radiator sizzling?

2002 YZ 426 - completely stock.

I was out on a trail ride yesterday - 41 degrees, great weather. Did a 1st gear hillclimb a 2-3 times, didn't seem like a big deal - slow going, but not a terribly long climb. Then I parked the bike to help a buddy get up the hill & another rider heard my radiators sizzling and/or boiling inside.

I noticed a week ago some radiator fluid was dripping out the overflow hose after a slow ride through all the parked trailers out the desert.

Bike seems to run fine & the coolant didn't appear to be low at all (I topped it off & perhaps I filled it right up to the cap, so the dripping was just the bike correcting the coolant level itself).

Is the "sizzling" thing a common occurance with this bike, or should I have waterpump concerns ... other concerns? My other bikes are a KTM 640 Adventure & a Gas Gas 280 trials bike - both have fans, so I'm not sure what to expect as normal from a "fanless" bike...

You'll get some of that with the big YZFs. They'll blow coolant if you just let them sit and idle for more than 3 minutes. They really like to keep moving.

It might be worth your while, though, to have the pressure cap checked out

It's normal. Even though it's water cooled it needs air running through the radiators to get rid of heat.

All the big thumpers will do that if you lug up a big hill climb and then park it. Even my 620 which is geared so low its sick will act up on me if I come to a stop a the end of a hill climb. Your definitely better off to keep on trucking if at all possible, if not shut her down and expect a little hissing and steam. I always carry the biggest MULE water pack I can so I have a little water to spare for my ride.

apparantly there is a new waterless coolant out from evans (NPG i believe?) that has a boiling point of around 350 deg. i'm gonna try some as soon as i figure out where to get it,maybe zip-ty? has anyone used it yet?? :cry:

I used to have the same problem until I switched over to Engine Ice. Since then I have never seen it boiling over or heard it hissing. Great stuff. If you do a search you will see many people have done it with great results.

I used to have the same problem until I switched over to Engine Ice. Since then I have never seen it boiling over or heard it hissing. Great stuff. If you do a search you will see many people have done it with great results.

Good to know about the Engine Ice. I tried that with my 640 adventure, but it actually performed worse than regular antifreeze(!?) Everything I've heard about that stuff is great, but other 640 Adventure rider have reported similar stuff - that bike is just particular to regular antifreeze (I use the Motorex stuff).

The sizzling didn't bug me, but the other guy pointed it out - he's on a 250, so I assume the smaller bike doesn't get as hot. I'll probably flush the system & switch to Engine Ice when spring/summer rolls around.

I had a problem with my bike boiling over even at very cold temperatures that never seemed to be a problem before. One TT'er suggested that the rad cap may be releasing pressure too soon. I bought a couple of used rads for spares and tried the other cap and it was better but did not cure the problem.

After trying everything I could think of I decided to take a look with an Infrared camera I use at work. I set the camera up on a tripod and the bike on a stand and fired it up and let it idle. The IR camera is digital and provides a live view like a dig vid cam. I watched the engine start to warm and then could see the rads start to warm up. Then I see a scortching hot spot on the bottom of the right hand rad that was much hotter than the lines coming out of the engine.

Turns out the rad had been bent a bit too close to the header pipe from crashing and the rad was heating the coolant instead of cooling it. I took the rad off and straightened the brackets as best I could. You've gotta be careful because the brackets are surprisingly stiff and you can easily distort the core and cooling fins and get leaks.

For extra protection I put some reflective aluminum duct tape on the bottom reservoir of the rad to reflect header pipe heat away. This may sound a bit hokey and I few years ago I would have thought it to be a waste of time, but I have a Level 2 infrared certification now and heat isn't what I thought it was. The surface color and thermal reflectivity of an object plays a huge part in radiated heat transfer between 2 objects if there is no contact. For you non believers, try this simple test. Look in your kitchen to see if you have a frying pan with a shiny side and a dark side. Non stick pans sometimes have a black cooking surface and a shiny metal bottom. Heat the empty pan up on a stove burner until it would fry bacon. now carefully pick it up with one hand ( by the handle) and gradually move your free hand towards the black cooking surface. You can be several inches away and still feel the radiated heat coming fromthe surface. Now turn the pan over and very, very slowly and carefully start moving your free hand towards the shiny bottom of the pan, but be careful not to touch it. You will see that there's a big difference between the heat coming off the shiny side and the black side, even though it's the same metal and very thin.

Thermally shiny surfaces have trouble giving up or taking in radiated heat waves. Chroming oil pans and valve covers traps more heat in the engine. Most rads are painted black to help them radiate heat away, but it works both ways and the black will cause them to pick up if there is something hotter than the rad close by.

I rechecked the rad with the IR cam after the tape and bend job. I probably only gained 1/2 inch clearance from the pipe but the hot spot and my boiling over trouble dissappeared. Of course the bike will still boil over if you let it sit still running for a few minutes since there is no airflow over the rads, but my problem was much worse than that.

Check your radiator overflow hose to ensure that it hasn't melted itself shut by contacting the exhaust header.

Out on a trail ride a couple of weeks back, super hot day, 50 miles from nowhere - I nearly crapped myself when I heard sizzling after dropping my 426. Thought I'd busted the radiator, but turned out to be a melted overflow spilling coolant on the pipe. Phew!

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