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RS Rider

RS Rider
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I wasn't a member here last year but this problem goes back to last year which was the first year owning/riding the bike. As you can see there is coolant escaping from underneath the filler neck cap. The threads inside the cap are wet, as are the threads on the filler neck. You can also see that the coolant gets onto the foam padding that sits on the top of the frame onto which the fuel tank rests.

I have tried two different caps and rubber seals (the large one which seals the cap to the top of the filler neck).  The only thing I can think of at this point is to replace the plastic filler neck. I did inspect the filler neck last year and could not see any issues with it at all. The orifice in the filler neck for the overflow tube is not plugged. The overflow tube was new last year and it is also not plugged and I've verified that it is not getting pinched by the fuel tank.

Do I have too much coolant in the bike? I was always under the impression that with no air in the system and the bike cold the coolant level should be topped up to the bottom of the filler neck.

Anyone here seen this issue before or have some ideas?
1985 K100RS coolant leaking from under filler neck cap. MWUrcL

Below, the current level of coolant can be seen in the filler neck. The bike has sat overnight and is dead cold.

1985 K100RS coolant leaking from under filler neck cap. SEeou4


Below is the sight tube on the overflow tank. The lower ink mark indicates where the coolant level was when I went for a ride yesterday and you can clearly see its current level. Both are levels on a cold engine. The top mark represents what the level was on the bike with the engine hot right after riding it yesterday.

1985 K100RS coolant leaking from under filler neck cap. JRepOQ

    

Point-Seven-five

Point-Seven-five
Life time member
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First question, is the engine running at normal temperature or is it overheating? A blocked radiator will overheat the coolant on a hot day and make it puke out of the overflow. I had that problem with one of my bikes and had to do a vinegar soak both internal and external to remove the mineral deposits in the radiator and the bugs blocking the fins.

Second, have you changed the coolant recently? It's possible you have an air bubble in the system that is expanding when hot and pushing the coolant out of the engine. An air bubble can also reduce the cooling capacity of the radiator, causing overheating.



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Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

Laitch

Laitch
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Like Point-Seven-five, I also think it's likely there is air in the system, escaping as vapor and condensing when reaching cooler air. Is the fan operating more frequently than usual? Is there any change in oil appearance? Were the replacement cap and gaskets OEM?

The coolant level in the coolant expansion tank should be at or near the MAX level in a cold engine before riding. It isn't surprising that the level would be higher in the tank when the engine is hot than when it's cold; that is operating correctly. Coolant is topped up in the expansion chamber, not at the filler neck, on a cold engine.


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1995 K75 81,000 miles
1985 K100RS coolant leaking from under filler neck cap. Usa-lo10
    

RS Rider

RS Rider
active member
active member
@Point-Seven-five wrote:First question, is the engine running at normal temperature or is it overheating?

Normal temperature. I installed a temperature gauge and it sits in the middle while riding. Of course that increases somewhat with slower speeds. My fan runs flawlessly and the bike doesn't overheat.

 A blocked radiator will overheat the coolant on a hot day and make it puke out of the overflow.  I had that problem with one of my bikes and had to do a vinegar soak both internal and external to remove the mineral deposits in the radiator and the bugs blocking the fins.

My original 3-core was replaced a couple of weeks ago with a 2-core because that is all I could find. The original had the same issue so I don't think it's the rad. I soaked the 2-core in my bathtub for quite some time and ran hot water through it. I also used the shower nozzle to extensively clean in between the cooling fins.

Second, have you changed the coolant recently?

Yes. It was the same coolant that was in the 3-core and it was fresh. I use Yamalube Yamacool. It's a 60/40 premix.

 It's possible you have an air bubble in the system that is expanding when hot and pushing the coolant out of the engine.  An air bubble can also reduce the cooling capacity of the radiator, causing overheating.

My first thought too. I suppose it's possible but after changing the rad and refilling the system, I squeezed and squeezed and squeezed and...... the rad hoses as the engine was running and coming up to temperature. Saw lots of bubbles. I'd be flabbergasted if somehow there is still air trapped in there somewhere.


    

5Back to top Go down   1985 K100RS coolant leaking from under filler neck cap. Empty Leak Sun May 23, 2021 8:02 pm

daveyson

daveyson
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I'm pretty sure the filler neck and cap are not original. The seals don't look good at about the 7 o'clock position. I think the level is too low, I adjust mine so that the level is at the full line when hot.

I think you got the cap and neck at different times so they might not be compatible, and both might not be compatible with an '85 brick. You shouldn't be able to see any bubbles in the overflow tank when the engine is hot or cold, or idelling or at revs.


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11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

RS Rider

RS Rider
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active member
@Laitch wrote:Like Point-Seven-five, I also think it's likely there is air in the system, escaping as vapor and condensing when reaching cooler air.


Is the fan operating more frequently than usual?

No it isn't. Even though I have a 2-core in it now it seems to behave about the same as the 3-core.


Is there any change in oil appearance?

Not at all.

Were the replacement cap and gaskets OEM?

I think one set was from Euro Motoelectrics and the other from Motorworks.

The coolant level in the coolant expansion tank should be at or near the MAX level in a cold engine before riding.

