Does overflow put coolant back into radiators??

I went riding the the desert with a friend who's WR450 had gotten pretty hot on a couple of trails and finnaly quit working. When we checked the radiator, after it was cool, it was very low and there was plenty in the overflow. Is the overflow only to catch it when it gets hot?

I had a similar experience with my WR426 where the radiator was low, but plenty of coolant in the overflow.

I also had this problem with my XR650R, so I am beggining to wonder if the overflow is only to catch the coolant when it gets hot. I thought when it cooled in the radiator it would suck the coolant back but I have not seen this happen.

It should refill as the engine cools and the coolant contracts. Sounds like you may have a leak or a bad radiator cap that is not sealing (maybe some gunk on the gasket) or maybe a kinked/crushed hose between the tank and radiator.

When you say "low", can you still see the coolant over the top of the fins ?

Or can you not see any coolant at all when you look in there ?

As the coolant drops back down in temperature, I don't think there is enough vacuum in the system to fill it back all the way to the top.

(I could be wrong, my YZ doesn't have a catch bottle like the WRs.)

I could not see the coolant when i took off the cap. It was down a little past where you could see it. I don't think there is enough vacum to return the coolant from the overflow to the radiators even with a good seal.

In a car engine, as the coolant gets hot, it expands and gets pushed out past the cap to the reservoir. After the engine is shut off and begins to cool, the coolant should be sucked back into the radiator, as long as there are no air leaks. I assume the same applies on our bikes with a coolant reservoir. Check for air leaks, bad cap, probably a pinched hose.

Actually it should behave just like your automotive system. My '03 stays full to the top. My '04 does not have an overflow so it always has a small air gap but still covers the core (unless I get it really hot). There is something definitely wrong if the level is below the core but it should stay full right to the top with the overflow bottle.

The main advantage of a coolant recovery system is just that, i.e. to maintain a cooling system filled to maximum capacity for maximum cooling efficiency. :ride:

On my 06 wr450 I experienced something alittle different my hose melted that runs off the cap on the reservoir and supposedly caused a vacumm then when it opened back up from more melting. Before I knew it it threw all of the coolant out of the reservoir. This is what I was told from the Yamaha dealer tech.Temps during the event was middle 90's here in Phoenix.

The tech told me to disconnect the top hose and it should prevent this problem from occuring again, additionally I did have the idle too high so when I stopped riding those few times it must of cherryed the exhaust eventually melted the hose which I could see. This hose is right behind the first exhaust connection coming from the head. :ride:

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