Radiator Cap 'ted?

[vifferman]

I generally haven't had any reliability issues with the VFR, but now that the weather's getting warmer, I have a concern with the engine running a little hot. It seems to be running about 5 degrees warmer than usual, even on cool mornings, and the other day when I was stuck in traffic the bike spewed about 1/4 to 1/2 a litre of coolant on the road. The temperature read about 106C at the time, and the fan seemed to be going. The reservoir / overflow bottle level was just above the "Low" mark prior to this.

Now, I flushed out the cooling system a few months or maybe a year ago, the pump's not leaking, and when I checked the level in the radiator, it was full. Squeezing the hoses indicated they were all full (made appropriate swishy noises).
The only thing I can think of is maybe the radiator cap's not keeping enough pressure in the system. (I'm not thinking about the thermostat, coz that's too scary...)

Do the symptoms fit this (i.e., the spring in the cap is tired so the system's not up to pressure?)
If so, can I buy a new cap at an auto parts store, or is it likely to be a Honda/VFR-specific cap?

[98tls]

Caps do indeed wear out,for sure the easiest/cheapest option first up.Then id be looking at the thermostat.

[Max Preload]

You haven't said what part of the cooling system the coolant came out from. If from the coolant reservoir overflow (not the hose to the radiator filler neck - they have a vent), you may have a very serious problem, especially if the reservoir was only on the LOW mark before it filled and overflowed.

In any case, check the cap seals for cracks (both the seal that closes the radiator neck and the seal that is immediately under the top of the cap) and confirm the operation of the check valve in the bottom of the cap that allows the coolant to be drawn back onto the cooling system as the engine cools.

If those are all good get your cap opening pressure tested at your local friendly radiator repair shop. While you're there, get the cooling system pressure tested.

The cap will likely be Honda specific but I know my Kwaka has a standard dimension cap (I recently had a hose split and once I'd done the repair pressure tested it overnight) although I haven't tried to source one that lifts at the specified 1.1 Bar. If you do manage to get one make sure it's the recovery type with the 2 seals and check valve as I've mentioned above.

[vifferman]

It was coming from the coolant overflow.

But it's okay now - it's all good.

I cleaned the radiator cap (again), and started the bike, and followed the procedure for blledin/burping the cooling system (blipping the throttle, squeezing hoses, topping up (none needed) etc.) The temperature seemed to rise more slowly than the other day (despite being in a warm garage, and not moving).
I was a bit worried for a while, as the radiators and lower hoses (left radiator to/from pump) and the one across the front of the engine between the radiators) didn't seem to be warming up, even though the temperature was climbing slowly towards 90C.
Eventually they did warm up, and the level in the coolant bottle slowly rose from between the marks to about a half cm above the upper level mark.
Then at 106, the fan kicked in like it should, and the temp started to fall. As it did, so did the level in the coolant bottle.
So, in that garage at least, it seems OK.

Thanks for the help!

[Max Preload]

It was coming from the coolant overflow.

So out of the reservoir?

...I flushed out the cooling system a few months or maybe a year ago, the pump's not leaking, and when I checked the level in the radiator, it was full.

When was it full? At the time of refilling a year ago or after it just dumped coolant onto the road?

But it's okay now - it's all good.

Well, not really. Depending on what the answer is to the question I've just posed above, you haven't actually found what caused the coolant to be dumped.

...temperature seemed to rise more slowly than the other day (despite being in a warm garage, and not moving).

It will warm much more slowly if the engine is just idling or revving without load.

I was a bit worried for a while, as the radiators and lower hoses (left radiator to/from pump) and the one across the front of the engine between the radiators) didn't seem to be warming up, even though the temperature was climbing slowly towards 90C. Eventually they did warm up, and the level in the coolant bottle slowly rose from between the marks to about a half cm above the upper level mark.
Then at 106, the fan kicked in like it should, and the temp started to fall. As it did, so did the level in the coolant bottle.

The transfer of coolant to the reservoir & retraction back into the cooling system suggests to a degree the correct operation of the cap. So I would suggest your thermostat is possibly getting a bit sticky or your thermo fan switch is getting dodgy. What ratio of coolant to water did you use?

[vifferman]

So out of the reservoir?

Yes, from the reservoir overflow. Note that this was only a guess, from looking down at the road while it was piddling out. There was no signs of leakage from the pump, which is the only other thing near there, and this was the most likely thing.

When was it full? At the time of refilling a year ago or after it just dumped coolant onto the road?

It looked pretty much full when I checked it after it had dumped coolant. Presumably, it had sucked some coolant back from the reservoir.

Depending on what the answer is to the question I've just posed above, you haven't actually found what caused the coolant to be dumped.

Well, it's not a leak, so that leaves either a bit of an airlock, which it doesn't appear to have now; a faulty/dirty radiator cap (which is what I think), or a faulty/sticking thermostat (I'm putting my fingers in my ears, covering my eyes, and going "La la la la la!!").

The transfer of coolant to the reservoir & retraction back into the cooling system suggests to a degree the correct operation of the cap. So I would suggest your thermostat is possibly getting a bit sticky or your thermo fan switch is getting dodgy. What ratio of coolant to water did you use?

The thermo fan switch is fine (or was when I tested it in the gargre. I couldn't hear it coming on the other day when it spewed coolant, but there was too much background noise. I thought at the time it hadn't kicked in, but as I was sitting in traffic, I couldn't check it properly. I forgot which way the fan blows (it sucks air in through the left radiator) and the lights changed before I had time to hold my hand over both radiators to feel for hot air being blown out.

At the moment, my feeling is it's either OK, or (given the slowness of the lower hoses etc to heat up), it's a sticky/faulty thermostat. I'm hoping it's not that, as it's a bitch to replace (have to remove fairings, gas tank, airbox, fuel rails, one or more throttle bodies, the hoses to/from the thermostat, and drain the coolant.

EDIT: Ok, I've gone back and had another look at the manual (didn't actually use it to do the work on the weekend). The thermostat starts to open at 84/85C, and isn't fully open until 95C, so thinking back about when the hoses/radiators actually warmed up fully, it would've been when the temperatures was in the low to mid 90s.
I'm going to just ride this for a few days (with breaks, of course), and see how it behaves.

[vifferman]

Rode home last night the s-l-o-w way, and the bike behaved impeccably: temperature got up to only 91 when sitting waiting in traffic, and once moving dropped reasonably rapidly to low 80s.

[Devil]

Had pretty much the same issue with the Trumpy. The rubber on the cap was disintegrating and not sealing properly anymore. Double check the "dirt" you might find around the cap hole, because what I thought initially was dirt was actually part of the rubber seal falling apart.

[vifferman]

It didn't actually look dirty - I just wiped my finger around it, gave the doofer a wiggle, licked the lime juice off my finger and wa-haaay!

Found out on one of my other forums (yeah, I'm moonlighting) that Honda car (Civic or Accord) radiator caps should fit.