Cracked Waterpump Impelor

My '03 YZ450 was losing alot of fluid out of the overflow. I replaced the cap with a cv4 30lb cap thinking this was the problem. It was still losing a lot of fluid so i pulled the waterpump cover and found the impelor cracked where the "nut" attaches to the impelor. My question is would the cracked impelor cause enough overheating to dump a constant stream of coolant from the overflow while riding, or could i have another problem which is causing enough presure to leak out of the new radiator cap that also caused the impelor to crack? I will obviously be replacing the impelor but regardless of the crack, it was still threaded on the shaft tight so i asume it was still spinning/pumping, so why the overflow problem? Any ideas? Thanks

The impeller can only have been cracked by some physical contact. If you bought the bike used, the previous owner likely did it while working on it. I doubt that either a crack or a missing vane would cause as much trouble as you describe. On the other hand, if the crack is below the hex used to tighten or remove the impeller, and it is such that the impeller is free to rotate on the shaft, that basically means you have no water pump, and it's certainly at least part of your problem.

You did not specify the conditions under which the coolant loss occurs. If you spend a lot of time idling, or going very slow, you will loose coolant regardless what you do to prevent it, short of putting a battery operated fan on the bike. If that is not the case, what you may have is a head gasket leaking combustion pressure into the cooling system. Normally, there is no reason to need anything like a 30 pound cap.

Thanks Grayracer.

I put new waterpump seals in the bike last spring and have riden quite a bit since. I guess its possible i cracked it while reinstalling the impelor and it has taken this long to start spinning free on the shaft. It was cracked at the base of the nut but was still in place and didnt look like the impelor had been spinning free. I also rebuilt the top end last spring and have ridden many hours since this with no coolant problems, the head gasket leaking pressure is a posibility i hadn't considered. Is there a way to test this?

Riding Conditions: It will not leak at idle, riding at the dunes with a paddle, it will start to leak as soon as i get moving. Even when the bike is fairly cool and running on the flats, so not under any heavy load which would cause excesive heat. I moved the overflow hose so i could watch it while riding and after leaving the truck with a cold bike i was getting a constant stream. stop and idle and the leak slows/stops. I let it leak past the point where the coolant was below the fins just to make sure it wasnt simply overfilled and that wasnt the case.

Thanks for the input.

If the impeller is spinning free on the shaft, you need to fix that before you do anything else.

A good shop should be able to test for the presence of combustion gases in the cooling system.

I thought id'e add i have had this bike for over three years, bought it slightly used from the first owner. I have ridden this bike on three trips to baja 700-800 miles per trip, and 10-15 dune trips per year, lots of desert riding, and Ive never had cooling problems. The most recent work i listed before : top end rebuild last jan, waterpump seals in march with probably 5 rides since then before it started giving me problems. It leaked minimally 3 weeks ago at the dunes, a little more the next weekend, new radiator cap before this last weekend ride, and i rode for 10 min and parked it because it was a constant leak, pulled it apart and found the cracked impelor. I also traded rad caps with my brother just to make sure the new cap wasn't bad.

Thanks Ill replace the impeller and go from there.

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