Cooling system question

[scooterollie]

Got something about the design of the cooling system on the CFMoto 250 scoots that has been stumping me for the last 10 months. I believe all the various scoots with this engine have basically the same cooling system setup - radiator, hoses, fill neck with pressure cap and overflow bottle. At first I thought the overflow bottle was actually a coolant recovery tank and that as the coolant heated up, it expanded into the tank. After shutdown, as the coolant cooled, it would be "sucked" back from the tank into the system. Mine has never sucked the coolant back into the system after it cools. If I follow the instructions (as per Baron Coolant Bulletin) for properly filling the system, after running the engine to full temp., the level of coolant in the tank goes up and stays up. When I remove the radiator cap, the level is down in the system by the extra amount in the overflow tank.

I have tried refilling the system several times with the same result. Stan Jessup and I had a conversation about this just after we both got our 250s. He thought the tank was just an overflow tank so coolant was not discharged into the environment and I thought it was a recovery tank as found in cars. However, if you really think about it, the coolant could never return to the system because the "vent" controlled by the radiator cap closes as the pressure in the system drops upon cooling. I initially replaced the radiator cap with a U.S.A. made "Stant" brand and the filler neck with one from Honda - no difference. When you remove the cap when the engine has cooled, there is a sound - not from excess pressure escaping, but from air being sucked into the system because of a vacuum! The pressure cap would have to be on the "recovery tank", not elsewhere in the system, for it to work like a car.

Excuse the long rant but it has always bothered me that the system would not stay full. The cooling system seems to function properly. I have one of the rare 250s with a temp. gauge that is calibrated properly - never goes above about the mid point, then drops back to about 1/3 after the fan cycles. I guess as long as the system "fills up" when the coolant heats up and expands, all is OK. You all probably figured this out long ago or never thought about as long as things were OK.

[WarrenS]

I think your assumption is correct that the expansion tank contains the overflow and it gets drawn back during cool down. My Helix worked that way and as far as I can tell the Burgman does too. I am not quite sure about the Morphous. Does the overflow tank have the hose from the radiator neck connected at the bottom of the tank? If it enters at the top it would have to have a tube going to the bottom of the tank. How about a fiber gasket in the pressure cap to seal the neck above the overflow? The overflow tank usually has a high and low mark on it.

[scooterollie]

warstein;
Thanks for the reply. Yes, I have even pulled that tube out, checking on it for the very reason you stated. All seems normal there, too! I would agree on the car-like function except for the fact that as the pressure in the system drops on cool down, the spring loaded seal in the cap won't allow any liquid to be sucked back into the system.

[Jacine]

I'll check mine and let you know. I use dexon gm coolant and it works fine.

[WarrenS]

The radiator cap has a metal disc in the middle. This acts as a one way valve allowing flow back into the radiator. There should be no vacuum when you open the radiator. That is the strange part. Your radiator hoses would collapse if this was the case.

[WarrenS]

I just had another thought. When you put the cap on it goes so far. This is a safety position to let off pressure when removing it when it is hot. You have to push down and turn further to compress the spring slightly. This compression takes the pressure off the center disc. I hope this is the answer.

[scooterollie]

Warstein;
AFAIK, the cap is fully seated but I will check again. I am aware of the two-position setting. Hoses do not collapse. Don't think they would unless the vacuum was excessive. I thought the spring mechanism in the cap operated in the reverse order, to release excess pressure from the cooling system into the overflow tank when the coolant gets hot, expands and pressure in the system builds up above the rating for the cap.

Jacine;
Thanks.

[WarrenS]

The spring is pushing on the rubber disc. When the designated pressure is reached it releases the overflow. There is no direct spring pressure on the center disc. When fully seated this releases pressure on the center disc allowing the reverse flow. I may have been overstating the case about the hoses collapsing.

[scooterollie]

Warstein;
Thanks, again. Interesting how the cap is supposed to work to allow flow back into the cooling system.

Quadruple checked everything and all is closed, etc as it should be. Even went through the fill procedure again, for the umteenth time. Did not get to run it to full temp. this afternoon.

[Jacine]

Mine returns normally. I just fill the bottle to the line and the radiator remains full.

[scooterollie]

Jacine;
Thanks.

I dug out my original Chinese filler neck and radiator cap to check them out. As Warstein said, there is a small spring loaded disc in the middle of the cap bottom that has a very light spring holding it down. As the coolant contracts and begins to create a vacuum in the system, that spring allows the center of the cap to open and the fluid from the overflow tank should be sucked back up and into the system. Didn't know about that!!! I was able to simulate the condition with my old parts. The spring on my replacement cap is even "softer" than the one on the original and there should not be a problem pulling the coolant back from the tank. After re-filling the system, I rode the scoot this evening. Cold, the level was up to the bottom line. After a several mile run, the fluid was only half way between the lines. Will see what happens after cool down. Something has been preventing the fluid from being pulled back. There was definitely a vacuum in the cold system when removing the cap. In the past I pulled the tube from the recovery tank and it seemed OK. Even cut the end on an angle to make certain it was not sticking to the bottom of the tank as the vacuum started to draw.

[scooterollie]

After watching the level in the recovery tank for several days, it is evident that something funny is going on.

