Common Causes of Motorcycle Coolant Leaks

Coolant loss on a motorcycle can be incredibly frustrating, especially when you can’t find where the leak is coming from or you have to keep refilling the reservoir.

There are several reasons a motorcycle will leak antifreeze/coolant, including loose or broken radiator hoses, or simply a loose or damaged radiator cap. Whatever the reason, the good news is most coolant leak problems are easy fixes.

Reasons for Leaking Motorcycle Coolant

If your motorcycle is losing coolant, here are some common reasons why it could be happening.

Loose or Cracked Radiator Hose

Always start with the simple stuff first and check the radiator hoses to see if they’re loose, or if there are any cracks in the upper (engine connection) or lower (radiator connection) hoses. 

The connection between the hose and the component it serves might simply not be tight enough. This could be because the metal hose clamp that keeps each end of the hose in place might be broken, corroded or loose, or may have slipped off altogether. 

Conversely, when the clamps are too tight, they can end up tearing or puncturing the hose. Do a simple inspection for any looseness, excess tightness or damage to the radiator hoses and their clamps, and replace them if needed.

Faulty Radiator

Next up, check the radiator itself. The radiator has small tubes or fins that run through it where the coolant circulates to cool off the engine. Many motorcycles have the radiator front and center, leaving it exposed to road debris. Because of this, debris such as rocks can punch a hole in the radiator and cause it to leak.

There’s also the case of the mysterious disappearing coolant, in which you know your motorcycle is losing fluid but you don’t see any signs of leaks. That can happen for various reasons, including radiator blockages from clumps of dirt and debris preventing it from circulating coolant. Corrosion can also eat through a metal radiator, and when that happens, you may find burnt coolant stains on the radiator’s surface or smell coolant coming from your bike. 

Whether it’s a leaking, clogged or corroded radiator, you’ll need to replace it. But before you do that, check that the seal between the radiator and the coolant tank isn’t worn out or damaged.

Leaking Drain Plug

One very simple and common cause of leaking motorcycle coolant is the drain plug to the radiator, which can easily get loose or break.

Drain plugs can wear out, corrode, or become loose from the heavy vibrations produced by a motorcycle. These plugs are cheap and easy to replace, so if you inspect the drain plug and it looks even a little corroded or worn, simply swap it out for a new one. 

Bad Water Pump

If you still haven’t found where the coolant leak is coming from, check the water pump, which circulates the coolant through the motorcycle’s cooling system.

Water pumps have a small weep hole, an impeller that spins to circulate the coolant, and a small bearing for the impeller. If any of the water pump’s components loosens or wears out, coolant will seep out of the weep hole. Inspect the water pump to make sure it’s circulating coolant properly, and of course have it replaced if it’s damaged. 

Engine Overheating

An overheating engine is a common cause of motorcycle coolant leaks, since the excess heat is creating pressure that causes the radiator cap to leak. 

If your bike’s engine overheats while it’s running, shut the engine off and wait at least half an hour before removing the radiator cap. That’s because the pressure buildup can spew coolant everywhere if the motor hasn’t cooled down. 

If the radiator cap looks good, the problem could be a blown head gasket, which would allow coolant to enter the cylinders and get lost in the system. When head gaskets fail, you’ll smell coolant, and it’ll make its way into the bike’s oil pan and contaminate the coolant reservoir.

The back pressure from an overheating engine might have nowhere to escape and could blow through the coolant reservoir or radiator cap seal. When head gaskets fail, so does the motorcycle’s cooling system, and an overheating engine could be catastrophic if the problem isn’t addressed promptly.

Motorcycle Coolant Leak Prevention

Coolant leaks can be prevented with basic maintenance. Start by inspecting the hoses and their clamps regularly to make sure they’re not getting loose or worn out.

Check the coolant level after every ride, and top it off when necessary to prevent overheating. You should also do a coolant flush after every 30,000 miles or every five years, whichever comes first. 

Remember the simple stuff if your motorcycle is losing coolant and you can’t find the leak, as something as simple as a faulty radiator or coolant reservoir cap could be the problem. Inspect both the radiator cap and its sealing gasket often to make sure they’re not worn out or damaged.

Another simple way to help prevent coolant leaks is to check your motorcycle’s service manual for recommendations on coolant flushes and general cooling system maintenance, and stick to the schedule. 

Finally, if you find yourself topping off or refilling the coolant more frequently than you would expect, or if you smell coolant during or after rides, take that as a sign that something’s wrong and take immediate action. 

Again, remember to start with the simple stuff. If it looks or smells like there’s a leak, start by checking the hoses and the radiator cap first before you dig deeper and start taking stuff apart.