How do I service the cooling system?

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You are working with anti-freeze/coolant, which is caustic. You should try to keep it away from any painted surface, and clean it off if you spill any. Likewise, if any comes into contact with your skin, clean it immediately. If discomfort persists, seek medical attention. It will also kill animals if they drink it, so don't leave any on the ground, and dispose of it at a recycling center.

How do I check the coolant level?

Fluid levels should always be checked with the bike straight up, resting on its tires. The slight elevation a centerstand or swingarm stand gives may give you an erroneous reading, but that has more to do with oil than coolant.

Your coolant tank will have two marks on it - a low or cold level, and a high or hot level. Overflow tank, behind left quarter (side) panel:


When the bike is cold (has not been run) the coolant level should be no lower than the low/cold level. When the bike is hot (just been shut off after a significant ride) the coolant level should be no higher than the high/hot level. This is a normal operating range - coolant level will fluctuate as it expands & contracts.

Adding coolant

If you need to add coolant, add a 50/50 mixture to the plastic coolant reservoir. Make sure the coolant that you use states that it is "safe for aluminum engines." Brand doesn't matter. It also doesn't matter if it states that it's for cars or motorcycles. Add coolant when the engine is cool up to the bottom (cold) level.

There are two types of anti-freeze: ethylene glycol (green) and propylene glycol (orange) (usually). Your EX250 has ethylene glycol, just like the Prestone that your Dad used to put in his '69 Camaro. According to the makers of Peak/Sierra and Arctic Blend coolant, you can combine the two without any damage to your cooling system, but it's not a bad habit to use the same kind of fluids in your bike as are already there when you top off. According to Prestone, you can keep an open bottle of anti-freeze for a couple years and not lose its effectiveness. Use that for topping off, but keep it away from kids and pets.

Adding coolant to an empty reservoir

This section applies if you have let all the coolant run out of the overflow tank, or otherwise suspect that there is air in your radiator. Adding coolant to the reservoir isn't helpful if the radiator isn't full or has air in it. If this happens, add coolant at the top of the radiator on a bike that is absolutely cold. The radiator cap is located behind the right side of the front fairing.


Fill that up first, and then add to the overflow until the coolant is at the proper line ('low'). You will then have to purge the air from the system (see below). If you check your system regularly this should not happen.

Questions on coolant? See the eet Corp FAQ

How do I change the coolant?

The coolant should be changed at regular intervals. Depending on the year, the Service Manual calls for a change at 18-24,000 miles. Since some people will take a long time to get to that number of miles, you should change it at least every 2 years, regardless of how many miles you have on it.

There are 2 coolant drain plugs. First, open the filler cap on the radiator. Then, remove the first one, which is on the water pump, below and behind the gearshift lever; it is a 10mm bolt (as opposed to the 8mm mounting bolts).


Watch out, as the water comes out with quite a bit of pressure; it goes straight out the side and will land a foot or two(!) from the bike. To keep coolant off the floor as much as possible, you can cut an old anti-freeze bottle as shown in the left picture. Make the cut toward one side and away from the cap, so you can pour the used coolant out without spilling. The flap that is produced by cutting catches the rather strong stream of coolant and diverts it into the jug (right photo).

PICT0439.jpg PICT0438.jpg

The alternative is to get a dishwashing-sized plastic tub and hold it at an angle to catch the first (horizontal) part of the flow. You can then move it as the flow slows.

After that part is drained, then drain the coolant in the block; the plug is on the front, between and slightly under the pipes.


Here's a suggestion for keeping the mess down while draining the block and head. There are only a few ounces of coolant left at this point, but cleaning it off the motor, starter, and floor is inconvenient at best. Take an oil bottle and cut it to this basic shape.


This is a catch pan that allows draining of the coolant from the cylinder head without the need for cleanup of the motor and/or floor. The bottle rests on the lower fairing, with the neck under the drain bolt. This allows access to the bolt with a socket on an extension. Let the bolt fall into the bottle; retrieve it when the bottle is removed.

PICT0426.jpg PICT0423.jpg

Next is the plastic expansion (overflow) tank. This is most easily drained by taking it off and pouring it out. If you want to be sure to get all the coolant out, you should probably flush once with water as well.

