I just fixed a persistent Hall Effect sensor problem, so now my bike will run.

However after about 30 minutes, when it got hot, the temperature sensor lit up and hot coolant started leaking (sort of shooting) out of the thermostat housing. 

Would this mean that my thermostat doesn't work, or that the housing and o-ring aren't dialed in enough, or something else?

I replaced the thermostat recently.

More info:
1. it has reached freezing temps here lately in case that has any bearing
2. I rode the bike for 15 mins and then left it running in the driveway so it wasn't getting air blowing through the radiator. I didn't check if the fan was running.

timz10000 wrote:However after about 30 minutes, when it got hot, the temperature sensor lit up and hot coolant started leaking (sort of shooting) out of the thermostat housing.

By writing that "the temperature sensor lit up", are you referring to the coolant temperature warning light in the instrument cluster?

Which model of thermostat did you install? What is its temperature rating? Did you install a new thermostat gasket, too? By "dialed in" do you mean tightened enough. That should be easy to determine. The thermostat housing shouldn't leak. Did you check the condition of the radiator cap gaskets?

When coolant was added, were steps taken to purge the system of air? Was the level in the coolant reservoir between the Min and Max lines when the engine was cool and before the engine began idling?

The fan should turn on automatically to inhibit overheating. It will activate before the temperature warning light is lit and will then regulate engine temperature. If the fan didn't operate during this event then the common reasons involve a seized fan motor and/or a faulty coolant temperature senor or its wiring. In my experience, a couple of minutes pass between the moment the temperature warning light is lit and moment the coolant boils over. Did that amount of time lapse in this case or did the light and the boil-over happen together?

Laitch wrote:

timz10000 wrote:However after about 30 minutes, when it got hot, the temperature sensor lit up and hot coolant started leaking (sort of shooting) out of the thermostat housing.

By writing that "the temperature sensor lit up", are you referring to the coolant temperature warning light in the instrument cluster?

Which model of thermostat did you install? What is its temperature rating? Did you install a new thermostat gasket, too? By "dialed in" do you mean tightened enough. That should be easy to determine. The thermostat housing shouldn't leak. Did you check the condition of the radiator cap gaskets?

When coolant was added, were steps taken to purge the system of air? Was the level in the coolant reservoir between the Min and Max lines when the engine was cool and before the engine began idling?

The fan should turn on automatically to inhibit overheating. It will activate before the temperature warning light is lit and will then regulate engine temperature. If the fan didn't operate during this event then the common reasons involve a seized fan motor and/or a faulty coolant temperature senor or its wiring. In my experience, a couple of minutes pass between the moment the temperature warning light is lit and moment the coolant boils over. Did that amount of time lapse in this case or did the light and the boil-over happen together?

Some answers:
- The coolant reservoir was low - at or around minimum but might have been just below min

- When I filled with coolant, I tried to follow the best practices listed but it was my first time so I may not have purged air sufficiently

- "Temperature sensor lit up" - I meant the red warning light in instrument cluster

- The thermostat was the correct one ordered from Euro Motoelectrics . As for the thermostat gasket, if you mean the rubber ring that goes around the edge I replaced that 

- The housing (the plastic cap) seemed a little well-worn though it fit back on seemingly properly

- I ran the motor for about 30 mins, then it sounded like it was going to cut out, I revved the throttle and the light came on and I noticed the coolant leak at the same time.

The coolant leak means the cap was not fitted correctly or is damaged, dont use any type of sealant.
overheating like that usually means the fan is not working, a common problem. Does it overheat when riding on the open road, if not then that also points to a cactus fan.

On any motor the fan will always come on before the over heat warning light.

If the warning light is coming on and your fan has not come on then you have a fan issue. As said, fan may be seized or just not working.

When you replaced the Hall sensor, is there a possibility you disturbed the fan to loom connection when you were connecting it?  Was the fan working before you replaced the Hall sensor?

92KK 84WW Olaf wrote:On any motor the fan will always come on before the over heat warning light.

