98 YZ400 boiling over in tight trails

I konw its been talked about before but my YZ400 is boiling over in a pretty fierce way in tight trails.

Background: I picked up a 98 YZ400 with a 420 kit in it from a friend of mine who has had it sitting for the past 3 years. My intention is to use it as a dualsport bikefor desert and trails and during the time of year when I cant ride my 04 CRF450 in OHV areas. I'm running a WB E-series full ststem with 6 discs in it. I did the grey wire mod and it starts and runs real well.

Problem: I've had it out twice now and the first time I had no problems pretty open trails hot weather and around 5000 ft. (Hungry Valley SVRA). Second time out was at Coral Canyon OHV area, Hot weather, 3000ft and very tight very rocky kick your :thumbsup::awww: type stuff. Sometimes you have to stop and wait for somebody to clean a section so you can get a good run at it. I boiled it over like 4 times to where it was raw steam coming out of the overflow. Is this common with the YZ400 when ridden like this? I was a little scared because I don't want to waste the motor and if I can't trust it in that kind of terrain I will sell it and get a bike that can cope.

How much will something like Engine Ice or Water Wetter help and is there anything else that might help?

Sorry this ended up being long and thanks in advance for any guindance.

id definately flush the coolant system and take the radiators off and make sure the fins are cleaned out... i dont think it should be overheating as long as you are moving reasonably.. id have to sit and idle for several minutes before mine would start

Your screwed. About the only thing that you can do is install a wr overflow bottle. Sure you can also install the oversize Wr radiators but that is major cash.

Water wetter aint gonna do it. Sure it helps. But not enough. As does engine ice. But it will still overheat. The yz400's need to move to stay cool. I have fought the same battle as you for years. My solution....I bought a CRF250X. No more overheating.

Take BOTH radiator halfs and uninstall them. Bring them to a radiator shop to get pressure tested. Check your water pump by removing the cover and doing a visual. Make sure you don't have a clogged radiator/cooling system hose anywhere on the bike. Lastly ensure your exhaust header is not heating up the bottom section of your right side radiator half as it does come in close proximity to the radiator. My stock header was REAL close to my radiator on my '03YZ450F. It caused the radiator to boil over occasionally, and the only solution was to install a different header. I choose WB Carbon Pro exhaust and the WB header had MUCH more clearance coming out of the engine block. I had about 3/16" with the OEM header and the WB header gave me about 1" of clearance.

If I were you I would take Chas YZF's opinion. This happened to me also. (99 YZ400f) We were riding the same trails as you described when overheating became apparent. I checked everything and all was in working order. The only thing I did was try the Engine Ice. My problem was solved immediatly. But like Chas said....check the pump, the header distance from the right rad. You may even want to put some heat tape on the bottom of the rad. Flush and fill with Engine Ice and make sure your water pump is working properly. Should be able to watch it run, once it warms up, with the rad. cap off. And replace the cap if it needs it.

I havn't had a boil over since and I have rode tighter trails also.

Keep in mind that not only do the 400's do this, but so do many others when riden on tight trails as you described. Many bikes NEED the air flowing over the rad. fins to keep them cool or they boil over after so long. A lot of guys run a heavy fan in front of the cooling fins when they need to have the bike running, but stationary, for testing of various things.

Good Luck :thumbsup:

Ya know nothing I tried on my 400 would keep it from boiling over when the trail gets super technical.

The cooling system is all working perfect. Its just that the bike is not meant to crawl. It needs to move. Some bikes are more forgiving in slow speed conditions than others.

My personal opinion is that all that displacment makes a lot of heat. The radiators arn't that big on the 400's. I have tried all kind of stuff to keep my bike cool. But in the end they were all bandaid fix's and didn't do a thing.

It also depends on the trails you ride. Some can get away with just engine ice and some of the other fix's. Others cant. I know mine would overheat in the really tight trails no matter what I did. If I can keep the speed over at least say 10mph it was happy. But if it droppes much below that for too long its a steam bath. Clutch abuse just seems to agrivate the situation.

Now with my new 250X I have no problems. Its smaller displacment dosn't heat up as bad. And the cooling system is designed for the slow going of offroad ridding.

Just my 2cents for what its worth.

Another solution may be....GET OFF THE TRAILS AND OPEN HER UP! :awww:


I feel you man! I have a 2000 426 and ran plain coolant when i first got it. i was riding some tighter trails and spurts of steam kept coming out of the overflow, not as bad as yours sounds, but scary. i put water wetter in the coolant at 50/50 and have never had the problem again, in any riding condition. I am a water wetter beleiver!

-Paul :thumbsup:

Trying to follow the dust trail from yzman usually helps to keep my 426 cooler :worthy:If mine starts to heat up i try to bounce between single track and the more open to cooler it down like fli-nbri suggested(if you have that option)

these are a few things that helped mine and i did them all at the same time. (1) took off my radiator crash guards and replaced the stock air deflectors in front of the radiators (2) drained and flushed the regular antifreeze and put in watter wetter and glycol water mix. It really helped it out but it still boiled over.

dont flame me for this but these are some other ideas that i have done or am going to do. (1) diffrent radiator cap with a higher pressure rating (2) overflow bottle (3) on the radiator shrouds, the portion behind the radiators i took a die grinder with a cutoff wheel and opened up the "holes" where the air comes out of the radiators to help air flow thru the radiators. :thumbsup::awww:

BTW the 450 exhaust cam is the best money i have ever spent :thumbsup:

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