yz 250 coolant leak

Ok guys new here

and new to MX bikes

i picked up a 2000 yz 250 last summer, guy owed me money

I got the bike, had it looked at by local shop

said all was well

good compression,no leaks ran very well and srtong

replaced plug, and oil, air filter, and coolant fluid.

rode bike about 20 times, before winter

stored bike with gas drained, and changed oil before storing

dug bike out yesterday

filled with fresh gas, installed new plug, check coolant level, basic pre ride stuf

bike fied up on second kick, blew a little blue 2 stroke smoke for a few seconds, and then seemed fine

took bike on a ride( old logging roads and coal bank hills)

10 min.s into it, I stopped to take a small break at the top of a hill i just climbed.

and noticed i was leaking coolant out the overfill line, as I reved the bike.

shut it down, waited a little for it to cool off.

then checked the coolant levels, they were low.

road the bike back nice and easy(only about a mile or so)

let it cool down

added more coolant, check the plug(was looking good0

started the bike up, and as it got warm and I reved the bike again it would flow out the over flow tube.

shut it down

decided to change the oil to see if i was getting any fluid in it

seemed just fine, replaced with new oil

I have no visable gasket leaks.

any ideas on what going on?

my budget is tight, as just lost a job after 22 yrs, so I am hoping its something simple so I can get it fixed


I would drain the radiators pull the drain plug at the water pump put a jug or container under the water pump put a garden hose in the filler and run water thru it to flush the system after it sitting for a while you could have a buildup blocking something i.e. Hose water pump impeller etc. Causing it to get hot and belch out the overflow

If it is blowing coolent out the overflow when you rev it I would guess head gasket (orings) is bad. Is there white smoke coming fron exhaust? Does exhaust have a sweet/antifreeze smell? Just pull the head and check the orings for a crack or pinch. Its a cheap fix and all you need is a torque wrench and the orings. If you dont have the manual ,I recommend that as your 1st purchase, or download one . Somewhere there is a free site, just search these forums for the site ,Im sure it will be easy to find.

Proper coolant level is just barely covering the tubes in theradiator and not full to the neck.

Get the cap tested too, to ensure it holds pressure.

Head gasket leak is my thought too, cheap and easy.

thanks guiys

no it has no swet smelling smoke or white smoke

just a hint of 2 stroke

I have never taken a bike apart at all, and right now no real place to do it

the bike will be going to a shop for this repair

any idea on cost to have the ehad gasket replaced at an average place?


Well they will probably charge you an hour labor + parts. An hour labor could be $60 - $100. It only takes about 15 minutes to change that gasket assuming thats what the problem is. If you can change the spark plug you can do it. Remove those 6 bolts around the spark plug, the bolt on the bracket to the frame, pull the head off and replace the o-rings. Clean any carbon off the head, make sure its not warped or damaged and reverse the process. Tighten the bolts in a criss cross pattern and your done.

Before you go dumping your bike at a shop for a simple repair, make sure you know what the problem is.

How much coolant did you loose? Where was the coolant level in relation to the fins in the radiator? In my bike the coolant level is just barely above the top of the fins.

If the bike was fine and was not burning coolant before the winter, the head o-rings did not pinch themselves from sitting. They may deteriorate, but that seems far fetched and may depend on the conditions the bike was stored in.

Ride the bike a little longer while constantly checking the fluid level. Zip tie an old water bottle to the frame and insert the radiator drain into a hole drilled through the cap. This is so you can keep an eye on the amount you are loosing without waiting for the bike to cool down. If the bottle fills up, you have a problem. If it only fills slightly, the cooling system is above capacity and as pressure builds, the excess is ejected.

Ok guy thanks for all the imput

I forgot to mention, that I had a shop install a new chain and both sprockets before i put it away, would have done it myself , but couldn't get the front sprocket bolt loose, no matter what I tried, even with a small impact gun I had at hand.

any way, I didn't think nothing about this, as I just figured it was done right

WELL, after giving the bike a close inspection on a stand, I found that my case saver(works connection) I am guessing made of aluminum, well, the sprocket thats on the bike now is either thicker, or they do not have the bolt on all the way

my case saver is rubbing very very bad on the bolt, so much so that after my short ride, it chewed up about an 1/8 of it so it sits flush with the sprocket bolt

now the bike rolled freely, thus I didn't notice it I guess

but I now think that maybe why my bike was running hotter, as it was having a load right at the front sproket.

I also changed the coolant today, new prestone50/50

after replacing it, while running it with the cap off, it bubble's a lot,but I think this is normal


I didn't take it out for a ride, but did let it het hot and ran in on the stand for a few minutes(1st,2nd and 3rd gears)

and didn't have any issues

could the case saver rubbing been the problem/

thanks again

OH, and I was planning on taking the bike to ashop to have a flywheel weight added to this bike, so while its there I am going to have them MAYBE fiix this issue if its still there>

Don't know about the case saver. GMO has some good points and tips. I don't think a lot of bubbles is normal, there should not be any air in the cooling system. Maybe after a coolant change as the air is pushed out but not on an ongoing basis. One of the signs of a blown head gasket is loss of coolant due to exhaust escaping into the cooling system and pushing it out, this would cause bubbles.

If you were doing the work yourself replacing the head gasket o-rings would be a cheap diagnostic, just to rule it out thing. I'd follow GMO's advice and play with it some more before spending the money. If your coolant levels out just above the fins and no more escapes your probably golden. Just be careful, there are no warning lights and bad things happen to aluminum engines that overheat.

here are a few pictures of it after i added the washers to get clearance

you can see how the nut's face is worn clean, and the metal build up on the case saver



The case saver did not have any effect on the cooling of the engine. It may have put a slightly higher load, but nothing near the mechanical abilities of the engine and cooling system.

bubbles in the cooling system is head gasket leak buddy or hopefully not but could be cracked head

no more bubble in radiator

that case caser was pressing a ton on the front sprocket nut, and I am guessing it caused the bike to run a bunch hotter, thus the overheating issue

ran the bike several times since the fix on the the nut/case saver

and not once did it get hot to the point of anything coming out the overfill, like it did before, or while idiling


I do plan a top end rebuild as soon money allows

so, will see the seals then if they are /were good or bad

or sooner if the bike overheat again!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now