If the warning light is coming on and your fan has not come on then you have a fan issue. As said, fan may be seized or just not working.

When you replaced the Hall sensor, is there a possibility you disturbed the fan to loom connection when you were connecting it?  Was the fan working before you replaced the Hall sensor?

The fan was working before I replaced the Hall sensor - I'll refill with coolant and run it again tomorrow and see if the fan turns on (I didn't notice today). I also have access to a parts bike and can try the thermostat cover from that to see if it seats better.

(I don't really know if the bike tends to overheat when riding because the Hall sensor problem would make it cut out after 10 or 15 minutes)

Were there any mineral deposits in the thermostat housing or in the hose under the filler cap?  That would indicate that at some point in the past someone used tap water in the cooling system and that the radiator is coated internally.  This will prevent proper cooling. 

Mineral deposits can be removed by flushing with radiator flush and distilled water.  In severe cases, you might need to do the flush 2 or 3 times to completely remove all the junk coating everything.

Point-Seven-five wrote:Were there any mineral deposits in the thermostat housing or in the hose under the filler cap?  That would indicate that at some point in the past someone used tap water in the cooling system and that the radiator is coated internally.  This will prevent proper cooling. 

Mineral deposits can be removed by flushing with radiator flush and distilled water.  In severe cases, you might need to do the flush 2 or 3 times to completely remove all the junk coating everything.

Doesn't look like any deposits in the hose under the filler cap. 

I have an auxiliary fan switch and the fan turns on. I can't test whether it goes on when hot until I fix the thermostat cover (now it's leaking when I run the motor after it sort of blew out when hot yesterday).

I will fix the thermostat cover, but what could explain what happened. Could it be that the relay that sends the signal to turn on the fan didn't work?

I'm thinking trapped air in the coolant lines prevented proper coolant flow.  Without the flow, the coolant overheated, maybe sending sudden steam pressure through the system when you blipped the throttle (water pump is mechanical) which might have blown the seal in the thermostat housing.

Best practice starting from a drained system is to carefully fill the coolant system with a cold engine until the level reaches the filltube.  Then run the engine and kneed the lower right coolant hose to release trapped air, while carefully topping up the coolant.  When the system is to capacity, fill the overflow tank to the MAX mark.   Is this the process you followed?

robmack wrote:I'm thinking trapped air in the coolant lines prevented proper coolant flow.  Without the flow, the coolant overheated, maybe sending sudden steam pressure through the system when you blipped the throttle (water pump is mechanical) which might have blown the seal in the thermostat housing.

Best practice starting from a drained system is to carefully fill the coolant system with a cold engine until the level reaches the filltube.  Then run the engine and kneed the lower right coolant hose to release trapped air, while carefully topping up the coolant.  When the system is to capacity, fill the overflow tank to the MAX mark.   Is this the process you followed?

I didn't knead the lower right coolant hose, but did the rest - maybe I didn't get all the bubbles out

Try having the bike on the side stand when you're filling the coolant - gets the air bubbles out easier.

Basic2 wrote:Try having the bike on the side stand when you're filling the coolant - gets the air bubbles out easier.

OK cool - I replaced the thermostat cover, refilled the coolant the way you recommended, and found that the fan works (when activated by auxiliary switch) but doesn't automatically kick on.

That being the case my next steps will be tracing the wires between the fan relay and the fan to see if they look OK, and swapping out a spare fan relay I have. (The spare I have is from a bike sitting for 20 years - think that's apt to deteriorate by itself over time? It was in a barn)

Aha! - so I looked at the fan relay and the previous owner had disabled the fan motor lead

That brings me to two questions

1. for now is it OK to ride if I manually turn on the fan with my aux fan switch once the motor gets warm?

2. to fix this, hm... I guess I need to figure out how they wired the aux fan switch. Is it OK to just wire the A2 fan lead in series into the circuit with the override switch? (Then I'd see if my fan relay is working and replace it if not)

I'm a novice at electronics