7 Symptoms of a Bad Radiator Cap (And How to Test It)
The part on your car called the “radiator pressure cap” contains two valves. Its purpose is to keep the coolant in the radiator and ensure the cooling system is pressurized. The radiator cap is sometimes called a coolant cap or antifreeze cap. All three names refer to the same part.
The cooling system can withstand a variety of pressures, but it’s usually between 13 and 16 pounds per square inch (PSI), or about 1 bar. This pressure is regulated by the radiator cap and must remain constant.
Excessive pressure exceeds the manufacturer’s specifications for the cooling system and can cause cooling system components to fail. Too little pressure can cause the coolant to evaporate. If there is little or no fluid to cool the vehicle, your engine may be overheating.
Below are some symptoms you can look for to determine if your radiator cap is bad: Signs of a faulty radiator cap
1) Coolant leaks
A stuck radiator cap can cause pressure to build up in the radiator, which can cause cooling system components to leak or burst. If you notice coolant near the radiator or radiator cap, you definitely have a coolant leak. Check the radiator for holes or if the cap is worn or damaged. If so, replace the cap.
2) White streaks on the radiator
When coolant leaks from the radiator filler neck and dries, it often leaves behind white streaks. You may not notice coolant leaking from under the radiator cap, but keep an eye out for those white streaks. These could indicate a pressured or intermittent leak at the cap.
3) Overflowing Reservoir
As coolant expands, it enters the reservoir. The radiator cap directs the coolant into the overflow tank, relieving excess pressure.
If the radiator cap is damaged, coolant can leak out rapidly and overflow the reservoir. While you’re at it, make sure your coolant overflow tank is working properly.
4) Crushed radiator hoses
If your radiator hoses are crushed, your radiator cap may be faulty. The radiator cap does not properly release the vacuum, causing the radiator hoses to collapse during the cooling phase. In this case, check the lid for damage. If so, replace it immediately.
5) Burst Radiator Hose
When the pressure in your cooling system is too high, you’ll notice one or more hoses start spraying coolant all over the engine compartment. In most cases, the pressure isn’t high enough to tear the hose in half. You’ll often see pinpoint leaks that only spray coolant when the car is warm.
In fact, a hose with a small hole can seal tightly when the car is cold. As you drive, pressure builds up in the cooling system. Eventually that pressure becomes enough to force the coolant through the small holes, and your car will slowly lose coolant.
In this situation, you’ll probably notice the leak in the floor or engine bay when you arrive at your destination, but not when you start driving. See if the overflow tank empties slowly over a few miles; this may give you a clue that this is the problem.
6) Engine Overheating
Leaking coolant or air in the cooling system can cause the engine to overheat. If you notice that the engine is starting to steam due to excessive heat, do not look under the hood until you have shut it off. Then wait a few moments for the engine to cool before opening the hood. This will help keep the engine cool while you inspect it. If coolant pools near the radiator cap, it can damage the pressure cap. Check and replace if necessary.
7) Air in the Cooling System
If the radiator cap is not sealed properly, air can get into the cooling system. This causes air pockets to form in the heater core, thermostat and radiator hoses. This prevents the engine from maintaining a consistent temperature and causes overheating.
How to Check Your Radiator Cap
WARNING: Never open your cooler when it is hot. Allow the engine to cool completely before opening the radiator.
The cooler is exposed to high heat and pressure. Trying to open a hot radiator can cause hot steam and coolant to spray out, increasing the chance of causing burns.
Check the Lid
Before digging too deep into a diagnosis, double-check that the pressure marked on the radiator cap matches the manufacturer’s specified pressure for your cooling system. You can find this information in your repair manual, factory service manual, or online. Visually inspect the lid to make sure the spring moves freely and that there is no debris or corrosion under the lid.
Radiator Pressure Test
If you have a radiator pressure tester, you can check the cooling system yourself. This can help you find leaks or determine if your radiator cap is bad. You may be able to rent this tester from your local auto parts store.
First, open the hood and find the radiator, which is usually located directly in front of your vehicle’s engine. Next, remove the radiator cap by first pushing it down and then turning it counterclockwise. Take a cap adapter and screw it onto the end of your pressure tester. Keep screwing it in until you can’t screw it in any further. If you’re not sure what a cap adapter is, check the label on the packaging of your pressure tester, or you can look online.
After attaching the cap adapter to the pressure tester, screw the other end of the cap adapter tightly onto the radiator filler neck. The pressure tester should be close to the radiator. If it isn’t, you may not be using the correct adapter for your particular cooler.
Pump the tester using the pump handle until it reaches the pressure specified on your radiator cap. Check to see if the gauge can hold the pressure.
Once the pressure test is complete, slowly loosen the radiator cap adapter to ensure that coolant does not leak out anywhere. It’s a good idea to have a pan or bucket ready to catch any overflowing coolant. Refill any lost coolant and wipe up any spilled coolant.
What does the coolant cap look like? Radiator cap
Because the radiator is under pressure, the top of the radiator cap may have a warning label instructing you not to open the cap while the engine is hot. A radiator cap is different from an overflow tank cap.
Is your radiator cap or cylinder head gasket bad?If your car overheats, it can be hard to tell if the radiator cap is bad or if you have a more serious problem such as a blown head gasket. To see if you have a blown head gasket, you can do a leak test.
The leak test involves sending compressed air through each cylinder through the spark plug holes (after removing the spark plugs). If air gets into the cooling system, you know the head gasket is leaking.
Doing a radiator pressure test can often tell you if the radiator cap is bad. After pressurizing the system, you may notice coolant leaking from or near the radiator cap. This can happen if the cap itself is not sealing properly.
Other Cooling System Issues (Faulty Thermostat)
If your radiator cap doesn’t heat up after running the engine for a while, you may have a problem with your thermostat. As a result, your vehicle may have a pool of cold coolant in the radiator even though the engine is overheating. Air pockets in the engine can create hot spots, leading to temporary overheating.
