I've noticed that when the bike is heating up coolant goes from where the radiator cap is into the overflow container, however, when the thermostat opens no water gets pulled from the overflow container back into the cooling system, and the bike slowly heats up until the overheat light comes on. The hose from the radiator cap to the overflow is fine, I checked it for leaks, so I suppose the radiator cap could be leaking air, although air bubbles bubble up in the overflow tank indicating air isn't leaving through the radiator cap. What could be causing the cooling system to lose its prime and prevent it from sucking coolant back into the radiator?

Its actually supposed to do that! The radiator cap has an overpressure and underpressure valve which allows the cooling fluid to flow into the container when the bike warms up and it will draw fluid into the radiator when it cools down again.
When the bike warms up, does the fan come on before the temp light comes on?
Air bubbles in the overflow tank/container means that there's air in the radiator that is collected in the top and is pushed out when the bike warms up. This will be pulled in again when the bike cools down. Are you certain the hose isn't porous?
Do you see waterlevel when you open the radiator cap?

Thank you for the response. I know how the cooling system is supposed to work, what I'm saying is that the radiator isn't pulling the coolant back out of the overflow container, it only sends it out as the water expands. This leaves me with an overfilled overflow container and an empty radiator. In response to your other question, yes the fan comes on well before the temp light. I am pretty sure there are no holes in the hose - I filled it with soapy water, plugged one end with a golf tee, submerged it underwater, and blew on the unplugged end to see if I made any bubbles in the water (there is probably a better way... ) and I didn't see any so I assume it's fine.

Then air is being sucked into the radiator as the engine cools, rather than the water being drawn back up from the expansion tank. You may have a leaking head gasket which is why air is being seen in the radiator and more water than normal is being pushed out into the expansion tank (water/air mix), so when the engine cools it doesn't draw back the full volume so over time the radiator level is dropping. My Nissan engine exhibited these exact symptoms and the head gasket was the problem. Some of the coolant ultimately ends up in a combustion chamber which accounts for the loss. Does the engine miss for a short time on start-up ?

I was kinda afraid of this possible result... I saw semi-continuous bubbling in the overflow after I put the coolant back in and was worried the bubbles were air leaking from the compression. The bike doesn't really have any problems starting up, but does the air leak necessarily have to be from the head gasket or could it be somewhere else? The bike is pretty consistently flipping on the overheat light after a set amount of time running, but the odd thing is then it will shut off for a bit, sometimes after I jiggle the throttle a little on and off, and then come back on, only to go off again later. Could a blown head gasket explain these symptoms?

The only place where the water could leak would be the water pump, but I don't think that if could suck air from this area.
Any water mixed with your oil?
Any water leak from the small hole on the front bottom of the engine (just on the back of the oil/water pump)?

I would tend to agree about a problem with your head gasket.

Hmmm, lost that last reply, try again. Your cooling system runs at around 0.9 Bar pressure with hot engine. Any air/gas finding its way into the cooling system needs to be above that pressure to do so. Here is a trick to confirm the problem - beg, borrow or steal a gas monitor which is used in industries to monitor the environment in confined spaces. Position the gas monitor over the coolant expansion tank filler hole and allow the engine to come up to temperature. The monitor will alarm when it senses carbon monoxide (in the combustion gas) and this will confirm the head gasket leaking. Hope this helps.

Good idea Dennis, I don't know if I'll be able to commandeer one of those gas monitors but I can try. I haven't noticed any water leaking from the water pump so I don't think it's leaking. One thing I wondered was whether or not the water pump might not be circulating and if this could cause the overheat and no circulation, and hence no suction.Would you guys recommend inspecting the spark plugs or anything else at the same time I inspect the head?

3 things -

1. Make sure you put the gas monitor in a plastic bag and seal the bag opening to the tank opening. This is to prevent the monitor detacting Co from the exhaust pipe (they are extremely sensitive).

2. Inspecting the spark plugs should show 1 to be cleaner/whiter/lighter than the others. This will be the cylinder where some coolant may be getting into when the engine is cold/cooling.

3. Water pump - you should be able to hear a grinding noise if the impeller has come off the shaft, this is not unheard of, and easy to check with coolant drained.

