Question about 2002 Yamaha YZ 426 F

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My YZF 426 2002 model has been rebuilt twice but radiators keep pressurising and pumping coolant out the over flow pipe, it has a brand new radiator cap and impella

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Sounds like the raditor could be blocked or the pump is spinning to fast install bigger radiator pipes also fill it with coolant completely to the lid and also bleed the air out off system get back to me if need any more help or you have done this

Posted on Sep 29, 2009


  • ollie reece-jones
    ollie reece-jones Sep 30, 2009

    the wrong gear in there or its too small of a gear it may be air in the system aswell or the thermostate not sure if these models have this get back to me



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Cooling system fault finding..

Your cooling system works by pumping coolant(water and antifreeze) through the engine where it absorbs heat, through the radiator (where it loses that heat) and back into the engine.
Main components of the cooling system.
  1. Coolant. It can be plain water. Adding Glycol based antifreeze is definately recommended as it not only raises the boiling point of the water and lowers the freezing point but also contains very important corrosion inhibitors. Adding too much antifreeze lowers the coolant's effectiveness so don't go more than a 50/50 mix unless stated otherwise by the manufacturer.
  2. Radiator. This acts as a heat exchanger between the coolant and outside air. It is actually a whole bunch of small pipes that have thin pieces of metal conncted to them to increase the effective surface area.
  3. Waterpump. Well that's what it is. A pump to circulate the coolant around the engine.
  4. Thermostat. This unit regulates the flow of coolant around the system. You need to get the engine to operating temperature as quick as possible, then you need to keep it there as this aids in lowering fuel consumption and emissions and this is the job of the thermostat. It is installed in the system in such a way that it can block the flow of coolant between the radiator and the engine.
  5. Cooling fan. Cooling fans help airflow through the radiator at low speed or when the vehicle is stationary This may be electrical - powered by a thermostatic switch and a relay, directly driven by a belt off the crankshaft or have a viscous coupling. The viscous coupling is also directly driven but contains a fluid which couples the fan to the pulley only when needed.

Reasons for overheating :
  1. Insufficient coolant
  2. Insufficient circulation of the coolant
  3. Insufficient airflow
  4. Excess heat generation
Do's and dont's
When topping up it is better to use antifreeze only, as you will not be diluting the mixture. When refilling use the correct mix of water and antifreeze or fill with pre-mixed coolant.
Replace coolant according to service schedule.
Never open a hot cooling system.

The first and most obvious thing to check is coolant level.
No reservoir : coolant is added directly to the radiator. Fill to the top. You may find a little fluid missing everytime you open the radiator. This is due to water expanding when heated which escapes through the overflow pipe at the radiator cap.
Non pressurised reservoir. This coolant tank is not pressurised and can be opened at any time. Usually has a cap that just clips on. Fill to the max mark when the engine is hot or at least to the min mark when the engine is cold. If this reservoir is filled it does not nescesarily mean the system is full since it depends on expansion and contraction of air and coolant within the system to get coolant back into the radiator. The radiator should be filled as well.
Pressurised reservoir : Most modern-day vehicles are fitted with this type of system. It is pressurised and should only be opened when the engine has cooled down. Do not overfill, always keep coolant levels between the minimum and maximum marks.
If coolant levels keep dropping with no visible signs of leaking have the engine tested for a blown head gasket.

The internal passageways that allow for coolant flow through the engine are metal and constantly exposed to coolant which causes rust which is transported throughout the system. The radiators' internal pipes have very small diameters and easily become blocked by this rust. This decreases the effective area of the radiator and the amount of water that can pass through it. Regular replacement of the coolant will prevent this build-up.
Waterpump impellers may break and decrease the efficiency of the pump although this is rare.
A slipping fan belt will also cause decreased coolant flow but is usually quite noticeable as a high pitched shrieking noise especially on startup and when revving the engine.
The thermostat is in effect a valve which opens and closes with temperature. They go faulty mostly because of rust and scale buildup and can get stuck in a closed or half-open position, thereby blocking waterflow to the radiator.

The radiator fins are spaced closely together so can easily catch debris such as bugs and grass which will impede airflow through the radiator. The fan assists with airflow so if not working will see the engine overheat when stationary or in stop-start traffic. If the engine cools down while driving above 35mph this is the obvious starting point.

Incorrect fuel/air mixture and incorrect ignition timing may cause the engine to generate heat in exess of the coolings systems' capabilities.

How to test and repair :
  • Check the fluid level. Also check for leaks. They will usually show up as brown or green streaks of dried coolant.
  • Check your fan belt/serpentine belt - is it still intact? Check the operation of the fan. On a direct coupling you should not be able to move the fan. A viscous coupling should not spin freely by more than a quarter turn. See below for the electrical fan.
  • Flush the system. Remove the upper and lower radiator hoses and if possible, remove the radiator. Stick a hosepipe into the outlet side of the radiator and let the water run until it comes out clear on the other side. Then repeat the process with the hosepipe in the inlet side. Continue with this until the water runs absolutely clear.Now remove the thermostat from the engine and do the same to the engine. Clear the radiator fins of bugs and debris.
  • Check the thermostat. It should be in the closed position. Drop it in a cup of boiling water and you will notice it opening. Since it is already out you may as well replace it since it is relatively inexpensive
Electric fan
On an electric radiator fan, let the vehicle heat up to just before overheating. If by this time the fan has not come on switch off the engine.Locate the cooling fan fuse and check that it is intact. Using a multimeter, measure between each of its' contacts and earth. See diagram. Measure between 1 and earth(bare metal point on vehicle body or on battery negative), and then between 2 and earth. Both readings should be 12V or more with the ignition on. If 1 reading is found only, the fuse is faulty. If no reading is found the wiring between the ignition and fuse is faulty.

