A motorcycle thermostat is a small temperature-controlled valve in a liquid-cooled engine that keeps coolant away from the radiator until the engine is warm, then opens to hold it in its normal operating range. When a thermostat fails, it usually sticks closed, which causes overheating, or sticks open, which makes the bike run cold. Both are fixable, and a thermostat is usually an inexpensive part.
Below is how a motorcycle thermostat works, the symptoms of each failure, how to test it and what replacement involves.
How a motorcycle thermostat works
Most engine thermostats use a wax pellet sealed inside a small chamber. As the coolant around it warms up, the wax melts and expands. That expansion pushes a rod, which opens the valve and lets coolant flow to the radiator. The temperature at which this happens is set by the type of wax used.
The thermostat doesn't simply snap from closed to open. It opens progressively and adjusts as the temperature rises and falls, balancing how much coolant goes through the radiator and how much keeps circulating in the engine.
Why the engine needs one
- Faster warm-up. A cold engine wears faster and runs less efficiently, so the thermostat keeps coolant in the engine until it's warm.
- Stable temperature. It helps the engine hold steady temperature whether you are stuck in traffic or on the highway.
- Better fueling. Fuel injection systems use engine temperature to set the mixture. A bike that never warms up can run rich.
Air-cooled and most oil-cooled bikes don't have a coolant thermostat. Some have an oil cooler with a thermostatic valve, which works on the same principle.
Where the thermostat is on a motorcycle
Location varies a lot. Common spots include:
- In a housing on the cylinder head, where the top radiator hose connects
- Inside the water pump housing
- Behind the radiator or in a hose junction, often hidden under the tank, fairing or radiator shroud
On many sport bikes and adventure bikes, reaching it means removing bodywork or the fuel tank. Check a service manual for your exact model before buying parts.
Stuck closed vs stuck open: symptoms
| Failure | What happens | Common symptoms |
|---|---|---|
| Stuck closed | Coolant can't reach the radiator | Rapid overheating, gauge climbs quickly, fan runs constantly, radiator stays cool while engine is hot, coolant pushed into overflow tank |
| Stuck open | Coolant flows to the radiator from cold | Slow warm-up, gauge reads low, poor fuel economy, rough cold running, idle stays high for longer |
| Partly stuck or slow to open | Erratic flow | Temperature swings, gauge climbs and drops |
A stuck-closed thermostat is the urgent one. An overheating engine can warp the head, blow a head gasket or seize. Our guide to motorcycle overheating explains what to do if your gauge climbs on the road.
Other problems that look like a bad thermostat
- Low coolant from a leak
- A failed cooling fan, fan switch, relay or fuse
- Air trapped in the cooling system after a coolant change
- A clogged radiator or bent fins
- A failing water pump
- A faulty coolant temperature sensor or gauge
- A head gasket leak pushing combustion gas into the coolant
Check coolant level and look for leaks first. A motorcycle coolant leak is a more common cause of overheating than a failed thermostat.
How to test a motorcycle thermostat
On the bike
- Start with a cold engine and check the coolant level in the reservoir (and the radiator, if it has a cap you can open cold).
- Start the engine and let it idle, watching the temperature gauge.
- Feel the top radiator hose carefully. It should stay cool at first, then get warm fairly suddenly as the thermostat opens.
- Watch the fan. It should come on only after the engine is well warmed up.
If the engine gets hot but the radiator hose stays cool, the thermostat may be stuck closed. If the hose warms up almost right away and the bike takes a long time to warm, it may be stuck open.
Never open a radiator cap on a hot engine. Hot coolant under pressure can cause serious burns.
Off the bike
- Remove the thermostat and check that it is fully closed at room temperature.
- Suspend it in a pot of water with a thermometer, not touching the bottom.
- Heat the water slowly and note when the valve starts to open and when it is fully open.
- Compare those temperatures and the valve lift to the figures in your service manual.
If it opens at the wrong temperature, doesn't open fully, or doesn't close again as it cools, replace it.
Replacing a motorcycle thermostat
The thermostat itself is a simple part, but the job usually means draining coolant and removing bodywork. A typical sequence:
- Let the engine cool completely.
- Remove bodywork, the tank or shrouds to get access.
- Drain the coolant into a clean pan.
- Remove the thermostat housing and the old thermostat.
- Fit the new thermostat with a new seal or O-ring, facing the correct direction.
- Refit the housing, torquing bolts to spec.
- Refill with the correct coolant and bleed air out of the system.
- Run the engine and check for leaks and normal warm-up.
Use the coolant your manufacturer specifies. Our guides on what coolant your motorcycle needs and how to change motorcycle coolant cover the fluid and the bleeding process.
Motorcycle thermostat replacement cost
Costs vary by bike, region and shop:
| Item | Typical range |
|---|---|
| Thermostat (aftermarket) | $15 to $50 |
| Thermostat (OEM) | $30 to $120 |
| Housing seal or O-ring | $5 to $25 |
| Coolant | $15 to $40 |
| Labor | About 1 to 3 hours, depending on access |
| Total at a shop | Roughly $150 to $450 |
If the cooling system is due for a coolant change anyway, doing both together saves labor.
Thermostat temperature ratings and "cooler" thermostats
Thermostats are made to open at a specific temperature that the engine designer chose. Some riders fit a lower-temperature thermostat hoping to run cooler in hot weather. On a healthy cooling system, that rarely solves anything, because the radiator and fan still set the upper limit once the thermostat is open. A cooler-running engine may also run less efficiently and take longer to warm up.
If your bike runs hot, look for the real cause first: low coolant, a weak fan, a clogged radiator or air in the system. Fit the thermostat your service manual specifies unless a trusted shop advises otherwise.
When to see a mechanic
Stop riding and get help if:
- The temperature gauge goes into the red or the warning light comes on
- Coolant boils over, steams or sprays
- You see white smoke from the exhaust or milky oil
- The fan never comes on, or never turns off
- You replaced the thermostat and the bike still overheats
Ride gently on a bike that runs cold, but get it looked at soon. A shop offering motorcycle diagnostics can pressure-test the cooling system and check the sensors, fan and water pump. You can find a shop in our motorcycle repair directory.
Frequently asked questions
Can I ride a motorcycle without a thermostat?
It is not a good idea. Without a thermostat, the engine may warm up slowly or run too cool, and on some designs coolant flow is actually worse without it. Replace it rather than remove it.
How often should a motorcycle thermostat be replaced?
There is usually no fixed interval. Most are replaced when they fail or when the cooling system is apart for other work.
Can a thermostat cause the cooling fan to run constantly?
A thermostat stuck closed can make the engine overheat, which keeps the fan running. A faulty temperature sensor, fan switch or relay can also cause it.
Is a cheaper aftermarket thermostat OK?
It can be if it matches the opening temperature and dimensions of the original. Many owners prefer the OEM part because of the exact fit and seal.
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References
- Thermostat (Wikipedia): wax pellet thermostats in engine cooling systems (en.wikipedia.org)