Author Topic: High Coolant Temp but Plenty of Coolant in Overflow Bottle ( ST1100 ) *  (Read 7692 times)

Offline KoTAOW

  • Keeper of the Archive of Wisdom
  • Moderator
  • STrider
  • *****
  • Posts: 465
Original article can be viewed here:

Submitted by Mike Martin.


High Coolant Temperature, but Plenty of Coolant in the Overflow Bottle

The problem addressed here is the one where there is no coolant leaking from the system (other than possibly at the small open-end hose attached to the overflow bottle), as detected by drips under the bike or an odor of antifreeze. The cooling fan is also operating when the temperature gauge needle reaches a point about 3/4ths of the gauge maximum.

First of all, it's necessary to determine whether the overflow tube or the radiator cap is the culprit. First an explanation of the cooling system on the ST. The radiator cap has a two-way function controlling pressure in the system. There is a relatively large spring holding a plunger with a rubber seal that will serve to release pressure from the system when it reaches the cap pressure rating. This is 16 to 20 psi (1.1 to 1.4 Bar), according to my Haynes manual. By design, the radiator is completely full of coolant at all times. When the engine warms up, this coolant will expand, generating a positive pressure. When this pressure reaches the cap setting, coolant is allowed to pass the cap and flow to the overflow bottle. Note that this occurs every time the engine is warmed up, not just those times when the system is "overheated".

When the engine is turned off and the system cools down, the coolant contracts. This produces a negative pressure in the system. A second check valve in the center of the cap then opens and allows coolant to syphon back into the radiator from the overflow bottle.

Perform a test beginning with the radiator filled and the proper reservoir level. (The system must be cold when you remove the radiator cap to check the coolant level.) Start the bike and let it run until the fan kicks on. When this occurs, the temperature should drop to mid-gauge position (if the coolant thermostat is not stuck closed) and the fan shut off. As the bike warms up and the coolant expands, some coolant will be expelled into the overflow reservoir. That's why there are two marks on the reservoir, a lower one for cool engine, an upper mark for normal operating temp.

If the radiator cap is functioning properly, no more coolant will be expelled. In this case, continued engine running will not cause the system to overheat. If the overflow hose is OK, when the bike is shut down and cools off, that coolant will be returned into the radiator. If the hose has a leak, the coolant won't return. This leak doesn't have to be large enough to exhibit coolant leakage. Just a tiny hole can allow air into the hose to break the syphon effect which is necessary to return coolant from the reservoir. Each subsequent cool-down and warm-up cycle will raise the coolant level in the reservoir even more. Eventually there will be insufficient coolant in the radiator and the fan sensor won't be in contact with coolant. Then the fan will not be triggered on, even if the engine temperature is sky-high. BAD NEWS!

If the radiator cap is faulty, the coolant will continue to be expelled once the bike gets hot. This is because there isn't sufficient pressure in the cooling system. A properly functioning pressure cap will raise the boiling temperature of 50/50 coolant from 223 degrees F. to 271 degrees F. The fan will kick on when the sensor in the radiator gets to 212 degrees F. However, local temperatures inside the engine are somewhat higher. If these local temps are higher than the boiling point, vapor bubbles will form and force more coolant out into the reservoir. This will continue to happen and the reservoir will get more and more overflow, and the radiator will have insufficient coolant.

In summary, a faulty overflow will allow the reservoir coolant level to rise an increment each hot cycle, becoming over-full over a number of hot cycles. A faulty radiator cap will allow the reservoir coolant level become over-full in one long hot cycle.

It's obvious that a faulty radiator cap must be replaced. But there is a fix for the faulty radiator overflow tube which connects the thermostat housing to the overflow bottle. It is most common for this hose to crack near its upper end.

This connection point is accessed by removing the right fairing pocket. You'll first have to remove the seat, saddle bags, both side covers, the top shelter, right-hand maintenance cover, the locking fairing pocket cover and the right hand fairing pocket. When you remove the screws which secure the right fairing pocket, notice that one of them -- the one on the inboard side nearest to the steering head -- has a pointed tip and different threads than the others.

Examine the hose where it connects to the radiator for cracking. If it is cracked there, you may be able to cut off a short section and re-attach the hose.

Another possibility is that the dainty wire clamp isn't strong enough to seal the hose to the radiator connection. A small worm-drive fuel line clamp will correct this problem.

2001 M. E. Martin, All rights reserved
« Last Edit: April 10, 2016, 03:29:38 PM by KoTAOW »