What Causes Too Much Pressure In Cooling System?

Barely anything can destroy an engine as fast as a faulty cooling system can. So, any issue related to engine cooling, including excessive pressure, should not be taken lightly. But on a positive note, too much pressure in the cooling system can sometimes be solved for ten bucks.

Here is what causes too much pressure in cooling system. The first possible cause is a faulty radiator cap, which is pretty simple to replace and cheap. However, it could also be a blown head gasket, cracked cylinder wall, or, more obviously, engine overheating.

What Causes Too Much Pressure In Cooling System

What Causes Too Much Pressure In the Cooling System

Before covering all the causes in more detail, it’s important to mention that unless your car is noticeably overheating, the most likely cause of high coolant pressure is a blown head gasket. Luckily, there are ways you can check that to be 100% certain, and if that checks out, start with the radiator cap.

Faulty Radiator Or Expansion Tank Cap

Although on the face of it, the radiator cap doesn’t seem to be doing much besides keeping the coolant inside the radiator, its function is much more complicated than that. The radiator cap basically serves as a coolant pressure relief valve.

Like all things in this world, the coolant expands as it heats up, and expansion in an enclosed space equals higher pressure. So, to relieve that extra pressure, the coolant cap has a spring-loaded valve, which opens when coolant pressure gets too high and allows the excess coolant to flow into the expansion tank.

Then, as the coolant pressure drops, the radiator cap valve closes, and the system continues functioning normally without you ever noticing. However, the radiator cap can fail or seize closed, which happens pretty often on older cars. And when that happens, the coolant pressure will become abnormally high at standard operating temperatures and even cause overheating.

Blown Head Gasket

The head gasket sits between the cylinder head and the engine block and stops the coolant from going into the combustion chamber. That coolant flows through passages in the engine block and the cylinder head. So, can a blown head gasket cause pressure in cooling system? The answer is positive.

Now, one of the most obvious symptoms of a blown head gasket is white or blueish exhaust smoke, missing coolant, check engine light, overheating,  and white/milky engine oil. But if the head gasket is leaking, the pressure inside the combustion chamber will enter the coolant passages and essentially inflate it as you drive. And that’s the head gasket pressurizing cooling system.

Cracked Cylinder Wall

As we mentioned, there are coolant passages going through the engine block right next to the combustion chamber. Now, although it happens very rarely, the engine block can crack, and that crack creates a path between the engine coolant and the combustion chamber.

The symptoms of a cracked cylinder wall are much the same as they are for a blown head gasket. That means exhaust smoke, missing coolant, white/milky engine oil, check engine light, poor performance, etc. Plus, the combustion chamber pressure will enter the cooling system just like it does with a blown gasket causing too much pressure in radiator hoses and the whole cooling system as well.


We already established that coolant, like all things in nature, expands as it heats up and increases cooling system pressure. Now, some cooling system pressure is normal, and there are systems designed to relieve excess pressure if the coolant starts heating up a bit more than normal.

However, if the engine overheats, the coolant pressure will get so high that it overwhelms the radiator cap relief valve. That said, engine overheating symptoms are pretty obvious, and you will see the temperature gauge go into the red, a check engine light, a coolant warning light, or an engine overheating light.

What Happens When Coolant Pressure Is Too High

When coolant pressure gets too high, the engine will start overheating. But for that to happen, the pressure needs to be abnormally high, to the point where steam is coming from the engine bay.

But if the coolant pressure is 20-30 percent over what it’s supposed to be, you probably won’t notice any symptoms for a while. But given enough time, high coolant pressure will cause leaks at different gaskets. Moreover, the coolant hoses can crack, the water pump can be destroyed, and even the radiator can start leaking.

Each of those leaks can leave you stranded or destroy the engine if it overheats more than once. So again, you should never turn a blind eye to any coolant system issues, even if the engine isn’t overheating at the time.

what causes pressure in coolant system

How Do You Fix Too Much Pressure In the Cooling System

To fix too much pressure in the cooling system, you first have to identify what’s causing the problem. That’s why we will cover each problem individually and explain how to inspect it, fix it, or how much it will cost to get fixed.

Faulty Radiator Or Expansion Tank Cap – Inspection and Fixes

First of all, to test a radiator cap, you need a radiator cap test kit. The kit is nothing more than a manual air pump with a pressure gauge and a radiator cap adapter. Therefore it’s pretty easy to use. However, it also costs at least $50.

So, if you plan on using it only once for this problem, it may be a better idea to just buy a new radiator cap without even testing it. That’s because a new radiator cap costs between $10 to $20, which is less than half the price of a tester kit.

