Coolant leaking from crankcase

Some time ago i discovered that coolan are leaking from the hole in the crankcase... bike was in the garage for a month, and i discevered coolant on the ground... why this small hole in crankcase is for??? Too big preasure in cooling system???



Bike Yamaha WR400F 1998.

That is a 'weep' hole.

It is there to let you know when the impeller shaft or impeller seal are wearing out.

If the bike sits for a long time, the seal becomes less compliant, letting it leak.

It usually goes away, or becomes minimal with use.



Just done mine, as it was also leaking on bike start up, but after about 10 secs of the engine running it would stop.

Ironically, this has happen after the winter lay up, and it was fine when I put the bike away last year

I decided to replace the seal, as I didn't want it to fail mid race, dump all my coolant and the engine seize

You don't even need to remove the clutch cover to replace the leaky seal (well at lest on the 450 you don't, don't know about the 400)

Drain the coolant

Remove the water pump cover (no need for a new gasket, as its sealed with an o-ring and hence is re-usable

Undo impeller with 10mm socket - its a normal RH thread

Remove impeller from shaft

you will now see the seal that is the cause of the coolant leak

Lever the seal out with a screwdriver or similar

To fit new seal, lube with grease the ID and OD of the seal (OD of the seal so it will slide back into the housing with minimal effort, ID so it lubricates the seal in use)

Press back in with fingers/thumbs, with numbers on the seal face, facing inwards

Refit impeller and torque to 14Nm - use a bit of locitie on the thread

Refit water pump cover

Refill with coolant

Start engine and check for leaks

Go Ride :thumbsup:

No more than 30 mins start to finish

Edited by GuyGraham

I have a question about removing the impeller. What holds the impeller shaft from turning when removing the impeller? I just replaced my stock pump and housing with the Boysen pump and housing but it was my understanding I had to remove the clutch cover as well as the side case to hold the impeller shaft. Now that I have done all of this, I have fluid leaking from the weep hole. I leave for Moab in 1 week so I need to get this corrected.

End of impeller shaft fits into a recess in the end of the crankshaft, and hence is driven by crankshaft

This stops the impeller shaft turning when undoing the impeller

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