Can I Add Coolant to a Hot Engine? Filling Process

Adding coolant to a cold engine is always less risky. But what if you discover the engine is overheating as you drive due to low coolant? Sure, you need to stop and top off the coolant.

However, the engine should be very hot at this stage, especially if you’ve been driving for a long time. Like yourself, many others are also asking, can I add coolant to a hot engine?

can I add coolant to a hot engine

Can I add coolant to a hot engine?

No, you shouldn’t add coolant to a hot engine. Note that, it’s not possible to fill coolant reservoir at this stage. Why? First, topping off your coolant reservoir requires you first remove the radiator cap. And at this point, the cap will be hot, just like the engine; touching it can burn your hand.

Secondly, your car’s cooling system is always pressurized and would boil out hot coolant if the cap is opened. So, even if you use a rag to hold the cap so you don’t get burnt, the pressure inside the reservoir can cause hot coolant or steam to spill out, inflicting severe burns. In cars like race cars, it will even explode.

Thirdly, engines react to temperature changes. So topping off coolant while the engine is hot can cause cracks and warps in your engine blocks and cylinder heads due to sudden temperature changes. What happens is that as cold coolant touches the engine, the hot metal contracts quickly. And eventually leading to engine damage. You may also notice white smoke from engine.

While overheating can result in cracking and warping of engine blocks and cylinder heads, rapid cooling worsens it. To understand better, try putting a hot drinking glass in cold water; what happens to that glass is exactly what happens to your engine.

For these reasons, you should wait before adding coolant to your car unless you have money to unnecessarily throw around. So how long should you wait?

How long to let engine cool before adding coolant?

It could take 20 minutes to 45 minutes for the engine to cool before adding coolant. However, the exact duration would depend on how long you’ve been driving, at what speed, and weather condition.

If you had only gone on a short trip and drove at moderate speed, your engine shouldn’t be too hot. So 20 – 30 minutes should be enough to cool your engine. This time could even shorten if the weather is cold as cars take longer to cool down during summer.

However, your car should be very hot if you’ve been driving for long and at high speed,. Here, you would need to let the car sit for at least 45 minutes, though it might cool faster than expected during the cold months.

While it’s advisable to wait for some time, it doesn’t necessarily mean you have to wait till your engine is cold. So you may want to ask can you put coolant in a warm engine. Yes, you can. However, if you choose to fill coolant into a warm engine, do so slowly with your engine running. This way, the cold liquids meet with the hot liquid already circulating in the system instead of entering the system at once.

How to add coolant to a car

Topping off coolant may look like an easy job to do. However, you might cause catastrophic engine damage if care is not taken. So to fill the coolant reservoir, here are safe steps to take.

  •  Get your manufacturer-recommended coolant and safety wear like hand gloves.
  • Switch off your engine and put your car in Park or neutral.
  • Allow it to sit for about 20 mins – 45 mins so it can cool down.
  • Check the hood to see if it’s cool enough to touch; avoid adding coolant to a hot car.
  • Partially unscrew the coolant reservoir cap but don’t take off the cap. This should let the pressure inside subside if it’s still high. In fact, step back a bit when doing this.
  • Now remove the filler cap.
  • Pour in Maxima Coolant but ensure you don’t overfill. Overfilling will let extra fluid expand and flow out of the overflow pipe. While this might not necessarily harm your engine, it could be dangerous to children and animals as coolant is acidic. In addition, coolant has a sweet smell, and animals or kids might be tempted to taste it.
  • Replace the filler cap, and ensure you tighten it properly to avoid coolant spilling out.

Can I drive right after adding coolant?

You shouldn’t drive immediately after refilling coolant in car. Here is what to do. Switch on your car’s engine and let it idle. Then check your temperature gauge on your car’s dashboard to see if the temperature is normal. If now in average operating temperature, you can drive.

But still keep observing the temperature.

This is also an avenue to check for coolant leakage under your car. If you’re experiencing fluid leaks, it won’t be long before your coolant goes down again and the car starts overheating. So try figuring out the problem, or take your car to a mechanic to check and possibly fix them.

True, coolant could get low maybe after 30k miles or so since coolant contains water that can evaporate. If you regularly commute in a hot climate, you may also need to top off coolant often. However, you shouldn’t be filling coolant now and then. If you’re doing this too often, there is probably a leak somewhere which could be due to a faulty radiator or water pump.

Can I add coolant while engine is running?

Ideally, your engine should be turned off when filling coolant for safety purposes. However, you may need to run your engine while adding coolant.

filling coolant in car

If the engine is cold, you don’t need to keep the engine running. However, coolant will not flow appropriately since the thermostat is closed, so you can fill it up and start your engine. This allows the car to heat up a bit for the thermostat to open and allow fluid to pass through it.

But suppose the engine is hot and you still choose to add coolant, keep the engine running during coolant refill to avoid thermal shock leading to cracks or warps. Usually, cast iron heads crack while aluminum warps. However, this is when you’re topping off.

If, however, your coolant reservoir is empty and your engine is overheating, allow the engine to cool. Except you choose to add coolant slowly with the top hose disconnected to avoid backflow of fluid. But even then, the blocks may be too hot for coolant and cause cracks.


This article has answered the question can I add coolant to a hot engine? A recap! No

Always wait before topping your coolant to avoid burns and engine damage. While even overheating can cause engine damage, rapid cooling fastens the whole process.

So if you’ve been asking, can you put antifreeze in a hot car? You know better now.  But some people will never adhere to safety precautions. For this reason, manufacturers have made recent radiators so that the pressure inside would hold the cap so tight you won’t be able to twist it open.

Solomon Osuagwu

Osuagwu Solomon is a vetted auto mechanic with over ten years of experience in the garage and five years of experience as a service writer. He prides himself in writing accurate information on professional repair guides, DIY repair guides, buyer’s guides, comparisons, and car reviews. If he’s not in his repair garage, he’s writing automotive blogs to help car owners and fellow mechanics to troubleshoot and proffer solutions to several car problems.

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