Excess Pressure In Cooling System – What To Do About It?

Engine performance is critical to any vehicle’s lifespan and power.

Nevertheless, many seem to forget about its operating temperature; if the engine is not kept in optimal condition, both under- and over-heating issues can lead to numerous technical problems for your precious car!

Hence, whenever there are symptoms of excess pressure in cooling system, detecting the cause and finding solutions to get rid of them is the key to survival. The question is: how to do that?

This article by Bryan’s expert team will lend a helping hand, providing insights into its mechanism, malfunctioning signals, and troubleshooting methods.

Understanding Cars’ Cooling Systems and Pressure

Excess Pressure In Cooling System

In short, this internal combustible engine is similar to a self-operating air pump. It takes charge of igniting sufficient air and fuel to let your car run smoothly throughout the entire trip.

However, one common byproduct of power generation is heat – which needs to get removed from your engine to avoid excess pressure that can destroy its compartments.

Here are several ways the cooling system can help reduce the engine’s air temperature:

The Cylinder Heads

A standard cooling system pumps fluids across the engine via the cylinder heads.

When they pass through, these cylinders absorb all the heat of the automobile combustion procedure, removing high temperatures from those fluids.

The fluid continues to get into the radiator hose – a mixture of fins, tubing, and tanks that serve as heat sinks. They will kill off the remaining heat, if any.

The Pressure Cap

Pressure caps (commonly known as pressure release valves) are often set at 15 PSI on most cars.

Like how pressure cookers increase their boiling temps when water is under pressure, the fluids in cooling systems also expand when heated, building up the pressure.

The caps are the only exit door for this pressure; hence, the settings on the cap’s spring help determine how much pressure is maximum in the system.

Let’s return to the 15 PSI example; once the pressure touches 15 PSI, the heater valve will be pushed open, letting the coolant flow escape.

This coolant flush streams through overflow tubes into the tank’s bottom, keeping air off the system.

After the radiator cores cool down, your cooling systems form a vacuum that yanks open a second safety valve to suck back water from the tank bottom, which replaces the previously expelled water.

Why Is There Excess Pressure In Cooling System?

What causes excessive pressure in cooling system?

The answer is simple and clear: heat. A rule of thumb is that any applied heat to exhaust gasses or liquid trapped in enclosed spaces will expand and create pressure – and your cooling system refrigerant is by no means an exception.

In most modern vehicles, no cooling system leaves any air room within the radiator, which is why there’s no space for the coolant to extend even amid the heat.

Thus, manufacturers always suggest you leave the coolant levels touching the cap’s top flange when the engines are cold.

Also, opening the cap to let these heats escape is a must; otherwise, radiator caps can push the boiling point to 45°F (or 25°C), worsening the buildup pressure in coolant system!

How to Fix An Over Pressurized Cooling System

How to Fix An Over Pressurized Cooling System

Opening the radiator cap to let the pressure in the cooling system escape is an obvious solution. But what are other tips?

Remember that system pressures are also influenced by:

  • Water pump output
  • Hose size
  • Thermostat design
  • Radiator configuration
  • Engine design

So have them adjusted, maintained, and upgraded at a trusted auto shop!

The coolants also play an important role; incompatible, substandard coolants only result in more pressure and heat. Here are small tricks to ensure you end up with a good coolant option: 

Check Coolant Water Color

Varied coolant colors correlate with varied car compatibility. For instance, IAT coolants are often green – more suited to older vehicles.

Meanwhile, HOAT enjoys a turquoise theme, working terrifically with BMW, Porsche, and other luxurious brands.

Still, note that colors are not the sole indicator of whether the coolant matches your car. Some coolants are specifically designed to fit certain brands, whose color varieties can be confusing.

So while we DO suggest using colors as guidelines, never forget to double-check their degas bottles to confirm their compatibility.

Consult The Manufacturer And Manual

The car’s manual can offer rich information, telling you which is the best coolant for a certain car model. Even if you have no copies of the manual, finding relevant information online is a piece of cake! 

Most suggested formulas are OEM (original equipment manufacturers) approved; nevertheless, there are high-performance aftermarket equivalents that you can consider as well.

Do Not Forget About The Water

While changing the car coolant, read the bottles to confirm whether you need to mix them with water.

Certain coolant types can pour straight into the system without additives, but other products are made to be used in a 50/50 mixture with water. If that’s the case, softened cold water will do.


1. How Many Minutes Should A Cooling System Sustain Pressure Buildup?

About two minutes at least, sometimes it will take couple of minutes more due to other reasons such as type of coolant, cover removal and technican skill as well. Otherwise, check whether the coolant is boiling or blows out of the reservoir.

2. Can Stuck/Faulty Thermostats Lead to Excessive Pressure in Cooling System? 

Yes, which is why we suggest checking leaks in the  bad thermostat housing to avoid too much pressure in coolant system.


How can we handle excess pressure in cooling system and ensure it no longer bothers the engine tank?

Bryan’s well-researched article has dived deep into this issue with tried-and-true tips even novices can adopt! 

Unless the weather condition in your neighborhood is always freezingly cold, the cooling system is always vulnerable to excessive heat posed by sunlight, dirt, grime, and other external factors.

Always check it carefully after every trip, and have professional services inspect them during their monthly maintenance procedure! 

For more questions and clarifications, you know the usual. Bryan’s inbox and comment sections are open 24/7.

1 thought on “Excess Pressure In Cooling System – What To Do About It?”

  1. Hi Bryan.
    I am having overheating issues. I have gone thru my entire cooling system on my 49’ chevy 3100 truck with straight 6 235 in it. Replaced radiator flushed system new hoses thermostat at 180. It has a small mechanical 15 in fan blade on the water pump with brass radiator. I start it and after 3 mins or so it’s warmed up and at 180 temp. From there it continues to rise to where I got to stop and shut it off or it will keep climbing. The pressure build up is really high and the cap on the radiator is 20 psi and once it reaches above and rises to 190-200 I have to shut it off. The other day it started running hot, when I turned it off, I just say a bunch of coolant running down the truck. I opened the hood and the lower radiator hose had popped off and it was clamped on the water pump tight. I also have an overflow tank but it isn’t always catching the overflow. So, I’ve been reading on these old trucks, they run a zero pressure cap. I’ve ordered one to see what that does. The new radiator I purchased did not come with a cap and the one I had put on it id a 20psi. Please tell me what you think my problem could be. Do you think running a zero psi cap would make a significant difference or I should be centering my attention elsewhere. Please provide any information as possible. I really appreciate it . Thank you.


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