2 Key Signs of a Bad Radiator Cap (And How Much a Reasonable Repair Could Cost)

Whenever I get my hands on a newly purchased used car, my checklist includes checking a few items under the hood as part of the “getting to know the car” process. For example, checking the oil dipstick will show whether the previous owner was diligent in changing the oil periodically. Opening the engine oil cap and looking inside the valve cover may reveal more, such as thick sediment or dirty oil, which signs of infrequent oil changes or owner neglect.

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One thing I always do is check the cooling system. What you should pay attention to is the condition of the radiator, the integrity of the hoses, the freshness of the coolant and the radiator cap.. The radiator cap is more than just a cap that covers the radiator filler neck, it is one of the most overlooked components of a car’s cooling system. Just like your brake pads, engine oil, and coolant, your radiator cap requires frequent inspection and replacement.

Failing to diagnose a faulty radiator cap is bad news for your vehicle. It seals the cooling system and ensures that the coolant remains under pressure as it expands and absorbs engine heat. The pressure increases the boiling point of the coolant, which is necessary for the smooth operation of the engine without overheating.

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The main symptom of a bad radiator cap is a persistent engine overheating. A failed cap means a failed seal, which means air can get into the cooling system, causing temperature spikes and costly internal damage. This also means it is impossible to maintain the system under pressure, which causes the coolant to boil and lead to overheating.

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A leaking radiator cap also causes loss of coolant. If you’ve been around cars long enough, you probably know this. Losing coolant is a sure way to overheat a properly running engine.. Another thing you should pay attention to are white stripes on the radiator or along the filler neck under the cap, which indicates a coolant leak while the engine is running..

Be sure to let the engine cool for at least 30 minutes to an hour before attempting to remove the radiator cap. It may be difficult to spot the damage at first glance, but rust, worn rubber seals, and excess dirt under the cap mean it’s time for a replacement.

Since a bad radiator cap will cause temperature and pressure spikes in the system, it can cause old rubber hoses to rupture, rupture, or leak. In addition to replacing the radiator cap, you should check all hoses to ensure there are no leaks in the system. Replace the hose if you find cracks, signs of damage or wear, even if the hose is not leaking. This will give you peace of mind that the entire cooling system is in excellent condition.

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An overfilled coolant reservoir may also indicate a damaged radiator cap. One of the radiator cap’s jobs is to direct coolant to a reservoir or reservoir for regeneration when the system begins to build pressure. If the cooling system is unable to maintain proper pressure due to a faulty radiator cap, it can prevent coolant from the reservoir from returning to the system and helping to cool the engine. In this case, the engine will overheat.

Radiator caps come in different sizes, primarily determined by the diameter of the radiator neck, and have different pressure ratings ranging from 83 kPa (12 psi) to 207 kPa (30 psi) or higher. You will need to find a replacement cap that is the same size and pressure rating as the original. Check your owner’s manual or refer to your car’s old radiator cap for the size and pressure rating that matches your car’s make and model..

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The good news is that replacing your radiator cap doesn’t cost much. A Genuine GM replacement radiator cap. costs under $12 on Amazon and fits most GM trucks and SUVs. Meanwhile, Genuine Ford radiator cap with built-in relief valve. sells for about $10. If you find rust or dirty coolant inside the radiator, it is best to flush the entire system and replenish it with fresh coolant before installing a new radiator cap.