There are four places to drain coolant:

1. The overflow tank (white, behind the left quarter panel).
2. The actual coolant drain bolt on the water pump, located below and behind the gearshift lever.
3. The bolt below and between the exhausts where they exit from the head. This one is important and often forgotten.
4. The hoses themselves.

If you're changing coolant types (say from ethylene glycol to propylene glycol, which is less toxic) flush the coolant system completely by filling it with water, running the engine for a minute, draining the water, and repeating until the water comes out clear. It should only take one or two flushes to completely clean out the system. For a coolant system flush, any kind of clean water can be used, but for the final water/coolant mixture, distilled water must be used. The flush water isn't in the system long enough to cause problems if it's not distilled. Most coolant comes premixed these days -- check the label on the coolant bottle to be sure, and only use distilled water if it's not already mixed and you have to mix it yourself.

You can't fill it at the expansion tank. Just fill at the filler cap at the radiator. Fill slowly; this will make purging the air bubbles from the cooling system (see below) easier. Fill the radiator to the top.

Purging the air from your cooling system

Traditional method

After you refill the cooling system through the radiator, you're going to need to get all the air out of the system. Like the brakes, the cooling system doesn't work well with any air in it.

There are two methods for accomplishing this. The traditional one involves putting the bike on the centerstand and leaving the radiator cap off. Start the engine and run it until the air bubbles stop coming up. You first have to get it warm enough for the thermostat to open, and it takes another few minutes of running after that to make sure it's purged.

After you stop seeing air bubbles come up through the radiator, put the cap back on, after first making sure the radiator and coolant overflow reservoir are full. Check the coolant periodically, but, unless you left some air in there, you shouldn't have to top it off.

This-is-my-first-time method

This way seems to work well for people who haven't done this sort of thing before and may be worried about the potential for the radiator to burp hot coolant on the bike or their person. It's also meant for those who don't like the idea of filling the radiator and purging air while the bike is running. If this sounds like you, try this:

Make sure your radiator and overflow reservoir are full. Put the radiator cap on. Start the bike and heat it up to operating temperature, preferably by riding it, not idling. The first time you do this after filling, it should reach temp very quickly, since there are many air pockets and it won't cool efficiently. Let the engine cool completely, and you should notice the level in the overflow reservoir has dropped. Refill the reservoir (NOT the radiator - leave the cap ON) and warm the bike up again. Do this until the level of coolant in the reservoir does not change and you're done.

After you shut the bike off and the engine cools down, the coolant in the engine shrinks and creates a vacuum. Since the combination of the top seal on the filler cap and the coolant in the reservoir prevents air from entering the system, the vacuum draws coolant from the reservoir through a small one-way valve on the lower seal of the filler cap to equalize the pressure - basically purging the air from the system every time it goes through a heat cycle.

If the temperature gauge is higher than normal on your first ride after purging, simply recheck the overflow tank and fill if needed.

That's all there is to it. It maintains itself. The only time you should need to remove the radiator cap is when you fill the system with coolant while changing it. If the level is ever low, you add coolant to the reservoir.

My bike overheated. What to do?

The Ninja cooling system rarely gives any trouble. The most common cause of overheating is, in fact, the wrong spark plugs. Take a look at your spark plugs. They had better be either NGK or ND brand spark plugs. If they are Champions, then you probably have more than a cooling system problem to worry about.

OK then. Start with the simple stuff. Check to make sure that the fuse for the fan didn't blow.

Checking the fan

On the left side of the radiator, there's a switch about halfway up. Pull the connector off the switch and put a jumper wire between the two terminals on the wire. This completes the circuit in the same way as a functioning switch. The fan should come on. If the fan comes on when you do this, the thermal switch in the radiator is most likely the problem.

The switch closes when it senses that the coolant is too hot, sending electricity to the fan, so if the switch is broken the fan will never come on. The switch circuit is 'always hot', which is how the fan keeps running even after you turn off the key.

Another thing you should do if the fan doesn't come on is make sure that power is getting to the switch. Check to see if the connection to the fan motor is properly hooked up. If it is hooked up properly and the fan doesn't come on, disconnect it, get a test light, and jumper the switch connector again, and see if there's power getting to the fan connector.

