MOTOBRICK.COM
TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: gard on May 06, 2025, 08:18:02 AM
-
1985 k100 RT with 142,000 miles. The bike appears to have original hoses, I did not see any obvious cracks or soft spots when I replaced the radiator fan. Still wondering if I should replace all the hoses while the fairings are off. Have others found they seem to last forever or have you gotten stranded? I noticed the hose behind the radiator is collapsed when bike is cold, it regains its round shape if the fill cap is removed. Is this normal due to the vacuum in the cold system which pulls fluid from the overfill tank or a sign of impending failure?
(http://)
-
If you don't replace the hoses now when the fairing is off, you'll never have any peace, gard. :laughing4-giggles: Get on it! It isn't a big deal.
-
you can also get pretty colours for your collant hoses too!
Who needs fairings when you have red coolant hoses. It will run faster too!
-
Thanks for that, I found the various color ones on the ebay, its strange on the same search on a different day comes up with a different result.
Ordered the red as recommended, I want those extra hp
zap bmw k100 silicon hose
-
For group wisdom: does collapsing coolant hose indicate problem with radiator cap? As in: it will let the high pressure coolant out but won’t bring it back from the reservoir?
-
Collapsed hoses are caused by a vacuum in the system- more than likely radiator cap related.
Frank
-
It makes sense that the collapsed radiator hose would be related to the radiator cap, not sure why I did not think of that. Can it be cleaned, repaired and tested or is it one of those items where you just replace it?
On a related note I ordered the new red silicon hose set (less than $60) but somehow forgot to check where it was coming from (China) so no idea when it will get here. The hose set did not include the overflow tube, about another $20. IIRC it was north of $200 from BMW.
-
Make a temporary mark of the coolant level when the engine is cold, then again when the engine is warm. Next morning it should be back at the cold level, then back again to the high level after a ride. If not, another possibility, but unlikely, is the overflow hose is blocked.
If the radiator hose looks a bit swollen or feels too soft, it's too weak to resist the vacuum and should be replaced (It shouldn't be too hard either, like if it goes snap, crackle and pop when you squeeze it, small pieces of rubber could have broken off and started circulating in the coolant, causing a blockage somewhere) I have five bricks, none of them have radiator hose problems, and most of them have had heaps of use and abuse.
Sometimes I think I should replace that part with one that uses a normal radiator cap, then I could easily use my pressure tester.
Normally you would just replace the radiator cap. Finding out the price of one of these BMW caps is something I wouldn't be looking forward to.
-
It could be worn hoses are the problem. It could be radiator cap gaskets. It could be the radiator cap itself; in Germglish the cap is apparently called "a lock". Photos are from the MaxBMW parts fiche.
(https://www.motobrick.com/gallery/4/1601-190525171228.png)
(https://www.motobrick.com/gallery/4/1601-190525172059.png)
(https://www.motobrick.com/gallery/4/1601-190525171408.png)
-
FYI: As Max shows, the last time I purchased a radiator cap from BMW it came with the gaskets so I did not need to buy them separately.
-
If the coolant level goes higher and lower, let's say about an inch, with temperature changes, the radiator cap and overflow hose seem to be good, then the hose is probably too weak.
If this brick has been sitting for years and you're just getting it back on the road, it could be that the valve in the radiator cap is stuck in the closed position. There are two valves, both should be in working condition. One allows coolant into the expansion tank, and should be closed at other times. The other allows coolant to return, and should also be closed at all other times. This is the one with the question mark over it. Check that it lifts up, and that it closes when let go. While you're at it, check that the other valve opens when you press down on it, and that it returns to the closed position when you let it go.
-
FYI: As Max shows, the last time I purchased a radiator cap from BMW it came with the gaskets so I did not need to buy them separately.
OTOH: $12.00 worth of gaskets could possibly solve the problem without the buying the cap. Consider it courting the thrill of low-stakes gambling.
-
I decided to try door #3, bought a used cap complete with the neck, hose and hose clamp for less than $10. I disassembled the cap, the center section just snaps out, it was all pretty clean and free but I soaked it overnight in vinegar anyway. The gaskets look good. I will install it along with the new overflow rubber tubing which is on order, on the slow boat from China.
Now I have a spare neck I could make a radiator cap tester, not sure what pressure it should relieve at or what vacuum level for the return.
-
. . . not sure what pressure it should relieve at or what vacuum level for the return.
Why is that? Not enough fresh air and shredded wheat in your youth?
From the miracle of the talking leaves within the BMW K75 K100 2V manual downloadable from the world famous MOTOBRICK.COM. from this list (https://www.motobrick.com/index.php?topic=11014.0).
Reservoir tubing could have come from any auto parts like NAPA or Baileys. You could have gotten by bicycle if you wanted it to take more time getting there.
:laughing4-giggles:
(https://www.motobrick.com/gallery/4/1601-220525102121.png)
-
Bicycles are easier to pick up, albeit somewhat slower fetching rubber hose.
-
It looks so comfortable, it would be a shame to disturb it.
-
Why is that? Not enough fresh air and shredded wheat in your youth?
