Overheating ST1100

Hi everyone
I have recently brought myself a lovely ST1100, I've been wanting one for years and have been a firm believer in the 1100 being touted as the better ST .
To give some kind of background; it was kept in a barn for 4 years, had 104k miles on the clock and was in pretty good condition once you got through the dust. A carb clean, replacement fuel pump and remedy for the famous seized alternator and I was on the road! I've done about 1000 miles on her now and despite the trepidation of such a big bike found that she handles beautifully and I am fully on board with those that say they are fantastic!
Sadly recently she has been overheating
It all started on a pretty hot day, I noticed that she was getting to about 3/4 on the temp gauge and that was unusual - normally she took the age to even register anything and then got to about 1/4 unless in traffic when she could climb to 1/2, possibly 3/4 at the most.
I had a look down (for some reason) and there she was loosing water from the overflow hose on the expansion tank.
Things have not got any better; I've had a good look round many forums and decided this was the best to join and also saw some other riders withsimilar issues and some 'cures' for it.
So far, I have:-

• Checked and topped up where necessary, the water/coolant levels
• Checked the return hose from/to the rad/expansion tank, it looked clean but a little bit perished at the join with the rad so cut an inch off.
• Brought a new rad cap (direct from Honda dealer)
• Taken out the thermostat and given it a boil to see if it opens OK - YES and I have not bothered to put it back in yet either.
• Lastly the other day I gave the whole coolant system a good flush out with the garden hose, replaced the coolant/ water at approx a 50/50 mix and also used some K-Seal as someone had mentioned the head gasket and figured it couldn't do any harm (also 'bled' the coolant system of air by allowing her to run up to temp with rad cap off etc).

Each step above, I have run her and included traffic and sure enough all the way up to the red line with temps at times - the needle will still move down a touch if I get to about 40mph+ but once anywhere near the red she will never get below 3/4 mark.
Yes, the fan IS running - I can't say exactly when it cuts in but it's there and sometimes has some visible effect on the temp gauge.
Last run out she was well up into the red and I was on the verge of pulling over when I got close enough to where I was going to gently get her the rest of the way. When I stopped her, you could hear the boiling water fizz and bubble back into the expansion tank and then see it spew over the floor
She still rides/runs OK, I think that she's possibly running a little rough when at high temp, however, that could just me being paranoid at that stage; if I wanted to give her some beans and go quick, I am confident that she'd still respond.


I'm at a total loss - head gasket has been mentioned to me as I say and I could really do without that expense and I am sure that the K-Seal would have had some effect even if only a temporary fix.
If this was a car I would be straight down the auto-factors for a new water pump, however, every bit of info I have heard or read says that it simply won't be that, it's certainly not leaking and I'm pretty sure that it's not noisy.


So, sorry for the essay, I just feel I can ask for HELP! better if I give all the facts/info straight off.
Please can anyone help???

Champton,

I see you stated :

I had a look down (for some reason) and there she was loosing water from the overflow hose on the expansion tank.

and then :

Checked the return hose from/to the rad/expansion tank, it looked clean but a little bit perished at the join with the rad so cut an inch off.

So, after this - are you still loosing fluid between the overflow tank and radiator ?

I had the same issue as you, but mine was simple, the hose you mentioned was leaking....

Good luck sir.... I am sure people with far more knowledge and experience than me will be chiming in..

Odie1

Check for a cracked return hose. If it can draw air it leaves your coolant in the reservoir rather than pulled it back to the radiator.

Check the radiator cap again. Mine was a really tight fit so I used silicone grease on the seal...was concerned that I may split the new one. This can cause low pressure in your symptoms without leaking water.

If your other inspections have been effective, you either have air in the system or a bad water pump.

I had a bad water pump and had similar (but not identical) symptoms. I never overflowed the recovery tank. I had the high reading temp gauge in warmer weather, along with the fan still working, but kicking on erratically. Sometimes the gauge would climb to the red line and the fan wouldn't come on, sometimes it would come on and lower the temp. With insufficient water flow you won't get consistent fan operation, so monitor for that symptom.

Thanks so far guys.
For some of the questions that came up; the return hose looked disappointingly good, I was simply being a bit picky - or more like hopeful - when I took the inch off at the rad end.
I have still lost some water from the expansion tank overflow hose when things have got 'hot'
Rad cap was a tight fit and threw me a bit when I first put the new one on, however, I had had a good clean around there and had a look to make sure that there was a clean smooth surface to make the seal to.
I should also say that I could see the water frothing around at the top of the rad when I ran her up without the cap on to dispel the air from the system.
Chris

dwalby said:

I had a bad water pump and had similar (but not identical) symptoms.

Click to expand...

Reading the OP symptoms I'd agree that insufficient coolant flow seems to be a factor... use of improper coolant, yet even tap water is known to corrode the impeller blades, so there is still *some* discharge, but certainly not enough flow for full load conditions, being caught in dense traffic or running in warm climate (didn't know that such exists in da UK though... ;-) )

So if everything else is exhausted, also the radiator fins are clean, a dodgy w/pump would be something I'd consider...

Oldbikefixr said:

Check the radiator cap again. Mine was a really tight fit so I used silicone grease on the seal...was concerned that I may split the new one. This can cause low pressure in your symptoms without leaking water.

If your other inspections have been effective, you either have air in the system or a bad water pump.

Click to expand...

Anyone else have trouble with the spell checker turning itself on AFTER hitting the 'Enter' key? 'symptoms' above was clearly 'system' when I entered the original response.

Oldbikefixr said:

Anyone else have trouble with the spell checker turning itself on AFTER hitting the 'Enter' key?

Click to expand...