After properly filling the system, each time I run the scoot, the level in the overflow tank climbs some until it is somewhere above the high line. It them remains stable at that point. When removing the radiator cap (cold) there is a sucking sound indicating a partial vacuum in the system. The system is low by the amount of excess in the recovery tank. Either there is a vacuum leak in the cooling system (as it cools) or a problem with the hose from the radiator neck to the recovery tank.

Next move is to try a new piece of hose from the radiator neck to the tank. If no luck there, maybe do a pressure test on the system, although, there is no evidence by smell or fluid, that I have a leak. Maybe some component does not leak on pressure buildup in the system but leaks when a vacuum begins to form at cool down.

Anyone have thoughts on this?

[WarrenS]

If you are still having trouble try checking the overflow tank for venting. They used to have a hole or 2 in the cap. Now they are getting more elaboate. Instead of opening the radiator cap you could open the overflow tank cap first and see if the level goes down as it is drawn back into the radiator.

[scooterollie]

warstein;
Thanks for your reply. My recovery tank is not pressurized and has only a rubber cap. Thought about the same thing a few weeks ago and punched a small hole in the rubber cap - no difference. I think that I either have a problem between the radiator neck and the tank or some other problem in the system that allows the vacuum created on cool down to bleed off, taking the path of least resistance and not sucking the coolant back from the tank. If the second were true, you would think there would be some kind of coolant leak when the system heats up and is pressurized.

[WarrenS]

I thought something like a flap of plastic flash from molding acting like a one way valve could be stopping the return. I realize this is far fetched.

[Jacine]

Try flushing the overflow line from both ends. It's not too far fetched to have rubber in the line that acts as a one way valve. You may have to replace the line.

[scootindan]

Here's some info that might be of use.

The cooling system is designed to be pressurized. Unpressurized, the 50/50 mix coolant would boil off at 226 degrees Fahrenheit. With the radiator cap properly seated and sealed, the boiling point is raised to 258 degrees.

The thermostat should open between 157 and 162 degrees.

The radiator cap is designed to release pressure in the cooling system. It should valve off pressure between 10.7 to 14.9 PSI. Pressure in the system should never exceed 14.9 PSI. Excessive pressure can damage the radiator.

Pressure is created in the system when the coolant heats and expands. Excess volume of coolant is valved off the to overflow bottle. As the system cools, a vacuum is created which should draw the coolant back into the system.

Although the Honda Helix manual states that coolant may be topped up through the overflow bottle, I have not found this to be reliable on my Baron 250. I check the level in the filler neck from time to time. It does seem to draw any overflow back into the system ok. The overflow bottle is not pressurized.

A point of interest is that the troubleshooting section of the Helix service manual lists, in order, the following causes for overheating symptoms:

- faulty gauge or sensor
- faulty radiator cap
- thermostat stuck closed
- insufficient coolant
- passages blocked in radiator
- faulty water pump
- faulty fan motor

That's all I have for now. Good luck with your problem.

DJ

[scooterollie]

scootindan;
Many thanks for your suggestions. I am going to start with the overflow tube and work backwards.

Warstein and Jacine;
Great minds think alike!!! I have also thought of this possibility recently. I do know that sometimes older brake hoses de-laminate on the inside, leaving a valve-like flap that can either prevent pressure being applied to the caliper/wheel cylinder or prevent pressure from being released. When I removed the overflow hose a couple months ago, I thought I checked it thoroughly but maybe I didn't.

Was also wondering if a faulty headgasket could do the same thing. Funny thing is the vacuum created in the system after the coolant cools and contracts. Makes you think the problem is somewhere from the cap to the overflow hose.

[scooterollie]

Update;
Pulled the complete overflow hose from the neck to tank. No problem there. Hose is fairly large diameter and fully open - no flaps, etc.

Saw some dried coolant residue at one of the hose clamps on a top hose at the radiator. Just in case, removed the O.E. clamp and replaced it with aircraft clamp.

Recharged the system and went for a ride. Same thing - purges the coolant into the tank when coolant gets hot, won't draw it back on cool down.

Sure is a stumper. Have not yet pressure tested the system since it seems to develpo a vacuum after cool down when I remove the cap but think that is next.

[fulltimerart]

My first CFMOTO 150 would push coolant into the overflow tank with enough pressure that it would shoot coolant over 2 feet high out the little hole in the overflow tank cap. This only happened while driving the scooter, it would sit and idle with no problems. Art

[Gary]

Ollie -
Another post made me think of something. Perhaps you have a small leak from your cylinder or exhaust into the cooling system. When riding, the pressure pushes coolant into the overflow (more than would go in from just thermal expansion). Then, on cooldown, some gets sucked back, but not as much as was pushed out.

Just a theory.

[scooterollie]

Gary;
I am beginning to believe something like that is the cause. I think I have exhausted every other possibility. Would like to do a pressure test out of curiosity.

[linda]

Hi,
I just purchased a new rocketa bali 250, and the radiator cap 14.9 psi would not fit properly. The only one I could find of that size was a 13.9. Still had problems with overheating and it also did not fit properly. Mangled the radiator neck. I found a 16psi that fits perfect. Will the extra pressure do damage to my radiator? I asked allnewsports to send me a new cap. Suprisingly they did, but of course the cap was to small. The neck is now shaped like an oval egg and pretty beaten up with all the coolant problems I've been having. I don't want to further the damage my having excess pressure. I took a few short rides and the temp seems ok so far. I'm leary to drive any distance.