Author Topic: Cooling System Pressure Test ( ST1100 ) *  (Read 3704 times)

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Cooling System Pressure Test ( ST1100 ) *
« on: March 27, 2009, 09:44:33 PM »
Cooling System Pressure Test

Original article on Mike Martin's web page here:

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. The system, by design, has the radiator 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 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 flow back into the radiator from the overflow bottle.

This explains how, if the overflow hose has a crack in it, air is allowed back into the radiator, reducing its cooling performance, and allowing overheating. An overflow bottle with excess coolant showing when the engine is cool is a symptom of this condition.

Now for the test itself. Not having a pressure tester in my possession, I rigged up a way to do it with a tire pump, a pressure gauge, and some hose, hose clamps, and fittings.

I disconnected the overflow line at the radiator filler neck, and connected a piece of hose connected to my tire pump. Into this line was inserted a tee connected to a pressure gauge. (The tire pump itself had a built-in gauge, but I found that the check valve in the pump leaked slightly.) I then pumped the system until the pressure gauge showed 15 psi (1.0 Bar). Then I clamped the hose from the pump with locking pliers.

I left the system in that condition overnight. In the morning the pressure hadn't changed, so I was assured that my system was leak-free! :-)

Please note that this test checks everything except the small check valve in the center of the radiator cap. If that valve doesn't seal properly, the system will not develop any pressure when the engine warms up. During a long period of operation, the bike may puke too much coolant into the reservoir and overheat. Conversely, if that valve is stuck closed, the system won't draw coolant back into the engine when it cools down. This second condition will exhibit collapsed radiator hoses when the bike is cool.

Last updated on February 24, 2009     2001, 2002 M. E. Martin, All rights reserved

Thank You again for your contribution Mike Martin
« Last Edit: January 02, 2013, 04:27:35 PM by Tom Melnik »