What Causes Back Pressure In Coolant Reservoir? Top 3 Reasons (And How to Fix it)

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back pressure in coolant reservoir

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The cooling system in your car is pressurized. This enclosed system should have a normal flow of coolant to and from the coolant reservoir. But when there is back pressure in the reservoir, you might see the coolant overflow or form bubbles.

So, what causes back pressure in the coolant reservoir? A leaked head gasket is the main reason behind the back pressure. Besides, trapped air and a blocked radiator can also cause back pressure in the coolant reservoir.

You need to know the detailed reasons with effective solutions to eliminate the back pressure. Let’s learn.

back pressure in coolant reservoir

Back Pressure In Coolant Reservoir: What Are the Reasons?

You might already know that the radiator pressure cap builds up pressure inside the cooling system. It increases the boiling point of the coolant so that the coolant can dissipate heat instead of evaporating.

When the coolant gets hot, it expands in volume. The excess coolant then goes into the overflow tank. After the engine cools down, the coolant should flow back to the radiator.

But sometimes, there is additional pressure in the system that prevents the coolant from going back to the radiator. It has 3 main reasons discussed below.

1. The Head Gasket Is Faulty

The head gasket in the engine works as a seal between the combustion chamber and the cylinder head. Inside the combustion chamber, the fuel and air mixture is compressed at a high pressure.

As long as the head gasket works fine, fluids like fuel and coolant stay separate. It also ensures the exhaust gas can leave the system properly. But sometimes, the head gasket can become brittle and develop cracks.

Due to these cracks or leaks, the exhaust gas will enter the cooling system at a high pressure. This will cause the internal pressure of the cooling system to rise. As a result, you will see the coolant overflow from the reservoir.

You might suspect that there is a hole in the coolant reservoir. But the coolant can escape through the overflow cap.

2. Trapped Air Inside the Cooling System

The cooling system in your car shouldn’t have any air. As long as the system is airtight, the coolant expands and contracts properly. Even though the expansion causes the coolant to go into the overflow tank, the coolant doesn’t leak.

But when there is air in the system, it also expands with the heat. The expanded air takes a large volume of the system and forces the coolant to go into the overflow tank. From there, the coolant can leak through the overflow cap.

You will also notice air bubbles in the coolant reservoir. If you see coolant coming out of an overflow tank, it is most probably caused by any of these two reasons.

3. Clogged Radiator

When the coolant goes through the radiator, it dissipates heat efficiently. The coolant then flows through the engine again to absorb the heat. Over time, there can be mineral deposits or debris in the radiator.

These contaminants will clog the radiator fins. As a result, the coolant can’t properly flow into the reservoir or get back to the radiator. A portion of the coolant will get stuck in the reservoir.

When the hot coolant expands, the coolant in the reservoir will face back pressure. As a result, the coolant may overflow from the reservoir.

How to Fix a Faulty Head Gasket?

Let me guide you through the steps of fixing a faulty head gasket.

Step 1

Check if there is coolant smoke coming out of vents, or if there is a sweet smell from the exhaust, the head gasket may be faulty.

Step 2

You should use a head gasket sealer to fix the crack or leak. I suggest using the Blue Devil Head Gasket Sealer. This is extremely easy to use and can efficiently seal the leaks. Pour the sealer into the radiator and allow it some time to bond the leaks.

Step 3

Even though the head gasket sealer fixes a faulty head gasket, you should take the car to a repair shop for a permanent solution. Depending on the severity of the leaks or cracks, you should replace the head gasket. It might cost about $1,000-1,500, including parts and labor.

See how to seal the head gasket.

How to Purge Air from the Cooling System?

You can easily let the trapped air out of the cooling system to prevent air pockets. Here are the steps to do that.

Step 1

Turn the engine off and let it cool down completely.

Step 2

Remove the pressure cap on the radiator. You may also need to remove the reservoir cap if it is pressurized.

Step 3

Pour fresh coolant into the radiator and make sure the coolant level is at “FULL”.

Step 4

Turn the engine on and let it idle for some time. It will initiate the coolant flow.

Step 5

Once the coolant starts flowing, its level will drop. At the same time, the air trapped inside will come out, which causes the coolant level to drop.

Step 6

Pour more coolant into the radiator until all the trapped air gets out.

Step 7

Once there is no sign of air bubbles, and the coolant level is steady, tighten the pressure caps. Before that, top the coolant off if necessary.

Step 8

Turn the engine off and then on. After driving for a few miles, check if the coolant level is constant and if the coolant has any air bubbles.

How to Clean a Clogged Radiator?

Follow these steps to clean a clogged radiator.

Step 1

Remove the drain valve below the radiator and drain all the coolant.

Step 2

Close the drain valve. Then, pour a store-bought radiator flush into the radiator.

Step 3

Turn the engine on and let it idle for some time. The chemical will clean the clogs in the radiator.

Step 4

Drain the chemical and then flush the radiator with water once again.

Step 5

Now, refill fresh coolant into the system.

People Also Ask

Here are answers to some commonly asked questions about back pressure in coolant reservoirs. Check them out.

FAQ Gear Head New

Can overheating cause back pressure in the coolant reservoir?

It is unlikely for an overheated engine to cause back pressure in the reservoir. But sometimes, the engine temperature can be too much to cause back pressure.

What is the ideal pressure in a cooling system?

The ideal pressure in a cooling system is about 16 psi. If the pressure is higher, there can be back pressure in the reservoir.

Can a faulty radiator cap cause back pressure in the coolant reservoir?

If the pressure valve that lets coolant go back to the reservoir is bad, it can cause back pressure. But this isn’t so common.

Conclusion

Back pressure in the coolant reservoir causes several risks. Due to the back pressure, the coolant can overflow. As a result, the coolant level will be lower than normal, and the car will overheat. So, you should take prompt steps to mitigate the issue.

I explained the most common reasons behind the problem and showed the detailed solutions. Apply these solutions to get rid of the back pressure in the coolant reservoir. Besides fixing the head gasket, you should clean the radiator and purge air from the system.

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