Yep... I sometimes need to edit up to 2~3 times to get rid of unwanted/not intended words...

Any chance the gage is reading too high and you really aren't overheating ??

Anyone know what the resistance of the thermo sensor should be @ room temp ?? The Service Manual doesn't give the spec.

You could measure yours and compare it to the reading of a new sensor at a dealer, if they have one in stock.

You could also start the bike ( with the rad cap off ) , let it "overheat' and measure the water temp at the rad cap port with a cooking thermometer or digital meter thermocouple.

I've not really considered the temp gauge to be at fault; the spewing water etc have been more of a givaway to the overheating teamed up with what the temp gauge has been telling me.

The consensus on here seems to be looking at the water pump actually being worth a look (Thanks everyone!), I'm a little surprised as everything I had heard before told me that "it's never the water pump" however, You guys have given me a ray of hope

Before tearing everything down I'd do as suggested by Jim: start the cold(!) engine with removed radiator cap and observe the coolant flow inside the neck (hint: do not blip the throttle with radiator cap removed, only idle rpm; otherwise the coolant will stain your garage ceiling and lots of other items...)
With the thermostat still closed the pump should press coolant though the 2mm bypass bore with a significant jet, when the thermostat then starts to open, the coolant flow should really start to gush through there (wise to either quickly put the cap back on or stop the engine by then...).

ST1100Y said:

when the thermostat then starts to open, the coolant flow should really start to gush through there (wise to either quickly put the cap back on or stop the engine by then...).

Click to expand...

I believe stopping the engine at that point, without putting the cap back on could result in a sudden gush of hot coolant, as it boils inside the engine without any circulation, or pressure being held by the cap, which keeps the boiling point of the coolant higher.

Bush said:

I believe stopping the engine at that point, without putting the cap back on could result in a sudden gush of hot coolant...

Click to expand...

Not in my experience as the engine had just warmed up (we started it cold), so isn't fully "hot" yet to actually cause the coolant boil over...
However will the level raise due the volumetric expansion, so putting the cap on prevents a smelly spill.

you never mentioned anything about the service history on the bike, do you know anything about it? If not, then you might as well bite the bullet and just tear into the front of the engine and replace everything anyway. If the timing belt has never been done you should go in there anyway to do that, so while you're at it change the water pump and the idler pullies. A bit of a hit to the wallet, but all parts that eventually wear out and cause big problems if they fail. Given the age of these bikes the two areas that require a bit of attention are this job, and the hoses under the carbs. Neglecting either is rolling the dice on an inconvenient breakdown along the road somewhere.

There will be no significant pressure build-up with the cap off - because the cap is OFF. Just keep the bike idling until it shows an "overheating" condition on the gauge. Then measure the temp with the cooking thermometer. And BTW, the boiling point of a 50/50 mix is 226 degrees at atmospheric pressure and 265 degrees at 16 psi.

But before I would do this , I would verify the temp sensor resistance at room temp.

As far as the water pump is concerned, there has been a post w/picture in the past of a water pump with the impeller literally "eaten away" , but I would suspect that last. If you can see good circulation with the cap off after the thermostat opens, I would rule out the water pump. It would really help if you could find another ST1100 near you to compare with. I would think there should be ST1100 practically on every block in London. Maybe you could "borrow" one for a few hours ???

Jim, he reports boiling off coolant with a new rad cap on and tight, wouldn't that suggest the coolant is overheated regardless of what the temp gauge/sensor tells him? Seems like calibrating the temp gauge isn't really much help at this point.

I'm wondering how to tell if some of the radiator passages are plugged. He already removed the t-stat, so either the pump isn't pushing enough coolant, or the rad isn't flowing enough coolant. If he can rule out a radiator flow restriction, then wouldn't the pump be the last thing on the list? Other than that the only other obstruction would be in the engine coolant passages themselves, but I don't even want to think about that.

Thanks Guys - Keep the ideas coming

As dwalby says - I've NOT replaced the thermostat and I have looked at the water bubbling and appearing to move around when running up to temp AFTER having done a flush through the rad with the garden hose.
I can't say there were jets of water flying out as I ran her up to temp without the rad cap on, however, I think I can say that this was water moving rather than bubbles from air trapped or leaking into the system. The garden hose flush of the rad looked clear by the end and I would say seemed pretty free flowing.
I have all the original doc's but am not certain of when or if the timing belt was changed and that's been on my mental 'to do' list - just overtaken by 'warmer' events!

If I'm stripping down anyway it would make sense to change that regardless of history.

As I mentioned before, on my last ride out after flushing etc, I could hear the water boiling away when I stopped the engine - this was just before watching it spitting out of the overflow hose. From that performance I'm not too concerned with the accuracy of the temp gauge - it said blooming hot - the water was boiling - that's accurate enough for me.

It's a good plan to find another ST, I don't know of anyone and I guess would be a little hard to find someone who doesn't know me and start stripping their bike of plastics... but I will try.

No one has mentioned head gasket and while I am desperate not to think that might be the cause, can anyone give me some signs/symptoms of that in relation to the overheating? Just seems worth asking before I start stripping to look at or replace the waterpump...

At what temp gauge reading does the fan start running ??

BTW, If you had a bad head gasket you probably would not be overheating, but loosing antifeeze out the exhaust and maybe smell antifreeze coming out the exhaust, too.

Have you measured the actual water temp with a cooking thermometer yet ???????? That is a really easy thing to do. Then you will know the actual water temperature .

Don't worry about head gasket failure, if that were the case you'd either be losing/smelling coolant out the exhaust like Jim says, or mixing oil and coolant together, depending on where the gasket failed. Both of those things are very obvious, and you're not seeing either.