What to Do When the Coolant Reservoir is Overflowing or Damaged

If you notice a vehicle's coolant reservoir overflowing, you should fix the problem promptly. Luckily, you can probably do it yourself with a little knowledge and a few supplies.

If you don't have experience with auto repair, you may need to take your car to a pro. Generally speaking, an independent shop will be cheaper than a dealer, but if your car is still under any kind of warranty, take it to the dealer instead. The fix may be covered under your warranty, and if it isn't, you don't want to void the warranty like you might by taking your vehicle to someone else.

How a Coolant Reservoir Works

A coolant reservoir is a fluid container, generally made of durable plastic, that's mounted in the engine bay of the car. It's used to store coolant for the engine, which helps as the engine goes through cycles of absorbing and then getting rid of coolant as the vehicle cools down.

The radiator cap allows some coolant stored in the reservoir, to leave the vehicle. The rest of the coolant stays in the vehicle's system until it is cool enough and begins to draw coolant from the reservoir back into circulation in the vehicle.

Coolant Reservoir Overflowing and Overheating

Constantly needing to add coolant to your car, visible leaks of the coolant from our car, and an overheating engine are all signs of an overflowing reservoir.

An overheating engine is one of the more common signs and one of the easier ones to detect yourself, especially if you don't have too much knowledge of the inner workings of a car, making it difficult for you to find leaks.

Overflowing reservoir fluids are generally flushed out of the overflow hose. This can result in coolant beneath your car, which often takes on the appearance of a dark puddle.

In more dire circumstances, there can be electrical damage. This happens when the overflow comes in contact with the electrical wires inside your engine.

An overflowing reservoir is not good for your engine and is an issue that will need immediate attention. Thankfully, this is generally something you can deal with yourself, though if the coolant has touched the wiring in your car, you should take it into an auto shop as a car's wiring mechanisms are not something the average DIYer can deal with without some help.

Here's how to look for and solve the issue of an overflowing coolant reservoir in less extreme circumstances.

Why Is My Coolant Reservoir Overflowing?

If your radiator cap has gone bad, it will allow too much coolant to pass by the cap and overflow around it. Fear not, though. Caps are easy and cheap to replace.

However, if it isn’t the radiator cap, it could be other problems, some more difficult than others to fix.

Broken Thermostat

If the thermostat is going bad, for example, your radiator won’t regulate properly, and the amount of liquid allowed to pass through at any given time may be too little or too much. Check the thermostat and have it replaced if need be.

Broken Water Pump

If the water pump isn't operating correctly and your coolant reservoir is overflowing, there may be too much coolant being pumped through, and pressure may ultimately be causing your reservoir to overflow. Replace it if needed.

Broken Radiator

If at this point your coolant reservoir is still overflowing, the entire radiator may need to be replaced. If this is the case, you can be sure that it will cost you more than just a couple of dollars for parts and labor, and it is recommended that you have the entire radiator replaced by a professional.

Damaged Coolant Reservoir

Step 1 - How to Remove an Old Coolant Reservoir

If the problem is just in the reservoir itself, you should be able to fix the issue.

Before you start, check the owner's manual to identify your reservoir's type and location. Make sure you have the right kind of coolant, and a space with plenty of ventilation and light. Finally, turn the engine off at least an hour before you start.

When you're ready, disconnect the negative cable from the battery of your car to ensure safety.

Remove the bolts that keep the coolant reservoir attached to the car body, but don’t disconnect the coolant hose.

Work the reservoir free of the car body.

Wrap a rag around the coolant hose and disconnect it from the reservoir, letting any coolant in the hose drain into the drain pan.

Now, unscrew the clamp on the other end of the coolant hose and let the coolant drain from the hose and the radiator into the drain pan.

When the old reservoir is completely drained, pour the waste into a container and screw the top on.

Make sure the container is secure and that the waste won't leak out. You'll need to dispose of this safely—don’t simply pour it down a drain since it can be highly toxic. There are a number of centers where you can take old coolant, so ask at your local auto parts store.

Step 2 - How to Replace a Coolant Reservoir Tank

To replace the reservoir, start by attaching the coolant hose to the radiator. In most models, it will clamp on, so tighten the clamp fully to avoid possible leaks later.

When it's on tight, it is time to attach the other end to the coolant reservoir.

Ease the new reservoir into place and attach it with the bolts until it’s firmly seated. Make sure it's secure.

Now, fill the reservoir with new coolant to the maximum level indicated on the side of the reservoir. Since you’ve drained the coolant, this will take quite a lot of replacement fluid.

Follow the instructions to mix the coolant together with the right quantity of water. Do not skip this step. Adding water is usually necessary to ensure the proper coolant mix is inserted into your car.

Step 3 - Test

Reattach the negative cable to the battery terminal.

Once it is reattached, turn on your car, and let the engine run for several minutes to check for any leaks.

The only possible sources of a leak would be at either end of the hose. If you find one, simply tighten the clamps.

Keep running the engine until you’re sure there are no further leaks. If there are, it may be time to take your vehicle to a professional who can diagnose the issue and fix it for you.

Maintaining Your Vehicle

Cars need regular maintenance. At least a few times a year, you should inspect your tires, add wiper fluid and change your oil.