs

When was the last time you changed the old freeze plugs? For sure, changing the aged plugs is a part of maintaining its health and a preventive method to get it popped out.

Use the Right Coolant

Never use random coolant for your vehicle. If you choose the wrong one, it can cause huge damage to the engine. So, you should check the car’s owner’s manual and find the recommended coolant for the engine. It will protect the engine from freezing when parked, overheating when running, and corrosion inside the engine.

Engine Block Heaters

Using engine block heaters can ensure cold weather starts. It may help in the prevention of freeze plug failures (9).

Correct Installation

Install the plug correctly to prevent it from popping out. Besides, make sure the plug fits snugly in the passageway and the seal is intact.

Final Words

So, I guess you have learned what causes a freeze plug to pop out. Yet, I am summarizing the whole thing again. Rapid temperature fluctuations, coolant levels, mechanical issues, or improper installation –are the most common reasons for the problem. You should recognize these reasons by their symptoms.

Follow the methods I have suggested to prevent a core plug from popping out. Also, I recommend you inspect and maintain the component regularly.

And that is enough for today. Thanks for reading it!

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