Yamah MT 125 overheating

[Texpis]

Hi all
Newby here both to the forum and biking, just passed my CBT and bought a 2017 Yamaha mt it has 40k miles on it.
It overheats (according to the gauge ) it seems a bit random when it does it but mainly on a long stretch in 4th or 5th.
I have changed the thermostat which would not open in boiling water but still the same. I’ve had the water pump out to see if it was worn all was fine the impeller was perfect, while I was there I back flushed the radiator, I have sprayed water through the radiator, I couldn’t find any no info on a bleed screw to bleed the system so I removed the temp sender to bleed it, but no difference.
So before I buy a new radiator any ideas.

Mick

[jaffa90]

Any other signs of overheating?

[jeffyjeff]

[stinkwheel]

Odd it's doing it on prolonged, higher speed runs. Normally a defective cooling system would cause more issues at low speeds or in standing traffic. Does it cool back down when you slow down again?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Texpis]

Hi thanks for the replies guys. There are no other signs of overheating, as soon as I slow down or stop the temp comes down. I’ll have a check of all the things you have suggested Jeff.

Mick

[that_impulse_guy]

you'll typically have two sensors.
One will be near the thermostat housing...its a resistor that changes its resistance based on heat. This is used for the temperature gauge...because its a smooth change, the gauge can move smoothly up and down (or the computer can display whatever number it wants on your display).


then, near the bottom of the radiator, the one you said you took out, would be another sensor. this is a binary thing...its opens/closed (Well, it grounds) a signal, and this turns the fan on or off.

I can offer two suggestions (well...three).
1) the sensor at the bottom of the radiator, you should be able to unplug the wire and tie it to ground somewhere, and the fan comes on. Ride around...do you still overheat? If no, that sensor was faulty.

2) sometimes...bikes need a bit of burping. Fill up all the coolant everywhere, go for a ride, at least enough to get it hot (20mins?)....come back, let it cool down....fill up coolant again.

3) buy one of these radiator caps, it should fit "most" bikes
https://www.ebay.co.uk/itm/115956298900

it gives you another gauge, in case the cluster gauge is lying to you. BTW, I linked you to a 0.9bar cap...yours may be different...buy the one your bike needs. What is also does it remove the possibility that your radiator cap is stuffed by replacing it with another one of extremely dubious and questionable quality. Your mileage may vary.
____________________
Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[stinkwheel]

[Texpis]

Cheers again guys I’ve run the bike on tick over until it got to temp (sits at middle temp for ever fan doesn’t come on)both sides radiator hoses were hot, so thermostat okay will try you’re other suggestions and get back

Mick

[Texpis]

Okay to finish this off after a lot of trying to diagnose it ends up an intermittent fault on the connector to the fans some how at speed the fault occurred stopping the fan, when stopping the bike the fan was on, cleaned and tightened the connector.
Rode for 2 hours temp rock steady, I am shocked at how much the fan is needed to regulate the temp I would have thought at hi speed the air through the radiator would be sufficient but obviously not.

Mick

[jeffyjeff]

[stinkwheel]

Agreed. I've had many water cooled motorbikes and the only time the fan ever comes on is either after prolonged, low speed riding in hot weather or after suddenly slowing right down or stopping after intense thrashing. The only time I've found the fan on while riding normally was when the head gasket blew on my GPZ500.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Texpis]

I would have thought the same hence the post but, brand new radiator with new cap, water pump removed and checked like new, new thermostat and temp sender checked in boiling water. Rode it for an hour and half last might different speeds temp stayed rock steady which is the first time since I got the bike
Mick

[jeffyjeff]

I don't mean to belabor the point, and beg pardon if I come across as unrealistically stubborn about this (not my intention); but ask you to think about this for a moment. Your engine has to reach approximately 100 degrees C before the fan will turn on, and it is not cooling down enough to turn the fan off (normally around 93 degrees C). In a well-functioning cooling system, the radiator can reject enough heat for the thermostat to control the engine temperature within a range of around 82-92 degrees C.

Once your thermostat opens fully, it sends all the coolant to the radiator without restriction and can do nothing more to cool down your engine. Your thermostat is doing nothing to control your engine temperature. It has opened fully and is now a spectator in your cooling system as the temperature continues to climb until the radiator fan turns on and stays on. You may not be overheating your engine, mate, but you are constantly running it at the very upper reaches of its design limits. Something is not right. If your bike was mine, I would be looking at it again. All the best.
____________________
History shows again and again how nature points out the folly of men - BOC