To test the switch, attach two wires to the terminals on the switch, and then to the test leads of an ohmmeter or multimeter. Drop the switch in a pan of boiling water, and watch the resistance go from open to near-zero. The switch is supposed to close (fan comes on) at between 94 and 100 degrees C as the temperature is rising, and open (fan goes back off) at 90C when the temperature is falling. Resistance spec is less than .5 ohm when closed, more than 1 megohm when open.

If, after all these tests, the fan doesn't come on (and the fuse is good), the fan is the issue. Look for a used one.

Checking the thermostat

Now, if the fan works properly, and the temp switch in the radiator is working properly, then it's time to test the thermostat. Pull the gas tank, then open the thermostat housing and remove the thermostat. Test it in a pot of boiling water to see if it opens. If it doesn't open, replace it with a new one.

If it does open: With the thermostat housing open, crank the engine over a little bit and see if you get coolant movement (this will be messy). If there's coolant, the water pump is fine. If not, then you need a new pump.

Early model radiators

There is an earlier switch with only one contact. In that case, shorting the connector contact to ground should turn the fan on. The old-style switch is on the right, with the newer switch on the left.


Single contact

  • Radiator diagram: JA004010-KA005001
  • Radiator part number: 39060-1072
  • Switch part number: 27010-1167

This was used on several bikes, but supposedly on the Ninja 250 only from 1986-89.

Two contact

  • Radiator diagram: JA004011-KA005002
  • Radiator part number: 39060-1087
  • Switch: 27010-1249 (89-90) superseded to 27010-1304 (through 2007).

It would seem that some 89s had one style and some the other, according to the parts fiche.

Cooling system electrical

Sometimes (and this is not a real common occurrence) your bike’s temperature gauge can give wacky readings. If your coolant system is perfectly filled and functional, then your problem could be electrical rather than coolant-related. In other words, the cooling system may be working properly, and you're just getting bad readings from the temperature sensor.

The problem could be that you've somehow messed up the ground path of the temperature sensor, which is critical to the proper function of the temperature gauge. Or you may have somehow compromised the gauge-to-sensor wire, so that it can't carry the proper amount of electricity to the sensor or it is now putting up too much (or too little) resistance to the electricity being sent through it from the gauge. This could happen if the wire is touching some metal part of the bike as it routes to the sensor. This would allow a path for electricity to go to ground without first passing through the temperature sensor.

The temperature sensor on the EX-250 functions as follows:

The various gauges and lights on the bike's instrument panel receive 12 volt electrical power from the bike's wiring harness. Inside the temperature gauge on the instrument panel, there is a bundle of copper windings that behaves like an electromagnet when a continuous flow of current is applied. An electromagnet is only a magnet while current is flowing through it. And, more specific to this problem, the bike's temperature gauge only functions as an accurate temperature gauge when the correct amount of current is passing through the gauge's copper windings.

During the operation of the bike, current is constantly flowing into the temperature gauge windings, then out through a wire to the temperature sensor and then to "ground" in the bike's frame. The rate of current flow and the resultant reaction in the gauge's electromagnet, which moves the needle to indicate the operating temperature of the coolant, is completely dependent on having a good, solid path to ground available.

Proper function of the temperature gauge is all about proper flow of electricity. Here's the routing: The temperature gauge functions by receiving 12 volt electrical power from the bike's wiring harness. There are two resistors in the gauge that reduce the amperage of the incoming power (to protect the gauge) and then a wire leaves the temperature gauge and proceeds to the temperature sensor.

The temperature sensor causes an indication of temperature on the temp gauge's needle by grounding the electricity supplied to it, and the heat of the bike's coolant controls how much or how little resistance the temperature sensor puts up to this electrical flow. More heat equals more ability of the sensor to send the gauge's electrical signal to ground. The ground path runs through the aluminum body of the thermostat housing, then through the steel bolts that attach it to the bike's frame, then into the bike's frame, which is in turn grounded through the bike's main grounding point on the rear of the engine block, and finally back to the bike's negative battery terminal post.

If anything has happened to reduce the temperature sensor's ability to ground the electrical signal sent to it by the temperature gauge, then you will get odd readings from the gauge.