If you do find it to be the head gasket leaking, inspect the cylinder head very carefully at the leakage site. Any flaws in the aluminium means the head needs to be machined to ensure a godd sealing surface. I would also get the head pressure tested while it's off the block.

Give it a leak down test. Can you pressurise each combustion chamber one at a time. If so when you do that you should see bubbles comming from the overflow bottle when the one leaking is tested.
I have a feeling that you have more than one problem and the overheating has damaged the head gasket. Is the thermostat OK, has it been tested.

Does anyone have any idea why the temperature light would go off and come back on, including when riding at freeway speeds, while the temperature gauge coming from the bottom of the water pump will stay at the same spot? Something seems fishy about it.

Also what is the worst that could happen if I take my thermostat out and ride the bike on my normal commute if the head gasket actually is blown..?

FreebasserSF wrote:Does anyone have any idea why the temperature light would go off and come back on, including when riding at freeway speeds, while the temperature gauge coming from the bottom of the water pump will stay at the same spot? Something seems fishy about it.

1. If the temp. management relay isn`t properly grounded, it would cause the fan and warning light to start prematurely.

2. You ain`t got proper coolant circulation due to less pressure...or.....air lock...or...malfunction pump.

Does anyone have any idea of what the worst that could happen would be if I took out my thermostat while I rode around with a blown head gasket? The bike might not get up to temperature but besides bad gas mileage what could happen?

You might make the leak worse and when stopped fill a cyl with coolant and hydraulic cylinder bending a conrod or maybe even blowing the cylinder wall out.
I can give you lots of horror stories.
I believe that if it aint broke dont fix it but if it is broke then fix it now because it will just lead to more expense if ignored.

FreebasserSF wrote:Does anyone have any idea why the temperature light would go off and come back on, including when riding at freeway speeds, while the temperature gauge coming from the bottom of the water pump will stay at the same spot? Something seems fishy about it.

I would say that more than likely you have an intermittent earthing problem with the wire from the sensor behind the radiator to the ECU which is a violet with a green trace and goes to terminal 10 on the ECU plug. Inge told me to do it or else It actually goes to the E terminal of the Temperature sensing unit. (sorry about that)
Connect a meter to the wire and to earth set it to 20 ohms scale, give the wrie harness a wriggling from one end to the other especially near the steering head, you may see the meter flicker meaning a short or that contact with earth is there. Also check for chafing of the harness along its length. This may or may not reveal a problem but is certainly worth checking.

Yeah I did some additional reading and I suppose I really shouldn't ride with water in my cylinders if that is what's happening. I guess I should go buy a cylinder compression tester for the 20 bucks they want at harbor freight. Do you guys think this is the best way to diagnose whether the head gasket really is blown? Many other signs indicate it but is there another reason a cylinder might have low compression besides a leak in the head gasket? Thanks for the help Kfreak/Rick G.

If you can try a leakdown as that is very definitave where a compression gauge won't give you a good guide as to what is wrong.
With a leakdown you usually hear the air leaking through the valve or past rings or see bubbles in the water but remember to remove the rad filler before you do it as a radiator will only take about 30psi tops.

Rick G wrote:I would say that more than likely you have an intermittent earthing problem with the wire from the sensor behind the radiator to the ECU which is a violet with a green trace and goes to terminal 10 on the ECU plug.

I do agree about the wire to be checked and its colour code, but this wire ends up at terminal E at the heat management relay.

Bad ground connection at the relay itself would also give similar symptoms.

Yes Inge you are right as usual I shall ammend that Put it down to a DOH moment

I've never heard of a leakdown... do I leave in all spark plugs except one at a time and hook some sort of pressurized air delivery system like an air compressor into where the spark plug goes? Wouldn't the piston have to be in between the intake and exhaust stroke so air wouldn't go through the valves?

Sorry I tend to forget that everybody doesn't know about these things.
A leakdown is when you put the piston on TDC compression connect compressed air to the combustion chamber and put for example 100psi then turn the source of air off and see how long it takes to leak down to zero. It is expressed in a percentage terms. A new engine will give a 3% which means it looses 3% of pressure in 1 minute but most used but good engines will show 10 to 15 % and be perfectly OK
You need a way to put the compressed air to the plug hole and listen for leaks or see bubbles in the coolant, you don't need to do the full test.
Making an adapter can be as simple as using an old spark plug with the ceramic centre broken out and an airline fitting welded to it.

