I bought Honda CB 400 SF, version S, used motorcycle. When I ride for about 15 minutes, the radiator fan starts running. While I am driving my legs felt warm air from engine. I am wondering if the engine is overheating.

How can I tell if this is normal?

  • When riding, your legs are close to the engine. It is not uncommon to feel warmth from the engine as you ride. So, we don't have any way to know if what you are feeling is normal or not. Does the bike have a temperature gauge? When was the last time the cooling system was serviced? You will have to do some troubleshooting.
    – CharlieRB
    Jul 31, 2017 at 11:41
  • I did ride other bikes like cbr 150r, dt 230 I did not feel my legs warm early. I have changed coolant 2 week ago. But no temperature guage on 1998 cb 400 super four, there is a temperature alert light on dashboard, no warning. I have to check engine oil, I changed to 10W/ 40. I might have to change it to 20w/50.
    Jul 31, 2017 at 14:32
  • Do you know for certain it's the fan kicking in? It seems at speed if the thermostat opened up and started letting coolant flow through the radiator at a regular amount, you'd get the rush of warm air over your legs as well. This would be completely normal. Aug 1, 2017 at 0:31
  • As my riding experience of my previous bike Honda CBR 150R 2016, the radiator fun was not started till I had ridden it for 1 hour. After searching in google, I found how cooling system works, there is a limited temperature reaches, thermostat opens and hot water is passed from engine to radiator, radiator starts cooling coolant. But the coolant get hotter and hotter thermal fun switch detects that and switch on to run fan. I found something cbr150r is a single cylinder, heat producing is less than cb 400 sf 1998, cb 400sf is an old technology bike and uses carburetor, cbr 150r uses EFI, so I t
    Aug 1, 2017 at 5:40

2 Answers 2


A good test if, if you're not losing coolant or having run issues, you're probably not over heating.

My fan turns on after a bit of riding (~5-10 min), and that's one of the indications that it's at full run temp. Once my bikes fully warmed up , and I continue riding, the fan mostly stays on as far as I can tell.

If you were actually overheating then you'd probably start boiling coolant which would splash all over the place when riding.

Summary, I doubt it's overheating, but if you want you can probably get a bolt-on temp sensor or even take external reading with a laser sensor (which won't be as accurate).


I had a CB400SF 2011 Model and experienced a similar occurrence. Although it is possible that everything is working as intended, I would recommend checking your coolant system. Ensure there are no blockages, and there is enough coolant in the system, and that the coolant is still effective in moving heat. Check that all the valves are operating. It may be that the engine is heating up, switching on the fan, but the valves that pass the coolant through the extended loop are remaining closed / are blocked.

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