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GT125
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GT125
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Hi all.. New here, but I can't seem to find any good suggestions for what to check next or if there is something I'm generally missing anywhere else online.

I've recently noticed that my GT125's temperature gauge creeps up fairly quickly into the red zone on every journey now after as little as 5 minutes.

The interesting thing with this is that the fan doesn't come on as I'd expect at any point, although I have tested the fan independently and it certainly works when connected directly to the battery.

The coolant has been recently changed using what I researched, flushed out and replaced with a recommended brand, and sitting at the max level nicely.

In an attempt to sort the issue too I've also replaced the temperature sensor (the one splitting the hose under the right hand radiator), and I've also changed out the themostat.

I'm assuming the water pump is working as expected, although I haven't fully tested this (as i'm not sure how without having to replace the coolant again), but the radiators are hot, which I'm assuming was a sign that the coolant was being pumped around somehow!

I also replaced the fusebox and all fuses in the glovebox, just to clean the thing up really as I'd noticed they were fairly corroded.

So.... would anybody have any idea as to something I could check next? Is there a relay i'm potentially missing somewhere in the circuit that might be giving me some hidden grief? Is there an easy way to check the water pump is functioning correctly without pulling it apart? or would the fan never coming on suggest it's maybe not overheating at all, and it could just be the gauge on the dashboard giving me the runaround?

Any thoughts, experience or ideas would be very much welcome!
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GT125
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GT125
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Just thought i'd continue my research on here and have a look again at the workshop manual for the GT200... Spotted that there is actually another temperature sensor potentially sat right next to the themostat.

Surely this might be the next replacement to try? Does anybody have any experience of these sensors going tits up? and what exactly is the difference between this sensor and the one sat under the foot panel in the right side radiator hose?

Any experience would be welcome! Thank you all!
Sensor location
Sensor location
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Hooked
MP3 500 HPE 2019
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Thermostat Fan Switch highly probable

How to test the Vespa Fan?
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The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
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The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
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I'd be changing the temperature sensor 82622R (right beside the intake manifold under the seat) as well before getting worried about head gaskets etc.

Page 29 of the parts list:
https://manuals.wotmeworry.org.uk/Vespa/GTS125/
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The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
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khakistocracy wrote:
Just thought i'd continue my research on here and have a look again at the workshop manual for the GT200... Spotted that there is actually another temperature sensor potentially sat right next to the themostat.

Surely this might be the next replacement to try? Does anybody have any experience of these sensors going tits up? and what exactly is the difference between this sensor and the one sat under the foot panel in the right side radiator hose?

Any experience would be welcome! Thank you all!
The one by the radiators is just a thermal switch to switch on the fan. The one on the cylinder head is a thermistor - a resistor that changes resistance with temperature. This is what drives the temperature gauge.
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GT125
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GT125
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Thank you for the replies.. I see the difference between the sensor and the switch now—and i'm just hoping that if I change out this sensor I might find out it's not overheating at all—and i'm guessing that the fact the fan never comes on is a sign it probably isn't (?).

Let's hope this works, otherwise I'll be back genuinely wondering why it's getting too hot too quick!
Replacement sensor
Replacement sensor
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MP3 500 HPE 2019
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That is my thoughts and certainly others here, hope it works ok.
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2018 GTS Super
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khakistocracy wrote:
The coolant has been recently changed using what I researched, flushed out and replaced with a recommended brand, and sitting at the max level nicely.
Issue started before or after coolant change? If after it's possible you have air in the system.
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GT125
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GT125
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That is a good question, as yes, it probably did start after i'd replaced the coolant. I'd forgotten about that little detail as I'd read that these are actually self-bleeding—is that actually the case? Sounds too good to be true!

Is there a way to bleed these properly then and make sure? I've got a feeling this probably is part or all of the issue now I think about it!
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Air in coolant or not, the fan should kick in, so this tells you that the sensor you ordered is probably the culprit.
khakistocracy wrote:
That is a good question, as yes, it probably did start after i'd replaced the coolant. I'd forgotten about that little detail as I'd read that these are actually self-bleeding—is that actually the case? Sounds too good to be true!

Is there a way to bleed these properly then and make sure? I've got a feeling this probably is part or all of the issue now I think about it!
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Vespa GT 125 L 2003
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Vespa GT 125 L 2003
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Yes it should be self bleeding.

Have you checked for coolant leaks at the elbow pipe near the thermostat? I had it on my gt125.

It only happened when I was riding because of the pressure and the heat made most of the coolant evaporate.
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P200E DN 1982
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all in all a ´great´ illustration of how munch more can and will go wrong with liquid cooling vs air cooled
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GT125
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GT125
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Back again with this after now changing out the temperature sensor that finally arrived. That now pretty much feels I've changed everything that could make a difference apart from the water pump..

After a bit of a test ride the temperature gauge still creeps up towards the red within about 5 minutes. Although on a decent blast on an open road with the wind whistling through the radiator it can slightly creep back down and hover around the 3/4 mark.. still no sign of the fan coming on at any point though, which still seems odd.

So, does anybody have any more ideas to help me with this? Is there a way to try to make sure there's no air stuck in this 'self-bleeding' system? Otherwise do I need to start looking at anything more sinister?
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