I understand everything you said but have one question: How would you put compressed air into the cylinder AND measure the pressure? Would the air fitting need a pressure gauge between it and the incoming air? Here's a schematic of what I mean:

AIR ->-> PRESSURE GAUGE ->-> AIR FITTING/OLD SPARK PLUG ->-> CYLINDER

Usually you would need a pressure gauge but in your case the main thing is watching for bubbles in the coolant.
You would need a tap to turn the air off to watch the fall in pressure as well.

Have you tried running the motor with the radiator cap off, and the coolant topped up so that you can see it? Any cylinder head gasket leaks into the coolant passages will manifest itself as bubbles or disturbance on the surface of the coolant in the radiator filler orifice.

Rickmeister.

But wouldn't there be bubbles in the top of the radiator filler orifice if I just filled up the radiator from empty? I thought there was due to dissolved air bubbles in the cooling system...

One other thing I noticed I hope someone can answer: At the radiator stub where the temperature sensor that goes to the relay box is, on the actual thermistor I noticed there is no copper washer that I have heard people talk about in other posts. It doesn't leak so the seal is fine, but could this potentially cause some sort of different signal to the temperature relay.

One final thing I was just wondering if someone could do: I don't have an infared thermometer but when I put my hand to the valve and camshaft covers they are too hot to touch, and their temp measures around 190 F (88 C). Does anyone know if this is normal?

The bubbles will keep comming if the compression is leaking and 88 deg is exactly the correct running temp. Charlie had a problem with the copper washer and had to clean the area very carefully to get a good reading on the sensor.

Thanks for being so helpful Rick. What I meant to say is that I don't even have a copper washer there. Will that going to give a false reading?

Not having it will not in itself be the problem but someone has been remiss when working on it and making it right is just another making sure that is not the problem. It aint called elictrickery for nuthin. Try to get an aluminum washer if possible as there is far less chance of galvanic corosion from dissimilar metals.

Ulrich_Grabau wrote:Have you tried running the motor with the radiator cap off, and the coolant topped up so that you can see it? Any cylinder head gasket leaks into the coolant passages will manifest itself as bubbles or disturbance on the surface of the coolant in the radiator filler orifice.

Rickmeister.

By the way I did do this, and there was some bubbling but it went away after awhile I think.

Ulrich_Grabau wrote:Have you tried running the motor with the radiator cap off, and the coolant topped up so that you can see it? Any cylinder head gasket leaks into the coolant passages will manifest itself as bubbles or disturbance on the surface of the coolant in the radiator filler orifice.

Rickmeister.

By the way I did do this, and there was some bubbling but it went away after awhile I think.

Inge K. wrote:
1. If the temp. management relay isn`t properly grounded, it would cause the fan and warning light to start prematurely.

2. You ain`t got proper coolant circulation due to less pressure...or.....air lock...or...malfunction pump.

I read from a Crazy Frog post that the temperature sensing unit only activates after the bike has reached 100 degrees C. So if the bike reaches this temperature and is not properly grounded it might turn on the light right at this point, even if this is below the temperature it is supposed to be at, correct?


One other thing I thought about: What if the spring valve in the cap of the radiator filler stem is stuck closed? Couldn't this prevent the system from pulling in water from the overflow tank?

Hi there, have you checked the radiator cap gaskets? your symptoms may be caused by the cap not keeping pressure.
Best regards
JAP

I only really checked the gasket visually. It looked okay and not very worn but it's hard to tell if it is right where it needs to be when the cap is closed. I'm considering buying a new radiator filler cap just to be sure.

FreebasserSF wrote:I only really checked the gasket visually. It looked okay and not very worn but it's hard to tell if it is right where it needs to be when the cap is closed. I'm considering buying a new radiator filler cap just to be sure.

Hi, you are talking about one gasket, there are two of them in the cap. Not counting pressure valve which are not seen.
Hope this helps
Best regards
JAP