From the miracle of the talking leaves within the BMW K75 K100 2V manual downloadable from the world famous MOTOBRICK.COM. from this list (https://www.motobrick.com/index.php?topic=11014.0).
Reservoir tubing could have come from any auto parts like NAPA or Baileys. You could have gotten by bicycle if you wanted it to take more time getting there.
:laughing4-giggles:
(https://www.motobrick.com/gallery/4/1601-220525102121.png)
Why is that? LOL Well I was just finishing my favorite IPA and it was raining outside and the hardcover book is out in the barn with the bike and I was too lazy to go look. I have tried downloading the official 2V version a couple of times but have each time gotten some kind of error message. Not sure what that is about.
I guess I could read from that table that the pressure relief should be between 1 and 1.15 bar, (about 15 psi).
Thanks for the table, I thought I had read someplace it was 10 psi.
I still do not know at what vacuum level the return should open up at.
-
Thanks for the table, I thought I had read someplace it was 10 psi.
Data from a source that cannot be identified closer than "somewhere" is nowhere.
-
From the Clymer manual, the pressure relief valve opens at 100-114 kPa (14.5-16.5 psi)
The vacuum valve opens at 10.34 kPa (1.5 psi) but these vacuum numbers should have a minus sign in front of them.
If the valves are closing under spring pressure, and the coolant level is changing as it should, that should be ok.
-
The vacuum valve opens at 10.34 kPa (1.5 psi) but these vacuum numbers should have a minus sign in front of them.
Minus signs are unnecessary because the word vacuum itself expresses a state of being below atmospheric (or ambient) pressure. High quality vacuum gauges don't have minus signs and pressure gauges usually don't have plus signs.
-
Ok, I'll take the bait, so now we're looking at the minus sign. So now I've looked at four brick books I have, Clymer's BMW, Haynes, and the Rider's Handbook. They all have a minus sign, except for Clymer's. You're right, we should all be aware that it's negative, then the minus sign would be unnecessary. So that's a lot of waisted ink there, one more first world problem.
-
Ok, I'll take the bait, so now we're looking at the minus sign. So now I've looked at four brick books I have, Clymer's BMW, Haynes, and the Rider's Handbook. They all have a minus sign, except for Clymer's. You're right, we should all be aware that it's negative, then the minus sign would be unnecessary. So that's a lot of waisted ink there, one more first world problem.
It really sucks. :laughing4-giggles:
-
They all have a minus sign, except for Clymer's. You're right, we should all be aware that it's negative, then the minus sign would be unnecessary.
You're right; they do. I should be more positive than negative. Pray
-
Bicycles are easier to pick up, albeit somewhat slower fetching rubber hose.
Why would you take a picture of a motorcycle cover on the pavement without a motorcycle?
-
I got some more work done on the bike. I replaced the alternator monkey nutz, it went pretty easy, I made a couple of long pins to guide it back on with a mark on the alternator to align to the gap nutz. The old ones had some wear and there was black greasy powdered rubber like crap under the alternator. The rear battery tray vibration mounts were free from the transmission so I drilled and tapped the holes to 5/16-24 and made a couple of helicoil type inserts with the 6x1 mm thread in the inside and 5/16-24 on the outside.
[ Invalid Attachment ] [ Invalid Attachment ] [ Invalid Attachment ]
The ebay used radiator cap and neck with hose arrived so I attached it to some gauges to check. I slowly increased the pressure and both caps release pressure at about 16 psi. When I close the valve and the pressure drops to about 9 psi in 6 seconds then slowly drops to about 5 psi over the next minute. The book says it should hold the 16 psi for 6 seconds so it is out of spec. Not sure if this will cause any big problem. The original cap had a swollen inner gasket that would not stay in place so I swapped the ebay one back and forth.
[ Invalid Attachment ] [ Invalid Attachment ]
I also checked the vacuum relief valve with a hand vacuum pump and a digital meter that is set to the psi range. The valve opens around 1 psi. When I stop pumping the vacuum drops to zero within about 30 seconds. So nothing about the radiator cap would explain the collapsed radiator hose.
[ Invalid Attachment ]
I did find some smutz in the overflow hose which was also very soft, I cleaned everything and installed a
new hose. I guess I will hook the gas tank up, adjust the throttle bodies and go for a ride if it stops raining.
Any opinions on if I should spring for a new cap?
-
Hopefully the cap is ok and a bit of air is escaping at the hose clamps or the air hose connector.
The vacuum valve probably closes before the vacuum reduces to zero, so if the hose is only partially collapsed, that's probably ok (assuming the coolant level reduces after cooling down) But since you have new one's on the way, then it's good to replace them anyway.
-
Pretty sure there is no leakage from the clamps or hoses, I did not mention above but I repeated the pressure tests while holding my finger over the overflow tube barb on the top of the filler cap neck, the pressure and vacuum levels held constant as long as I kept the barb sealed.
In my first post there is a photo of the radiator hose, it was partially flattened, not totally compressed. I will keep an eye on it the next couple of times I have it running. I guess an an experiment I could hook up the vacuum pump to the bike and measure how much vacuum it takes to flatten the radiator hose.
I have not ordered a new cap yet, not sure about that yet.
