Coolant Overflow after Bike sits and idles

[KJM00]

Hey Guys,

Looking on poking the more mechanically inclined brains (than I) on the sub.  My water pump failed probably 2,000 miles ago, I rebuilt it with the seal kit and advice from you folks and it repaired the bike for a year without issues. (Thanks again!) Just recently an issue pertaining to the Cooling system popped back up.   There's no coolant in my engine, nor oil in the overflow, but a few days ago while stuck in downtown traffic my overflow started to shoot coolant out.  (Puddling underneath the bike.) 

I rode it home the last two miles and parked it in the garage, refilled coolant and decided to test it out a few days later.  Today I took it out, rode it for 30 minutes and no issue at all, I left it parked in the sun for 5 minutes and bam the issue is back.  I'm not getting a Temp alarm, but it seems like the bike is overheating or theres issues with the coolant circulation. 

I'm probably missing something obvious, but hoping this isn't a sign of a cracked head.  Where should I start the next investigation steps?

Thanks guys,

[frankenduck]

Replace the radiator cap - part 17111464720. That part includes new gaskets do you don't need to buy them separately.

[KJM00]

will do thanks, will report back.

[Laitch]

I'm not getting a Temp alarm, but it seems like the bike is overheating or theres issues with the coolant circulation.

You definitely have issues with cooling but circulation is only one of the factors that could be at play.

As frankenduck implies, radiator caps and gasket can wear out, but regardless of whether the radiator cap is faulty, the temperature sensing relay is not triggering the temperature warning lamp in the instrument cluster as it should when the engine overheats. We know that because coolant boiled over without warning. Failure to light the warning lamp could be caused by a faulty coolant temperature sensor located behind the radiator.

The sensor has two wires; one sends a signal to the fuel injection control unit to adjust mixture according to engine heat. The other wire sends a signal to the temperature sensing relay that responds to heat by triggering the fan. If the engine continues to heat, the relay then triggers the warning light in the cluster. There could be breaks in the continuity of those wires, or the coolant sensor itself could be sending faulty signals. While you can ride after you install the new cap and gasket, the engine will still overheat if the fan isn't working. That will cause coolant boil-over if you're trapped in jammed traffic on a hot day. You might like a warning before that happens so you can move to the shoulder and shut down your engine for a minute or two. I suggest this malfunction be remedied soon.

The fan could be seized. It can be checked for operation. Use a dowel for spinning the blades to determine if they are seized. If they spin, terminals in the temperature sensing relay can be jumped to determine if the fan motor works.

[The Mighty Gryphon]

Besides the fan's airflow, you need the fins fairly clean so air can circulate over the heat transfer surfaces in the radiator.  You may need to clean a bunch of insects and dirt from the fins.  I know this because I had a K100 that overheated even when moving at 65mph on 70 degree days.  There were enough insects in the fins to feed a family for a couple days.

Think of how many bugs you have cleaned off the windshield over the past 30 years.  They're still in the radiator.

[frankenduck]

the temperature sensing relay is not triggering the temperature warning lamp in the instrument cluster as it should when the engine overheats.

If the cap is bad then it blows before the coolant gets hot enough to trigger the idiot light.

[Laitch]

If the cap is bad then it blows before the coolant gets hot enough to trigger the idiot light.

Under other circumstances, that's what I'd call cold comfort.    My experience has been that if enough coolant has blown out to lower the pressure, the remaining liquid and air within the system is going to keep heating up and trigger the light. Replacing the cap and gasket is a good start though—especially if money is no object—but verifying the rest of the system will keep KJM out of mischief, at least for a while. 

[frankenduck]

When the radiator cap blows I turn off the bike. Once the bike is turned off the water stops circulating and there's no more heat being generated by the motor.  How is the coolant going to get any hotter?

I've had rad cap issues on a couple of bikes and the idiot light never came on.

[Laitch]

When the radiator cap blows I turn off the bike.

I wasn't making observations about what you do; I was observing what KJM did and what I've experienced, but I agree. If coolant isn't heated, it won't get hotter. 

[daveson]

If you turn the bike off, it can get a bit hotter before it cools down because the pump circulation stopped.

[frankenduck]

If you turn the bike off, it can get a bit hotter before it cools down because the pump circulation stopped.

There's nothing generating more heat if the engine is turned off. The water sitting still at the temp sensor will decrease in temperature.

[daveson]

There's nothing generating more heat but the reduced cooling efficiency with the pump stopped can result in a small increase in temperature.

This applies even more so if the coolant circulation stops while riding, the reduced coolant efficiency results in the temperature rising.

The circulation doesn't stop, it's just greatly reduced when the pump stops, cause the hot coolant will rise into the radiator and is replaced by the cooler coolant from the radiator. I think this is how it worked back in the very early days before someone improved the efficiency by including a pump in the system.

[frankenduck]

There's nothing generating more heat but the reduced cooling efficiency with the pump stopped can result in a small increase in temperature.

This applies even more so if the coolant circulation stops while riding, the reduced coolant efficiency results in the temperature rising.

The circulation doesn't stop, it's just greatly reduced when the pump stops, cause the hot coolant will rise into the radiator and is replaced by the cooler coolant from the radiator. I think this is how it worked back in the very early days before someone improved the efficiency by including a pump in the system.