The manual does actually say to top it up to the MAX mark, so I'll make sure of that before the next ride. Practically speaking, though, you'd think that anything between MIN and MAX should be perfectly fine.


It isn't surprising that the level would be higher in the tank when the engine is hot than when it's cold; that is operating correctly. Coolant is topped up in the expansion chamber, not at the filler neck, on a cold engine.

Yup, just like in any vehicle that I've ever had.

    

RS Rider

RS Rider
active member
active member
@daveyson wrote:I'm pretty sure the filler neck and cap are not original.

The cap isn't as it's the newer style. I still have the original, old style. I've not touched the filler neck so I can't attest to its provenance.


The seals don't look good at about the 7 o'clock position.

I'll have a closer look. I might have another.

I think the level is too low, I adjust mine so that the level is at the full line when hot.

The manual actually says to do that when the engine is cold. Regardless, I will make sure it's topped up prior to my next ride.


I think you got the cap and neck at different times so they might not be compatible, and both might not be compatible with an '85 brick.

I've not replaced the filler neck.

You shouldn't be able to see any bubbles in the overflow tank when the engine is hot or cold, or idelling or at revs.

That certainly makes sense but I've never actually checked.

    

Laitch

Laitch
Life time member
Life time member
@daveyson wrote:I think you got the cap and neck at different times so they might not be compatible, and both might not be compatible with an '85 brick. 
K100RS2V, K100RT, K100, K75 all have the same cap and neck part numbers.


__________________________________________________
1995 K75 81,000 miles
1985 K100RS coolant leaking from under filler neck cap. Usa-lo10
    

Laitch

Laitch
Life time member
Life time member
You have just about ruled out every cause for this problem, RS, and yet, there it is. Smile Unless the cap has been cross-threaded or the neck has cracking that opens during heat this could turn out to be another elusive condition like Danish Sprang Clutch Syndrome. drunken


__________________________________________________
1995 K75 81,000 miles
1985 K100RS coolant leaking from under filler neck cap. Usa-lo10
    

10Back to top Go down   1985 K100RS coolant leaking from under filler neck cap. Empty Leak Sun May 23, 2021 9:08 pm

daveyson

daveyson
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Is that a normal looking cap in the US, I've got five bricks, none have a cap that looks like that. The rubber seals don't look good to me. I'd fix that. 

I'd be putting more coolant in the filler neck and overflow tank, and checking for bubbles.


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

Laitch

Laitch
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Life time member
@daveyson wrote:Is that a normal looking cap in the US . . .
I don't have a close relationship with my moto's coolant filler cap. I look at it every couple of years but it just doesn't seem to make a lasting impression. I try not to look at others' caps; people can get edgy about stuff like that.


__________________________________________________
1995 K75 81,000 miles
1985 K100RS coolant leaking from under filler neck cap. Usa-lo10
    

12Back to top Go down   1985 K100RS coolant leaking from under filler neck cap. Empty Leaky Sun May 23, 2021 9:42 pm

daveyson

daveyson
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Come on now, have a quick little peek-a-boo, I won't tell anyone.


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

RS Rider

RS Rider
active member
active member
@daveyson wrote:Is that a normal looking cap in the US, I've got five bricks, none have a cap that looks like that.

https://www.euromotoelectrics.com/product-p/radcap720.htm

The rubber seals don't look good to me. I'd fix that. 

I'd be putting more coolant in the filler neck and overflow tank, and checking for bubbles.

    

MartinW

MartinW
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+1 on replacing the seals.
Regards Martin.



Last edited by MartinW on Mon May 24, 2021 8:39 am; edited 1 time in total


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K75s Hybrid
    

92KK 84WW Olaf

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Seal in cap is Kaput.


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1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 42,640 miles.

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

RS Rider

RS Rider
active member
active member
So I just got back to having a look at this today. I reinstalled the original style filler cap that came on the bike with the large rubber gasket from the newer style cap. The rubber seal looked fine actually but I flipped it over anyways.

I topped up the overflow tank to the MAX mark and fired up the bike from cold. Moments after starting up the bike I got a bubble in the sight tube every several seconds and then what appeared to be a flurry of them when, I believe, the thermostat opened? Didn't see a single one after that.

Not going riding today but I plan being on the bike all weekend. We'll see what happens when I start it up cold tomorrow.

    

Laitch

Laitch
Life time member
Life time member
@RS Rider wrote:Moments after starting up the bike I got a bubble in the sight tube every several seconds and then what appeared to be a flurry of them when, I believe, the thermostat opened? 
How much time elapsed from the engine's starting until the bubble flurry?


__________________________________________________
1995 K75 81,000 miles
1985 K100RS coolant leaking from under filler neck cap. Usa-lo10
    

18Back to top Go down   1985 K100RS coolant leaking from under filler neck cap. Empty Coolant leak Fri May 28, 2021 7:44 pm

daveyson

daveyson
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Goodo, hopefully the replaced seal doesn't leak, and you don't have any more bubbles, unless you still have bubbles, then hopefully it is a leaky seal. 

If you have bubbles over a period of time (say 1,000 miles) but you're not loosing any coolant, that could be, and hopefully is, due to a leaky filler cap gasket, then you need a new one.

Sometimes a used gasket can look perfect, but leak because it has swelled in diameter, so it distorts when the filler cap is installed.


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

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