Yz Motor Problems

Hello everyone,

Since I purchase this hybrid quad I've had nothing but problems with anti-freeze leaking out of my weep hole and it's getting in my oil. The first time I replaced the waterpump bearing, shaft, inner and outer seals. I figure this would take care of the problem. But it did not so I had a local shop replace a new outer and inner seal and it's still leaking and getting into my oil. I'm running a oversized crossflow raditor with TRX250R cap and raditor scoops. But I'm not running an overflow bottle! Could this motor be over heating? It's not leaking anti-freeze through the overflow.

Please post a reply if anyone has experience this problem or knows how to fix .

If this helps out the motor is YZ400 with a YZ426 crank.



Hey Guys,

Today I went to a indoor race and I noticed a white milky oil residue coming out of my overhead valve overflow hose. Could this be because of moisture in the head? And the anti-freeze was boiling still after the motor had cooled down a bit! :applause:

I just drained my oil after today's race and noticed no more white milky residue in my oil so is this problem coming from my anti-freeze expanded and has no place to go expect the weep hole. I checked my anti-freeze level and noticed it was down 1" from the top.. So if I run my anti-freeze level down a bit will this keep the anti-freeze from expanded and leaking out of the weep hole? ( I think someone else mentioned this to me in the past but I like to keep all of my fluids top off before ever race)!

This motor uses the same seals as the YFZ450.

Should I try the double lipped seals??

Should I also try the Engine Ice and 16lb radiator cap first before replacing my counter balance bearings and rebuilding my bottom end complete?

I just replaced my base gasket because it was leaking so could this motor be over pressure? Please HELP...

I assume that you changed your head gasket at the same time you did the base gasket, right? A blown head gasket will pressure up your cooling system and cause you to loose coolant.

In regards to the coolant from the weep-hole - If you've replaced everything in the water pump (including the pump shaft), then check the pressure of the radiator cap that you're using and compare it to that of the stock YZ. If it's a very large difference then it's possible you're building more pressure than the YZ pump was designed to handle, forcing a leak.

In regards to coolant in the oil; as stated above, get a leak-down tester and see if you can find any head gasget leaks. You MIGHT find a leak by firing it up in neutral with the rad cap removed and look for air bubbles in the radiator. There is a weep hole for the coolant to leak out when you have a pump leak, so if you're getting it into the oil the either both pump seals are very very bad, or you've got a leak somewhere else, such as the head gasget. Hope this helps and good luck!

I did replace the head gasket the same time as I did with the base. I guess my problem might be the Honda TRX250R radiator cap. What's the PSI rating on the stock YZ400 or YZ426 radiator cap??

If it's my radiator cap would you still do a leak-down test to make sure the gaskets or seals did not burst again because of the pressure. Thanks for all the help!

First off you will never be able to keep the coolant at the top of the radiator after you get the bike to operating temperature, then check it when it is cool. There is no recovery tank on the YZs. Easy to rig one up like the WRs, especially on a quad. So as long as there is coolant above the rows in the radiator, you are running the proper level. YOu have to remember it will spit out any excess and the fluid expands when hot.

Milky looking stuff from the valve cover vent, normal. Probably got water and or condensation from washing the bike.

Milky stuff in the oil, that is not normal. But if it is a small amount, it may be because of condensation getting sucked into the breather again. If you milkshaked the oil, be concerned.

What PSI is that cap? Reference that to the stock pressure of the YZF radiator cap. I believe it is 1.1 on the stock YZ caps.

So every time you guys race the YZ426 motor's you have to top off the radiator because of the antifreeze expanding that much?

My antifreeze level was down almost 2" from the top of this radiator I'm running and it's a double row radiator with inline coolers. It probably holds approx 1/2 of a gallon, if not more.

But what's the real reason for it expanding through the weep hole? Instead through the overflow. My radiator cap that I'm using is 16psi. ... Stock YZ opening pressure is 13.5-17.8psi.... I know these radiator caps have worked on other YZ motor's in the past.. Thanks again guys I know we're going to figure this thing out sooner rather than later! I can tell you one thing this motor is very strong and can't wait to get this problem fixed because I'm going to kick some a$$ on this machine this year...

I see everyone point on the condensation most likely getting in my breather or from excess antifreeze at one time.

no you dont need to fill the radiators every time. You fill it to the top when cold, run it. The fluid will expand and excess comes out the drain tube. Like said as long as you have fluid over the rows inside, you are fine.

If you were to check it hot, you would find the level is full again.

Did you possibly put ne of the seals in backwards and the shop just re assembled it the same way it came apart? Lip towards oil and lip towards the coolant?

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