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My '01 ZX9R suddenly overheated after a ride a couple of weeks ago while I was letting it idle to burn up the gas in the carbs (a habit I've developed whenever the bike is stored for more than a couple of weeks) before putting it away. The temp soared to 239 F rather quickly and the fan never kicked on. The overflow tank puked on my driveway, so I know the temperature sender is working.
Step 1. I drained the coolant, replaced the fan switch which worked intermittently in testing, completely cleaned the radiator (it had a lot of crud stuck in it on the outside after 89,000 miles) and filled with new coolant. Still overheated.
Step 2. Drained the coolant and removed and tested the thermostat, it passed the test. Put coolant back in, and of course it still overheats.
Step 3. I put pressure tester on the radiator and pressurized it to 20 PSI. It dropped 3 PSI in an hour and a half with no signs of external leaking. I pulled the spark plugs and cranked it over. I am 99% sure I saw mist come out of the #3 hole. I re-pressurized it and I'm giving it another hour and a half too see if it does it again.
I assume it has a leak in the #3 hole that's letting combustion gases into the coolant causing the overheating; and I figure it needs a head gasket or a new head, but just in case, I thought I'd ask if anyone else has other ideas.
NOTE: I'm due for a new head (or engine depending on the condition of the cylinders). Some of my valves are on the smallest shims available and it's cheaper to throw a used head ($150) on there than it is to buy 16 new valves (at least $200). The timing could not be much better either since it's still Winter. If the engine is shot, I'll have a bigger decision to make...