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Bad Radiator Cap or Blown Head Gasket: How to Tell the Difference

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  • 4 min read

Last Updated on: 7th September 2023, 12:37 am

A bad radiator cap and a blown head gasket can lead to overheating and other issues, it’s important to know the difference so you can take the appropriate action.

The best way to tell the difference between a bad radiator cap or a blown head gasket is to check the oil filler cap. If it’s milky or frothy looking then it’s likely the issue is a blown head gasket and not the radiator cap.

In this article, we’ll go over the signs of a bad radiator cap and blown head gasket and how to tell the issues apart.

person opening a radiator cap

Signs of a Bad Radiator Cap

Below are some signs of a bad and faulty radiator cap.

  1. Overheating
  2. Low coolant
  3. Burst or collapsed hoses

1. Overheating

A bad radiator cap can cause the engine to overheat, this is due to the radiator cap not creating a complete seal and causing the coolant system to lose pressure.

This leads to coolant leaks and reduced coolant flow and effectiveness, resulting in overheating.

signs of car overheating infographic

2. Low Coolant

A bad radiator cap can cause coolant to leak from the cooling system.

If there is coolant leaking or if the coolant level in the reservoir is low, it could be a sign of a faulty radiator cap, among other issues.

3. Burst or Collapsed Radiator Hoses

The hoses within the vehicles cooling system may burst or collapse if the radiator cap is faulty.

This may happen if the pressure in the cooling system is too high or isn’t regulated properly, causing the hoses to expand or collapse.

collapsed upper radiator hose

Signs of a Blown Head Gasket

Below are some signs of a blown head gasket.

  1. Overheating
  2. Milky oil filler cap
  3. White exhaust smoke
  4. Reduced performance & rough idle

1. Engine Overheating

A blown head gasket can cause the engine to overheat.

The head gasket is situated between the engine block and cylinder head and is responsible for sealing the coolant and oil passages.

If this gasket fails, coolant can leak into the oil passages.

2. Frothy & Milky Oil Filler Cap

If the oil in the engine becomes contaminated with coolant it’ll be milky and frothy in appearance, it also reduces the effectiveness of the engine oil.

milky oil filler cap examples

3. White Smoke From Exhaust

White smoke emanating from the exhaust tip can be a sign of a blown head gasket.

The high water content of coolant fluid is the reason the smoke is white.

4. Poor Engine Performance

The head gasket also creates a seal between the cylinder head and engine block, maintaining compression in the combustion chamber.

If it fails, it can lead to uneven compression and result in poor engine performance and a rough idle.

How to Tell the Difference Between Them

These two issues can leads to similar signs and symptoms and it can be hard to tell the difference.

However, there is one clear way of knowing if its a bad radiator cap or a head gasket issue. Check the oil filler cap, if the oil is milky and frothy in appearance, it’s more likely to be a blown head gasket.

A blown head gasket usually presents with other issues too, such as white smoke coming from the exhaust and reduced engine performance.


A bad radiator cap and a blown head gasket are two important issues that can cause significant problems for a vehicle.

Checking the oil filler cap is a good way to tell the difference between the two issues.

  • 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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