Alternatively, you could take your car to a repair shop where they might test it for free because it doesn’t take longer than a couple of minutes, but there are no guarantees that you won’t have to pay something.

Blown Head Gasket – Inspection and Fixes

Although there are plenty of unmistakable head gasket symptoms, they might not be so noticeable in the early stages. So, to check if the head gasket is good, you can perform a compression test. Compression testers cost about 20 bucks and are pretty easy to use. Alternatively, you can have a professional do the test, but that will cost you between $50 and $100.

However, a compression test isn’t the best if you want to check the head gasket because the engine fails the test if the piston rings are worn out as well. Instead, you can get a head gasket kit which is even easier to use than a compression tester. You can see how it works in the video below, and it, too, is roughly $20 to $30 on Amazon.

Now, if it turns out that the gasket is faulty, it’s difficult to say how much it will cost, but it’s definitely not a DIY-friendly repair. What we can say, if it means anything, is that it can cost anywhere between $800 to over $2,000, depending on the car. That’s with labor and parts included.

Cracked Cylinder Wall – Inspection and Fixes

Inspecting a cracked cylinder wall is the same process as inspecting a head gasket. You either need a compression test or, better yet, a head gasket test kit. You can check the video above to see how the head gasket tester works and how much it costs.

Alternatively, you can remove the spark plugs and use a smartphone endoscope camera to look inside the combustion chamber. You can get those cameras for less than ten bucks on Amazon. But keep in mind that the cylinder wall cracks are often as thin as a strand of hair, so it could be difficult to notice.

And if the cylinder wall is cracked, the best option you have is to buy a used engine, preferably from a reputable used car parts dealer, or out of a crashed or salvaged vehicle. How much that will cost depends entirely on the model, so it doesn’t make much sense to try and guess.

Overheating – Fixes

Overheating is the whole problem by itself and deserves a separate article. A lot of things can cause overheating, including a rusted or slugged-up cooling system, a faulty water pump, a damaged radiator, collapsed hoses, a stuck thermostat, etc. So, the inspection, fixes, and cost to fix will vary wildly.

But we can say that a full cooling system inspection costs $100 on average. And how much it costs to fix depends on the problem and can be anywhere from $100 for a thermostat replacement to $500 or more for a new water pump.


Can a bad water pump cause high pressure?

No, a bad water pump will never cause high coolant pressure unless something else is wrong as well. However, the main symptom of a bad water pump is overheating, and overheating will cause high coolant pressure.

Can a blown head gasket cause pressure in the cooling system?

Yes, a blown head gasket will cause high coolant system pressure. When a head gasket is blown, the pressure from the combustion chamber enters the cooling system and basically inflates it. Then, when you turn the engine off, all that pressure reenters the combustion chamber, which is why there is exhaust smoke and coolant in the oil.

How do I know if my radiator is blocked and not circulating coolant?

The only way you can check if the radiator is blocked is to put your hand on the radiator exit hose and notice if you feel any vibrations, but keep in mind that the hose will have to get hot for the coolant to start circulating. But if the coolant isn’t circulating, the engine will overheat in a matter of 10 to 15 minutes, so that’s your obvious sign something is wrong.

Can a bad thermostat cause pressure?

Technically, a bad thermostat can cause high coolant pressure, but by the time it does, the engine will noticeably overheat and possibly even shut down. So, if your coolant is under high pressure, but the engine is not yet overheating, check the radiator cap, or you might have a blown head gasket.

How do I know if my radiator has water circulating?

To see if the radiator has water circulating, place your hand on the radiator exit hose and notice if you feel any vibrations, but keep in mind that the hose is hot. Also, if your car has a transparent expansion tank, check if there is coolant flowing out of the return line. That said, if the coolant isn’t circulating, the engine will overheat in a matter of 10 to 15 minutes.

Final Words

So, now we know what causes too much pressure in cooling system. The best possible outcome is that the radiator cap is faulty, but if that’s not the case, I’m afraid the engine has a blown head gasket or a cracked cylinder wall. And, of course, if the engine is overheating, then that’s why the coolant pressure is high. In the end, hopefully, our inspection guides were useful, and our ideas on what might be the cause.


Ibro Cehic

From a young age, I was captivated by cars and motorcycles, and my first driving experience only fueled my passion further. By the age of thirteen, I was already tinkering with vehicles and knew that my life would revolve around them in some way. Combining this passion with my love for writing, I now share my automotive expertise with fellow enthusiasts through my articles.

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