What nonsense. When the water pump stops water/coolant does not keep circulating. You need the pressure generated by a spinning impeller for that to occur. Momentum may keep it flowing for a fraction of a second but even then the engine is no longer firing and therefore no longer generating heat to transfer additional heat to the coolant.

Where did you study thermodynamics and fluid dynamics? (I studied both while getting an engineering degree from The University of Washington.)

[daveson]

I'm surprised by your claim that circulation can't be created by differences in temperature, I believe it can. There are plenty of examples of water cooled internal combustion engines that don't have a pump, where the circulation is due to temperature differences. And for example there have been plenty of hydronic heating systems that worked without a pump, especially back in the days when efficiency wasn't considered too important. But this is a side issue.

The point I was trying to make is that the temperature can rise after the engine is turned off. I have noticed this in the past on cars with heating problems.

Frankenduck, I like your idea of considering the cap as the cause. And I like the other ideas too, especially Laitch's, that maybe the fan simply isn't working, since it seems to me like the problem is while stationary. I like these ideas cause it would be good if the problem is due to a simple cause that is easy to fix. If something like this would happen to my brick, I would probably straight away assume the worst and go "oh no I've done a bloody head gasket"

[Laitch]

Where did you study thermodynamics and fluid dynamics? (I studied both while getting an engineering degree from The University of Washington.

I'm surprised by your claim that circulation can't be created by differences in temperature, I believe it can. There are plenty of examples of water cooled internal combustion engines that don't have a pump, where the circulation is due to temperature differences.

Here's a little something from the Henry Ford Institute. Of course YMMV. 

[The Mighty Gryphon]

To successfully circulate hot coolant the system needs to be specifically designed with large passages as vertical as possible, with gentle bends among other criteria. As a result, these passages will not produce turbulent flow for ideal heat transfer either.  I seriously doubt that the K bike block was designed with gravity circulation in mind.  That is why it has a coolant pump.

Bottom line is that I seriously doubt that the residual heat in the head(which by the way will be pretty close to the coolant temperature) will heat the coolant enough to overcome the resistance to flow of the system.  While it may be fun to kick back and "blue sky" scenarios that could cause a particular problem, it is always better to go for the most simple explanation as per Occam's Razor.  It makes it a lot easier for the guy with the problem we are trying to help.

[Laitch]

While it may be fun to kick back and "blue sky" scenarios that could cause a particular problem . . .

It is fun. KJM already received his simple solution, and a check list (if KJM wants to undertake it). We're just passing time here until the results come back, Mighty Buzzkill. 

[blackie1]

if the cooling system is not completely bled of all air with the radiator cap removed and engine is run up to temp so the  thermostat has opened, prior to final top up of coolant and then radiator cap put back on after all air has been belched out.
 could failure to properly bleed all air from the system cause these symptoms?
i know i have had similar problems to this in some cars that i have owned.

[The Mighty Gryphon]

if the cooling system is not completely bled of all air with the radiator cap removed and engine is run up to temp so the  thermostat has opens prior to final top up of coolant and radiator cap then put back on after all air has been belched out.
 could failure to properly bleed all air from the system cause these symptoms?
i know i have had similar problems to this in some cars that i have owned.

Very possible, it is amazing how much a tiny air bubble can expand in a hot engine.  It's kinda what I thought when I first saw this thread.

[KJM00]

Super long delay, but thank you very much Frank so far so good.

I order the cap and seal replacement kit that Frank recommended, it took about 10 days to receive, apologies for the delay everyone!  After replacing Seal and Cap I let t he bike idle for 15 minutes, then when for a 20m Ride, left it idling as I got food from a fast food joint. (5-10m of idle and staring at it while i waited for food) 20 minutes home, then left it idling for 15 minutes in my garage after the trip.  Zero overflow so far!

Going to take it for a 45-60 minute ride this monday, if that goes well, I'll take her back to work as my primary commuter. 

Thanks guys!  So far Cap seems like the solution.

[Laitch]

Thanks for the update. 

[daveson]

Good work, hopefully the problem is sorted, and the fan was working.

Just to add something I'm thinking about the T model comparison. The T model Ford being a three litre engine with a maximum twenty horsepower at 1,800rpm, a slow lazy coolant system without a pump was good enough. A K100 having a one litre engine with  ninety horsepower at 8,000rpm, I guess a slow lazy coolant system wouldn't be good enough, so it has a pump.

[Laitch]

Just to add something I'm thinking about the T model comparison.

There was no comparison to the Model T; there was an introduction to the use of thermo-siphon as an engine design principle that eventually might become useful now that there's a nascent movement to convert Bricks into carbureted engines. Can eliminating two—or more—of their cylinders be far behind? Snip,snip here; weld,weld there and Bob's your uncle. 

[Past-my-Prime]

Although I could be mistaken, and I'm too lazy to go and find the reference, I seem to recall strongly worded suggestions somewhere in the BMW manual *NOT* to let the engine idle for prolonged periods of time. While the fan can bring air through the radiator especially on hot days in slow moving traffic (my favorite riding circumstances) the additional air movement brought on by actually driving is important in effective cooling.

My lesson: start it, minimal warmup (couple minutes, tops) and drive it!

[The Mighty Gryphon]

PMP, I think you are referring to the instruction to ride off as soon as starting a cold bike rather than letting it sit and idle to warm up. 

I suspect this was to get the engine to operating temperature faster so as to get out of the cold engine enrichment regime as soon as possible to avoid fouling the plugs and building up carbon in the combustion chamber.