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Missing Coolant Reservoir Cap: What to Do? (Explained)

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Last Updated on: 7th September 2023, 12:38 am

The coolant reservoir cap is a crucial element of a car’s cooling system, it maintains the pressure and the quantity of coolant within the system.

Despite its small size, it’s a significant part of the car that frequently gets neglected.

Understanding the consequences of a missing coolant reservoir cap and knowing what to do if the cap is missing is important.

a yellow coolant reservoir cap

What Happens if Missing a Coolant Reservoir Cap

The coolant reservoir cap, also referred to as the coolant overflow or expansion tank cap, holds the responsibility of sustaining pressure within the coolant system.

If the cap goes missing, there is a risk of coolant leakage from the system, causing a drop in pressure. This can trigger a variety of issues, depending upon the type of coolant system present in your vehicle.

Pressurised Cooling System

In a pressurised coolant system, the coolant is maintained at a pressure above atmospheric levels.

This increase in pressure elevates the boiling point of the coolant, enabling it to draw more heat away from the engine, improving the efficacy of the coolant fluid.

However, if the coolant tank cap goes missing, the system’s pressure could decrease, leading to a lower boiling point for the coolant. This can result in engine overheating and potential damage.

Non-Pressurised Cooling System

In a non-pressurised coolant system, the coolant isn’t maintained at a pressure above atmospheric levels.

If the coolant reservoir cap is absent, it could lead to coolant leakage from the system, resulting in a reduced coolant level.

Consequently, the engine may overheat because the coolant’s capacity to absorb heat from the engine is diminished.

Can You Drive Without a Coolant Reservoir Cap?

Driving without a coolant reservoir cap is generally discouraged as it can result in engine overheating and damage. The absence of a coolant reservoir cap can cause a reduction in the coolant level.

Why the Coolant Cap Is Important

A coolant reservoir cap, along with a radiator cap, plays a significant role in preserving the pressure (in the case of a pressurised cooling system) and the level of coolant in the system.

It’s the point of entry for adding coolant, and without the filler cap, debris could infiltrate the coolant system. Furthermore, the coolant reservoir cap helps prevent coolant leakage and spillage.

What to Do if You Lose the Coolant Cap

In case you lose the coolant cap, it’s crucial to replace it immediately. The absence of a coolant reservoir cap can result in engine overheating and potential damage.

Meanwhile, you can apply some of the emergency solutions provided below to ensure you reach your destination.

Temporary Fix for a Missing Coolant Reservoir Cap

If you happen to misplace your coolant cap and cannot replace it right away, a temporary solution is to use a cloth and duct tape to seal the opening of the coolant reservoir.

This helps to prevent coolant leakage from the system, but it should not be considered a long-term fix.

Non-Pressurised Cooling Systems

If your vehicle features a non-pressurized coolant system, there are a few stopgap measures to help prevent coolant from leaking out of the system.

  • One way is to cover the opening of the coolant reservoir using a cloth and duct tape.
  • Another option is to use a heavy-duty rubber glove and zip ties to seal the opening of the coolant reservoir.

Pressurised Cooling Systems

A pressurised coolant system operates differently from a non-pressurised one, needing a more robust temporary solution to prevent coolant leakage.

The methods for a temporary fix of a missing coolant reservoir cap are similar to those used for non-pressurised systems.

However, with these temporary solutions, the coolant system may not function correctly, potentially leading to engine damage or overheating.

Therefore, only drive for short durations and consistently monitor the coolant temperature.


The coolant reservoir cap is a key part of a car’s coolant system. It’s crucial for preserving the system’s pressure and coolant level.

If it’s missing, it can’t do its job, and debris might get into the system and tank. If you lose your cap, replace it immediately to avoid engine damage or overheating.

There are several temporary solutions to prevent coolant leakage. However, you must use the right fix for your specific coolant system.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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