Question about 2002 Yamaha YZ 426 F

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Coolant fluid leaking when warming up

When my yz426f is warming up coolant comes out of somewhere & steams up off the header. Once its warmed up it seems to stop. Is this bad?

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  • danny
    danny May 11, 2009

    Its leaking only when its warming up not cooling down, but I can understand the theory in your solution

  • danny
    danny May 11, 2009

    Haha, Have you ever seen a 426 start cold & idle right away without reving it?! I think its coming from the overflow it stops when the bike is at running temp



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Tighten all hose clamps ,  this is caused by the hoses expanding and contracting with the water temperature , when it is cold the hoses have cooled down and leaking will happen, 

Posted on May 10, 2009

  • Paul G
    Paul G May 11, 2009

    yes but when it is warming up the hoses are cold and the the head is only just starting to expand with the heats as well,  
    Try this .
    from a stone cold position start the bike don't rev it just watch it idle , look at the hoses watch where the water is coming from it can also be a gasket (same principle )very important that you see where it is coming from if the hoses are all good then the water pump gasket is not sealing properly .



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Hi, Rick and the usual suspects are:
1. Low coolant level.
2. Restricted radiator air flow.
3. Faulty thermostat.
4. Coolant pump or fans inoperative.
5. Vent hose crimped.
6. Air in the coolant and needs bleeding.
7. Engine idling too long or bike moving too slow.
For more information about your issue and valuable "FREE" downloads that you will need please click on the links below. Good luck and have a nice day.
Help Grayracer or anyone Coolant leak from overflow tube YZ 400 426 450
Yamaha YZ426F Owner Service Manual
OEM parts for Yamaha
Yamaha YZ450F Owner Manual Page 3

Btw, I’m available to help over the phone in case u need at

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Radiator checked and no leaking - collant light comes on in dash-board display and the coolant in resovoir is low with no apparent leaking found???

You might have the leak in the system which comes effective as the engine warms up and the SEALED SYSTEM oozes off as steam from any of the joints, the radiator or the engine gaskets.
Top up the coolant and let the engine warm up on idling while you check for possible leaks in all the areas , water pump, engine and radiator.
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Also could be a headgasket failure between coolant channels and a combustion chamber. This latter problem sometimes shows itself as vapor at the tailpipe after there should be no more visible. You may also detect the odor of coolant there too.

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Jetta GLI 2005

leak in the actual system, try running the car as it warms up and look underneath it too see if any water leaks out. Some water will seem to disappear if the radiator cap is worn out also.

It has to go somewhere!

its either
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2) leaking out through overflow/radiator cap
3) going through the engine (unlikely if its going through a lot quickly) check your exhaust to see if steam keeps coming out when warmed up (steam can come out on cold says and be normal)

remember as the car warms the pressure will rise, so the leak might not show up for a while and DONT open the radiator cap when the engine is hot!

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