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Discussion Starter · #1 ·
So, I started out by searching the forum and the internet and tried a few things but have not resolved the issue yet.

2006 GSX-R600, 22K miles

Starting with the Radiator full, cooling system bled, and coolant in reservoir at "MINIMUM" level.
Start the bike. Run it until it heats up. The coolant in the reservoir starts to rise slowly at about 180 deg F. When it reaches 220 deg F the fan turns on (which is also normal) and then the temperature will slowly drop back down below 200 deg F. Shut the bike down. Wait for the bike to cool down to room temperature.

At this point the reservoir is now at about the full line indicating that the coolant has not been sucked back into the radiator, like I had thought it should. If I ride hard or ride for a long time, eventually the reservoir will overflow and push coolant out the overflow line onto the ground.

Even after the bike is completely cool, when I opened the radiator cap I hear a pfft sound and the system spits some air and coolant droplets out indicating that the radiator was still pressurized.

Coolant not being returned from the reservoir is confirmed by seeing the coolant a couple of inches low inside the radiator. So I refill the radiator, go through the service manual procedure (including bleed screw on pump housing) to get the air out, and try again with the same results.

It appears air is being added to the cooling system somewhere. Enough air gets added in that as the system cools the pressure never drops back to normal atmospheric pressure (I let it sit for a few days after one run and the system still held pressure). If it was just an air bubble I would not expect the system to remain pressurized after cool down, especially with the coolant not being returned. But whatever mechanism is letting air in, does not allow air or liquid to escape over time.

It does not appear to be leaking or otherwise losing coolant. When I purge the air from the cooling system and then siphon coolant back out of the reservoir and add it back to the radiator, the levels end up about where they should be. Also, there is no sign of coolant in the oil (or vice versa).

The radiator cap seems fine, it holds pressure, but allows outflow, and the return spring and seal look and feel fine. The accumulation of pressure inside the cooling system seems to be preventing the return of fluid from the reservoir.

The air that comes out of the radiator does not smell like gas or exhaust, so I have been avoiding the headache of pulling the head to replace the head gasket.

Any other ideas? Is there any way to rule out the head gasket without pulling it?
 

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Good write up and description of what you've done and observed.

I was going towards a bad radiator cap, but the system being pressurized even after the bike is cool leads me to the unfortunate idea that the head gasket is leaking.

There was a thread about 6 months ago with the same issue (it had some good pictures also) as I remember and I also had a friend's track bike do the same thing. Both were the head gasket.

The only way I know to confirm a head gasket leak this small would be a one of the tools that detects combustion products in the coolant. I think you can rent the tool at auto parts stores and just buy the chemical.
 
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Here is that thread.

 
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Cap. There's 2 seals on it. One is the pressure, and the other is to close the chamber which is linked to the expansion tank. It's a bit strange it would boil over, and not leak at the cap though. If the expansion tank is doing it's job, then that's what should be drawn back into the system. The only other point for air to get back in... the cap. Is the radiator neck dinged up at all?
 

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@Chuckster did you catch that it stays pressurized after cooling down, even after a day.
 

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Discussion Starter · #6 ·
Thanks for the responses. I read through that other thread and it was good information, though I can't say it was what I wanted to read. I borrowed a radiator cap from a bike that is working well and it did not have any impact. The filler neck looks clean and I cannot find any deformation, either on the inner opening where the pressure relief seals, or on the outer rim where it enforces the path between the radiator and the reservoir. Also, with the bike at temp all the hoses going between the bike and the engine were hot to the touch. I also noted that when the bike was hot the hoses were firm and harder to squeeze, which I guess I would expect under the circumstances.
 

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The sign of a bad radiator cap is similar. The engine cools down and as the coolant contracts it should pull the fluid from the reservoir back into the engine, but due to a leaky top gasket it pulls in air also. The cycle repeats until you have lots of air in the cooling system and it starts running warm or overheats.

Your key observation is that it stays pressurized even after cooling down. And you had repeatable results.

If it were my bike, I would not be in a rush to pull the head either but would do a test on the coolant system.

It looks like the tool and fluid is under $30 and you might be able to borrow the tool and just buy the fluid from a parts store.

295912
 
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Discussion Starter · #8 ·
Thanks. I'll see what I can find in this area as far as the the combustion products test tool. The other thing I might try is to siphon off some of the coolant and pull the small hose from the upper left side of the radiator. Then cap, plug, or otherwise seal off the end of the hose and try to draw a vacuum on the radiator and verify that it pulls from the reservoir.
 

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Just a wild throw it out there guess. John set me straight if I am leading him astray... Could the issue be a leak in the hose from the canister to the reservour? That may break the suction on the tube and not allow fluid to be drawn back in. It doesn't explain the rad staying pressurized unless the air contracting in the rad is not strong enough to lift the spring where a fluid contracting would be. In that case you are hearing air sucking in not out.
 

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Discussion Starter · #10 ·
Just a wild throw it out there guess. John set me straight if I am leading him astray... Could the issue be a leak in the hose from the canister to the reservour? That may break the suction on the tube and not allow fluid to be drawn back in. It doesn't explain the rad staying pressurized unless the air contracting in the rad is not strong enough to lift the spring where a fluid contracting would be. In that case you are hearing air sucking in not out.
Thanks for the idea. I did verify that I could pull coolant out of the reservoir through the port inside the radiator filler neck, so I think the hose is good, but I couldn't do it with the cap on to test the cap upper seal.
 

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Staying pressurized after cool down is something I'd like to see. Even a working system will create a negative pressure when it cools (to pull back from the expansion tank). Just don't see how it's even possible.
 
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