radiator and overflow problem have me stumped

i started noticing this problem about a month ago.For some reason my radiotor will not suck coolant back from overflow bottle.radiator fills overflow bottle but will not let water return to radiator so after a few hours water just ends up pouring out overlow bottle's overflow outlet.i've checked routing of hoses to make sure they are not being pinched,ive also replaced all the hoses,i've replaced radiator cap and inspected overflow bottle for leaks or damage.neither left or right radiator is damaged or leaking.waterpump seems to be working fine and motor shows no sign of head gasket leak.the no water in crankcase oil and no oil in radiator coolant.after a day of riding i usually lose about 1/2 quart of water.

are you sure you don't have a compression problem, take cap off, let the engine warm & see if you get bubbles coming from radiator. if so you have compression back feeding into the coolant,, possible cracked head, or bad head gasket can leak into engine & not have any external leaks.

Just some food for thought: Look at all the "external" water connections.

My bike started doing the same thing last year. I did everything you did to fix it and I even got a higher pressure KX500 radiator cap. The coolant still wouldn't suck back up into the radiator. I began to notice a teeny, tiny little wet spot at the base of the cylinder on the right side. The wet spot would never get big like it was leaking a lot, the dust on that area would just be a little darker. That's where the system was losing pressure. I replaced the o-ring where the water pipe at the base of the piston joined the crankcase water pipe (and $head$ and ba$e gasket$ :)) and the problem was solved. It took me a couple of months to figure this out because there was no oil in the coolant or coolant in the oil.

One easy way to check for oil and coolant leaks is to use aerosol foot powder spray, of all things. Clean the bike and motor real well and let it dry completely. Then spray all around the motor with foot powder spray in areas that are likely to have a leak. (e.g. at mating surfaces) It will stick to everything you spray it on and if a leak occurs you'll be able to see it as a darker stain in the powder. Then you can get to work and fix it.

The cooling system may be over pressurized due to a faulty head gasket causing leaking and over filling the coolant reservoir. But since you’ve ruled out the head gasket, a faulty radiator cap could also over pressurize the system or in your case not maintain the correct pressure.

Try replacing the radiator cap and see if that helps. Yamaha also sells a tool to check the radiator cap, but I would think the inexpensive route would be to just go buy yourself a new radiator cap.

my 2 cents

this problem came up before w/ a fellow TT'er. After LOTS of comments/suggestions, it was a gasket/o-ring leak somewhere. The engine was pressuring the coolant system >> the overpressurized system then dumped coolant to the reservoir via the radiator cap.

thanks for ideas.im going to start trying them out.i really didnt want to pull head and cylinder till a last resort .i had Eric Gorr do a complete motor lower end rebuild and install big bore kit less than 3 months ago.

Not to sound to stupid but could it be as simple as the wrong lines connected to wrong side of the overflow bottle?

i think i might of found problem.i decided to re-torque head and found that 12mm nuts located at front and rear of cylinder were both only finger tight.i just finished torqing everything down and putting bike back together.hopefully the head gasket is still good and tightening bolts will solve the problem.i'll find out tommorrow when i go riding. thanks again for the help.

12mm nuts located at front and rear of cylinder were both only finger tight.i just finished torqing everything down and putting bike back together.


Sounds like your problem...

Don't forget to torqe the head bolts under the cams! I got an big bore done by Eric gore and it started leaking at the head gasket. he had me 3 bond the head gasket. What ever you do, NEVER EVER 3 bond the base gasket. It will disolve the base gasket as soon as it gets warm. It may be simpiler than that though, check it out good. Black flecks in the coolant, you know etc, etc. Hope you get it nailed down. I like the foot spray Idea! Easy to find die check. ---Mike

Another obvious problem could be the type of Radiator cap. If its a YZ one it may not be a 2 way cap, ie. it lets water out, but wont suck water back in. Just a thought .... Like most older cars without coolant reserviors the cap is different.

The again, by the sounds of things you may have found the culprit. Good luck and Torque evenly


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