Now check the operation of the fan itself. Disconnect the fan. Connect a lead wirefrom the battery negative to one of the fan connector pins and from thebattery positive to the other pin. See diagram: 1 - Fan motor
2 and 3 - Connect directly to battery


This is a DC motor so polarity doesnot matter. If the fan operates reconnect it to the harness. If not, replace the fan.
Locate thefan relay. You will need a multimeter to test. Remove the relay. Switch on the ignition. Measure voltage at allthe connections in the relay socket to earth. Two of them have to be at 12V or more. If not, the problem is between thefuse and the relay. Re-insert the relay.
Disconnect the thermoswitch. Usually located on the radiator. See photo : 1 - Thermostatic switch. 2- Leaked and dried coolant on a neglected radiator.

Shortcircuit the 2 wires from the harness that were a moment ago connected to the thermo switch.(With a single wire thermo switch the wire needs to be connected to earth or battery negative). Switch on the ignition. The fan should start running again. If not switch off the ignition and remove the relay again. You already know the connecting point with 12V. Measure the other 3 wires for resistance to earth. One of themshould read more than 5 ohm. Connect a lead between the "12V" connector and the "5ohm" connector. Switch ignition on. If the fan runs the relay is faulty. If not the wiring between the relay and fan is faulty.
Simplified wiring diagram:
A- Battery
B - Ignition Switch
C - Fuses
D - Relay
E - Thermostatic switch
F - Radiator fan


Leak detection
The most obvious tell-tale would be a puddle of coolant under the vehicle after a trip. Don't confuse this with condensation dripping from the aircon.
Inspect pipes for cracks and leaking at joints.
Inspect the radiator for dried coolant. (See photo above)
Inspect the engine block for coolant streaks. Leaks can start on the block itself through the welch plugs which are also constantly exposed to coolant.
Another leak point is the water pump shaft seal which cannot be repaired. Replace the whole pump.
Remember that the cabin heater is plumbed into the cooling system and can also leak. Wet carpeting in the footwell is indicative of this although such wet carpeting may also be the result of condensation from the A/C.

How to wire a relay

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2008 grand caravan 3.3 L engine blows the plastic Y-coolant pipe twice.


Oct 17, 2016 | 2008 Dodge Grand Caravan

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I have a 1994 ford Taurus that has new plugs new thermastat new water pump new coil wires but car is still overheating

does putting heater on hot prduce hot air.on full with fan on does it reduce temo?is ur radiator fan coming on? hard to give one dont know if its overheating in traffic or on the move?there could be air stuck in pipes slowing water flow.or a pipe/radiator blockage so coolant doesnt get to flow around engine so it gets hot.u could check radiator by touching see if hot or cold where pipes enter(prob switch off engine first) try bleeding system squeesing pipes to push air out

May 20, 2015 | 1994 Ford Taurus

1 Answer

2001 Mazda MPV overheats. radiator hose doesnt feel pressurised ie very soft to squeeze. when i put coolant in after 5 mins of driving is backflows out of the reservoir and causes heavy smoking from t

sounds like the radiator is blocked either coolant flow or ext air flow through the matrix the thermostat can get ruined when the system overheats so allways change the stat after a overheat problem GET THE RAD CHECKED FIRST if ok check the coolant pump

Aug 22, 2017 | Mazda MPV Cars & Trucks

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I have a 1994 chevy caprice classic. Put brand new radiator, water pump and thermostat even put new fan motors but it keeps running hot and water in the collant jug keeps bubbling after ive been driving...


Jul 05, 2013 | Cars & Trucks

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Just replaced temp sender unit and changed coolant. Temp gauge works fine, but when up to temperature, water spews past radiator filler cap. does not discharge into expasion tank although the small pipe to...

Hi, why did you change the sender in the first instance? If you mean the water is pushing past the radiator cap when the engine is hot, and suspect the head gasket is the cause of the water pressurising the system, run the bike with 1 plug removed if the cooling system is getting pressurised from a leaking gasket, then with the plug removed no pressure will be forced into the system from that cylinder and the water should not push past the radiator cap, try this on all cylinders one at a time to eliminate them, if the pressure does stop at any time then the head gasket COULD be at fault,you can buy a kit which measures water in th exhast system

Sep 25, 2011 | 1996 Triumph Trident 900

1 Answer

Even after replacing the radiator, thermostat and

The problem with overheat is due to poor coolant circulating. The main culprit is the Radiator. When the radiator is clogging up, coolant can't be circulating. The stagnant coolant inside the engine water jacket keeps absorb heat from combustion chambers having no way to dissipate the heat then the engine is overheated. You can test the water pump by removing the thermostat, then hooking everything back. Open the radiator cap, then turn on the engine to see if water flowing down to several small tubes inside the radiator. If water can't go down fast enough to cause over flow at radiator opening then your 90 Legend Radiator already got clogged up. Replace with a new radiator, not even a rebuilt one. Good luck.

Dec 13, 2009 | 1991 Acura Legend

1 Answer

Coolant filling up overflow bottle and not returning Started intermittently No coolant leaks Changed Hoses Cap Thermostat Cooling system has always had good quality coolant and is clean inside...

You may need to ask AAA to put a radiator pressure tester on the resevoir bottle and pump it up to operating pressure(Pressure cap rating) and see if it over pressurises while it is running at operating temp,rev engine up and down and watch what the testers pressure readings do,it should move up and down in sync with the water pump.If the pressure keeps building i would have to believe that combustion chamber gases are over pressurising your cooling system.Also test the pressure cap is functioning within factory specs.

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