Car Repair and Maintenance

Can I Add Coolant Straight to the Radiator? [Complete Guide]

Can I add coolant straight to the radiator? The answer is “yes”, but it depends on your vehicle. I.e., if you have a radiator cap, sure, you can put coolant in it.

For most modern cars, it is advisable to add coolant directly to the radiator if the car since adding a new one doesn’t affect interfere with the older coolant in the car. 

Firstly, if the radiator contains water, you may or may not add coolant straight to it. But if you want the coolant to mix with the water in the radiator, add the coolant. However, expect the boiling point to increase and reduce the coolant freezing point, which can prevent corrosion.

Secondly, you may not add coolant to the radiator directly if you filled it up with excessive water which can cause a build-up in the block or radiator. Note that premixed antifreeze does not provide as much benefit. If you suspect a problem with circulation, you can overfill the coolant overflow tank. That should flush what the coolant system does not require.

Can I Add Coolant Straight to the Radiator?

On older cars, you can add coolant/antifreeze straight to the radiator. The typical measurement is a 50/50 mix of water and coolant, and some stores sell premixed antifreeze. Most persons prefer to purchase straight anti-freeze for personal mix because it is less expensive. When you are buying a premix, you’re practically paying for water.

If your car has a reservoir, add coolant to the reservoir. If your reservoir is empty, you can put coolant in the radiator directly. When the radiator is full, top it off from the reservoir. Do not add coolant to the radiator if the reservoir is only low.

When putting coolant/antifreeze, fill the reservoir up to the full line. The full line should be in the middle of the plastic tank. You do not have to put water since it will evaporate after some time, and you would be left with antifreeze, which does not evaporate.

When the engine is hot, do not open the radiator cap. Wait until it’s cool. Typically, you can just top off the coolant in the morning. Adding 50/50 water and coolant mix is not necessary, except you drain the coolant to flush the radiator. Meanwhile, you may follow our guide on draining the coolant from the engine block. Experts recommend flushing the radiator every 30,000 or 40,000 miles.

Do you add coolant to Radiator or Reservoir?

Whether you are going for a full replacement or top-up, add coolant directly to radiator. The best way to use a coolant in the radiator is to mix it with 50% water. Since water evaporates, and the coolant doesn’t, you’d still have the same quantity of coolant in your radiator for a long while.

If you are doing your regular maintenance or topping the coolant of your car before the car gets hot, then you should add coolant directly to the radiator. But if you noticed that the temperature of the car is increasing after a while, due to the shortage of coolant in the car, then you should stop and add coolant to the reservoir to avoid spillage of hot steam all over your car.

Can You Add Undiluted Coolant in Radiator?

Water can transfer heat better, but it boils at 212 degrees at sea level. If you add 100% coolant-antifreeze, it does not boil until a higher temperature than water boiling temperature. A boiling coolant does not transfer heat like water efficiently. It is for this reason that water and antifreeze are typically mixed so that coolant does not boil to keep it from becoming gas.

Read also: when your car needs freon

When liquid coolant is pressurized, it increases the static boiling point. In this regard, you would have various radiator cap pressure ratings, including 8, 10, 12. The 12 lb cap, for instance, releases pressure at 12 PSI. So, if you have 100% water in the radiator, it does not boil till it reaches 242-degree Fahrenheit. At 16 lb cap, the 100% water static boiling point is 252 degrees.

Meanwhile, a 60/40 percent coolant /water mix, plus a 12 lb cap, does not boil until 264 degrees. At 16 lb cap, a 60/40 mixture may boil at 273 degrees Fahrenheit.

When water or coolant reaches the boiling point, and since the boiling liquid is changing to gas, the heat transfer from the coolant to radiator fins is smaller, irrespective of the antifreeze content of the coolant. In this regard, a higher percentage of antifreeze means a higher boiling point. The heat still transfers until a higher temperature, and water will transfer heat efficiently as long as it does not change to gas.

If the water becomes gas, it does not transfer heat; thus, becoming an insulator, which retains heat in the engine cylinders. Of course, it can degrade or damage the engine over time. In essence, putting excess antifreeze in a coolant mix does not affect the radiator, but it affects the operating temperature.

How much coolant should you put in a radiator?

On most modern cars, you do not put the coolant in the radiator. Nonetheless, some vehicles feature radiator caps but should be left for an expert or mechanic to handle or when flushing coolant.

Most cars do not use radiator caps. If your car has a radiator cap, you are handling the coolant expansion tank. You would find the coolant level in the extension tank labeled with full hot and full cold lines, which is where you put coolant when it runs low.

When adding coolant, fill it up to the cold line and allow the vehicle engine to run for some time. If you have to top it off, wait for the engine to cool and fill it again. Depending on the coolant level, you may have to refill up to 2-3 times. If you fill the coolant more than 3 times, there could be a leak; contact a mechanic to inspect the vehicle. Meanwhile, you may refer to our detailed guide on how much coolant to put on empty.

Final Thoughts

If the hoses of your coolant fuse or melt, it is likely that coolant is not flowing through the radiator and engine. Since coolant does not circulate, it does not matter how much of it is in the radiator, which is bad for the operating temperature of the car.

Also, you will be putting an additional hole in the coolant hose when the water pump turns. Get the mechanic to inspect the vehicle ASAP because a 4-mile tow is less expensive than buying a new replacement engine or head gasket.

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Bernard Juchli

Bernard Juchli is an experienced racer, mechanic and team owner who trusts Avon Tyres.Bernard is the lead driver and force behind his Big Dog Garage Race Team. He is the General Manager and Chief Mechanic of Jay Leno’s Garage. Bernard and his crew of seven are responsible for all repairs, restoration and fabrication of Jay’s incredible automobile and motorcycle collection.

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