What to Do if You Overfill the Coolant! Expert Advice!

The coolant level can be tricky to get right; the reservoir tanks aren’t always transparent, and sometimes they’re so filthy you can’t see the level. So, what happens if you overfill the coolant, and how do you quickly put the level right without taking the car to a garage? Obvious questions all answered in this article.

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How to Remove Too Much Coolant?

Not all the methods below are suitable for removing a cup or two of coolant. If you find a little overfill, use a turkey baster, siphon or bottle spray head. If you can’t do this, try the drain plug in the bottom of the radiator.

Failing that, you may need to perform a coolant flush to drain almost the entire system to remove the overfill, which will require a full bleed afterward.

Turkey baster or siphon

The easiest way to remove coolant from the reservoir tank is to suck it out with a turkey baster. Or better yet get a specific siphon/extractor, which has the same effect of being simple. Make sure the engine/coolant is cold; coolant gets extremely hot, so do not attempt this with the engine running, or if the coolant is hot, you will hurt yourself.


Using a 200cc fluid extractor like the one above is a simple and yet effective way of removing excess coolant. Suck out precisely what you need, spray the excess back into another bottle for disposal or top back up to the required level. Just be sure to refit the coolant cap afterward.

On a side note, don’t use that turkey baster in the kitchen again. Coolant is poisonous.

Bottle spray head

Household cleaning products, detergents, or car cleaning products usually come with a manual pump head sprayer. If you remove that from the bottle and place the tube into the coolant tank, you can spray the coolant out of the tank. You will want something like a bowl to catch the coolant. It’s a very effective way to remove a small amount of overfill.

Drain plug

Some radiators have a drain plug underneath or on the side tanks of the radiator. It isn’t a great way to remove coolant, but it works. Once you remove the drain plug, most of the coolant will leak from the drain plug quickly, so make sure you have a catch pan.

You must be fast to remove a small amount of coolant and reinsert the drain plug. It will make a mess, and you can be sure you will have removed too much. Be careful not to overfill the level when topping back up cause you don’t want to do this twice.

Dump the bottom hose

The fastest way to remove coolant from an engine is to remove the bottom hose of the radiator. 90% of the coolant system will be released in a few seconds. This isn’t used to take a small amount of overfilled coolant out because, number one, it’s impossible to remove the hose and reinsert the hose while chucking out coolant. Number two, the coolant system will need to be bled of air after refilling. This means removing a little bit of overfill. It could take an hour or more to put it right again.

Bleed screw – bleeding the coolant system

If you aren’t sure what the bleed screw is, you could be opening a valve that shouldn’t be opened. In this case, refer to the turkey baster or a siphon method, as that is the simplest and safest way to remove coolant. Opening and closing a bleed screw, you need to be sure that is what you are opening, so someone shouldn’t carry this out without mechanical vehicle knowledge; you can cause more problems if this goes wrong.

The method of using the bleed screw is essentially bleeding the coolant system to remove air and overfilled coolant. If you dump the bottom hose to drain the coolant and start again, you will need to bleed the system afterward, in which case, keep reading.

A coolant bleed screw is usually located on one of the hoses at the top of the engine, although they aren’t on every car. With the bleed screw opened, it relies on gravity to push air out of the system when drained and refilled. Gravity will cause any remaining air and coolant out of the bleed screw if there is excess coolant.

radiator overfilled with coolant

How to bleed the coolant system

  1. Remove the cap on the coolant reservoir tank.
  2. Start the engine and leave it idling (you can rev the car to 3000RPM get it to warm up quicker)
  3. Locate the bleed screw and place or hold a container under the hose to catch excess coolant.
  4. Undo the bleed screw; it will not always need to be removed entirely; most bleed screws have a chamfered edge to allow air/coolant to escape.
  5. Wait for the air to stop spitting from the bleed screw, and you have a constant coolant flow. You may need to add more coolant, so keep an eye on the level.
  6. Tighten the bleed screw and make sure to refit the coolant reservoir cap.

What Happens if You Overfill the Coolant?

The thing with overfilling coolant and the need to rectify the problem is what can happen if you overfill the coolant and continue driving. The pressure buildup can burst water pipes, force the hoses off, crack expansion tanks, and force water out of vulnerable parts of the radiator. All of the problems will result in the car overheating through leaking coolant, and you may think this may happen slowly, but it can happen with a few moments of driving.

The coolant system on a car is under intense pressure when the vehicle is hot; most cars are now fitted with an expansion tank, also known as the coolant reservoir or overflow tank.

Precisely as the name suggests, it is an expansion chamber for the pressure build-up to be released or controlled. The sealing reservoir cap is pressure rated to keep the system closed, which is why you shouldn’t open the lid when the car is hot. All that pressure build-up will shoot the water out of the top of the tank if the cap is removed while the engine is hot – dangerous!

The coolant cap fitted onto the radiator on cars with expansion tanks is known as a pressure relief cap. This allows excess pressure in the coolant system to be released at the radiator if the pressure builds up in the reservoir tank sufficiently to open the release valve in the cap, usually from being overfilled. This means you will rarely have a problem with cracking bottles or overheating with this type of system if it’s overfilled because it will force excess coolant out, which is still not good. Most vehicles now only have one exit point for extra pressure, which is on the expansion tank.

Overfilled with coolant, the pressure in the tank can result in the tank and cap exceeding their rating, which can cause cracks and the coolant to leak. The area in the tank above the maximum level allows the coolant to expand under pressure; obviously, there will be no room for this to happen with the coolant overfilled. This will cause the opposite problem, and you will run the car low on coolant, which will cause the vehicle to overheat.

Can Overfilling the Coolant Cause a Leak?

If the pressure builds up too high in the coolant system, the fluid will force itself out of the reservoir tank. Worse, the tank could crack and cause more of a problem. Coolant leaking in the engine bay could cause issues with electrical sensors if they get wet.

Is It Safe to Drive With Too Much Coolant?

The problem with driving with too much coolant will be evident when the thermostat opens, which means the car’s coolant is up to temperature and under pressure. At this point, you can get a problem like a cracked reservoir bottle, weak water hoses can burst or blow off, and the car will overheat.

If the car is cold and you need to move it a few yards, it would be ok, providing the engine doesn’t get up to temperature. However, with how easy it is to remove excess coolant and no natural way of telling exactly how long until the car is at the operating temperature, you shouldn’t drive the car and fix the issue.


Overfilling the coolant can be a simple rectification. If you have a turkey baster to hand, suck out as much as you like from the coolant reservoir tank. It is essential to remove the overfill and drive the car. Ignoring the problem can cause a leak of coolant, potentially broken components, and an overheating engine, which would put the car further out of action.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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