-
The first thing I thought when looking at the photo was, it looks like the hose has a twist in it. It's as if the hose clamp at the standpipe was tightened before the hose at the radiator was properly located.
That's some good tests you've done there.
Which book states how long the cap should retain its pressure?
-
Thanks for your comment.
I found that info in the clymer book in the chapter 3 tuneup section, pg 55 cooling system inspection, "the cap must be able to sustain 16 psi for 6 seconds". With a quick look I did not find anything similar in the BMW shop manual pdf so perhaps that is a general instruction for radiator cap testers?
Thats interesting, I did not think about the possibility the radiator tube in my original post may be installed a little twisted, I did not remove the standpipe side when I replaced the cooling fan. The tubing returned to circular cross section when the cap was removed.
The existing overflow tubing seems to be a fairly soft degraded looking black unreinforced rubber, either original or dealer installed. The new tubing I bought and installed is also a relatively thin wall, easily crushed "blue silicone vacuum tubing" advertised for this purpose on ebay. I just noticed the photo in the Clymer book shows a heavier looking overflow tube, looks almost like a fuel line with the braded fabric outer layer. I think I will ride my bicycle to the nearest auto parts store and get something that has a stiffer wall that is listed for antifreeze. I am suspecting the tubing may be getting crushed someplace, it is not entirely clear to me where it should be routed along with all the other wires and tubes along the frame and under the tank.
Looking at the Clymer book they also recommend testing the cooling system by pressurizing it up to but not above 16 psi, it should hold the pressure for 6 seconds, I can do this with my homemade tester connected directly into the radiator hose after removing the filler neck.
-
Well, you could test it as far as Iam aware the system runs on 1 bar == 15psi on boiling point.
You spoke of caps, the radiator cap or the filling cap?
The thermostat in the radiator should be opening at 0.1 bar or at temperature levels of 82, 85 or 87 Celcius depending.. there are 3 of them. The valve in that cap should be checked
If that malfunctions or is clogged, busted or in any other form not working it could cause the hose to end in a vacuum state.
Also my aftermarket hoses are pretty soft compared to the oems, not sure if that was a lifespan thing as they were installed when i couldnt even walk. But mine dont flatten.
At this point you als might want to check for a clogged system, do you recall the amount of coolant that you Drained from the bike?
Also bleu hoses. I feel somewhat betrayed.
-
Goodo thanks I see that in the book now, was just being lazy, only looked at the table.
The hose returns to normal shape with the cap removed, so if it returns to normal shape with instead, the overflow hose removed, then yep it could be the expansion tank hose collapsing. The expansion tank hose on mine is also soft and unreinforced, I spose that's strange, thinking about it now.
With the pressure test of the cap, sometimes flipping the gasket around helps. If the hose seems soft, it might loose a bit of pressure as it expands, maybe wait a minute and repressurise it.
-
... I repeated the pressure tests while holding my finger over the overflow tube barb on the top of the filler cap neck, the pressure and vacuum levels held constant as long as I kept the barb sealed.
I think the results of this test shows that at least one of the valves in the cap is leaking. Hopefully a bit of a clean might help, yep probably a bit of wishful thinking.
-
The crap blue silicone hose as well as the black one removed from the bike overflow tank and the radiator hose that goes behind the radiator all collapse with a couple of pumps on the hand vacuum pump, around 5" hg. The overflow hoses are easily crushed between 2 fingers.
The auto parts stores both were unable to find 5/16 coolant, heater or radiator tubing. They do have what they call "vacuum tubing" which is a relatively stiff unreinforced rubber, about 1/2" OD, no way I can crush it between 2 fingers, vacuum has no visible effect on it. I bought a 3' long piece in a blister pack, only needed 2.5', $9.69, Gates 27555. The website recommends it for only air or water or coolant overflow tanks. I consider this a win compared to the around $50 from BMW.
I pressure tested the coolant system as recommended by Clymer pg 55 using my homemade tester. It is supposed to hold 16 psi for 6 seconds. Initially I found a small leak at one radiator hose, tightened them all a little, now it holds 15 psi, I figure that is close enough for a 40 year old bike. Glad I found that small leak now instead of along the road with all the fairings installed.
The radiator cap has 2 flat face type gaskets that must seal to 2 different surfaces that are at different heights. I think this is not a great design practice, dimensional tolerances of the cap, neck or gaskets will result in one gasket working better than the other. Redesigning it so one is a radial seal like a piston in a bore is outside of the scope. I removed the outer gasket to ensure good seal to the inner one. This reduced but did not eliminate the slow cap leakage in both the pressure and vacuum tests. So I am convinced one or both relief valves in the cap have a small leak. A new cap is on order.
I got distracted yesterday, someone showed up with a welding job on a dump truck. Today I intend to reinstall the gas tank with the cap I have, adjust the throttle bodies and go for a ride.
-
Went for a nice ride, some state roads, secondary paved roads and a steep narrow single lane dirt road 2-up. No overheating problems and the radiator hose no longer collapses. I think the issue was the degraded overflow tube along with some crap in the overflow tank. Still have the old radiator cap, a new one is on order.