ST1100 Overheating Enigma

Gman

Joined
Dec 21, 2024
Messages
8
Age
83
Location
atlanta, georgia
Bike
94 ST1100
I bought a 94 ST1100 a year ago which had not been on the road for 25 years. I finally got it running well, but it overheated in about three miles.

The thermostat opens and I can see coolant circulating. Possible causes for overheating: Water pump not moving enough coolant; dirty radiator,

blown headgasket dumping hot gases into cooling system. Which is more likely and how can I test for the latter? Thanks for your help.

Gman in Atlanta
 
I had a radiator that did not flow good enough.

When I removed it and tested with a water hose it seemed fine however it wasn't good enough. I had a known good radiator and it flowed only slightly better, I tested them side by side ..... however it fixed the overheating problem.

My bike did not overheat in 3 minutes, it took about 15 minutes before it would get to the hot mark.

Also when you drained and changed the radiator coolant, was it just water or was it coolant that was drained and was it rusty?

Good luck.
 
Lots of reasons why an engine may overheat. You have named a couple. but if it was blown head gasket, you would probably see a lot of air burping out the rad filler with the cap off, along with a possible oily sheen in the coolant, or you will find your oil contaminated with coolant and looking milky. Not likely the head gasket from what you say so far.

If that bike sat with either a weak watery coolant or even just coolant that was unchanged for that entire time, the water pump impeller may have corroded so much it doesn't move much coolant efficiently. The fan not coming on is already mentioned. A defective rad cap is a possibility, as is a defective thermostat not opening fully.

I would suggest you should plan to go through the entire cooling system and replace parts that will be well past their normal life. Hoses and metal elbows in the V of the engine for sure. The rad could be flow tested at a repair shop, if you can find one of those these days. If you also consider looking at the timing belt, which should be inspected at least, you could also pull the water pump to see the condition of that impeller.

Answering a few of the questions already asked may help move this along.
 
Check the overflow tube at the radiator filler cap. I think you should be able to see it without removing Tupperware. It’s a common occurrence that it gets cracked and plugged. It’s a very simple fix, cut off an inch or two of the bad section and reconnect. The not so simple is removing the right fairing pocket to get at it. ;)
 
I would probably drain the coolant out and see what it looks like.
Be sure on the refill you use the proper type of coolant - wrong one can wreck the water pump.
As MotorBikeMike pointed out you can get flow but not enough in the radiator to get properly cooled.
I had the same problem on one of mine as he did and it turned out to be the radiator was partially blocked.
Where does the temp gauge get to - over in the red?
Checking the overflow tube as Kiltman said can also be the culprit.
The fan should have kicked on before it got to the red if the fan it working.
Test it by grounding it - wire is on the sensor on the lower left back side of the radiator.
If it comes on when grounded but not when the engine is hot, you've got a wire problem or a sensor problem.
 
agree with the head gasket comments, they can randomly blow in various places, but oil in the coolant is a common symptom (in general, we don't have a lot of empirical evidence on ST1100 head gaskets blowing). Not likely the cause.

regarding the water pump impeller, I have first hand experience with that one. Mine corroded so badly that all the fins were gone, just stubs were left, and I'll be damned if that pump still flowed enough coolant that I rode it for years in that condition and didn't even know about it. I discovered it after the bearing started making a little noise, and when I pulled the water pump I was shocked at what I found. The only symptom at all was sometimes if I was idle in traffic and the fan came on it didn't work as well as it should have, but as soon as the bike started moving at speed again, the temp needle dropped back to normal.

OP sees coolant circulating at the radiator, so there's at least some flow, but given the length of inactivity its quite possible that parts of the radiator are clogged and not flowing coolant at all and its returning mostly hot coolant back into the engine. That's probably where I'd put my money if we were betting.

Another possibility, but probably unlikely, is that the bike isn't really overheating but the gauge is indicating that it is. Mine has been exhibiting weird temp gauge response for probably 20 years. It just randomly climbs to vertical or a little further, but at freeway speeds where there's plenty of airflow. Then it randomly drops back down. I thought after I found the water pump issue and replaced it with a new pump that weirdness would have gone away, but it hasn't. I checked the ground behind the center stand handle, replaced the temp sensor in the T-stat housing, and it still does it. But it never climbs more than a little past vertical on the temp gauge, never goes over into the danger zone.

OP can check the temp sensor in the T-stat housing pretty easily with a multimeter for resistance reading and compare to the service manual values. Then check the actual coolant temp in the radiator with a thermometer.

Last thing to mention is to make sure the fan is working, just as part of the cooling system troubleshooting process.
 
I would say the water pump impeller is toast or the radiator is partly blocked.

I seem to remember some early fans came loose from the fan motor shaft. The fan would rotate from the friction of the shaft on the hub but not enough to cool the radiator. Quick, easy and free check.
 
So, how do you know it's overheating?
Just by the factory coolant temp gauge? What if that thermometer is wrong, and lost its calibration?
Does your coolant system barf hot water / coolant on the street after a few minutes?
Maybe it's overfilled (I've done this with prior bikes I've owned).
Is your coolant boiling-over and steaming? Are you sure it's the correct stuff, mixed at the right proportions,
not decades old? Are you sure the system is sealed to hold the correct PRESSURE, as lower atmospheric pressure allows liquids to boil at lower temps?
On one of my prior bikes, I had two coolant leaks that kept the system from staying pressurized: Dry rotted rubber gasket on the radiator cap, and then a hole in the radiator due to age & corrosion (a local radiator shop did plug that with epoxy). And that bike was a 1996 model; newer than yours.
 
Thanks guys. Lots of good suggestions and information. I'll start systematically eliminating possible causes and report back later..
 
Sorry...late suggestion.
Take a look at the hose that runs from the thermostat housing back to the overflow tank while you check things out.
 
I recently experienced similar symptoms, I had already renewed the stat a couple of years ago so unlikely to be the problem. Turned out that the radiator had corroded to such an extent that although no leaks were apparent, all high pressure in the system was lost so that any slow speed running or stationary running caused an almost immediate temperature raise. Worth poking the radiator with your finger and if it goes through, that's your problem.
 
Figured I'd chime in, as I was just googling a similar issue, and it brought up this chat.
I have a 91, and have had no issues outside of replacing rear brake and both rubbers. That is until this week. I live in SE PA, and I started it up on full choke, as I always have to, on a 15 degree day, went inside and came out 2 minutes later to drop the choke, but I had puddles front and rear. I didnt pull plastics yet, just killed the bike and went in to change out of my riding gear disgruntled. I checked it again hoping it was a fluke. It was a 38 deg day, fired up and this time I got it running for about 15 minutes after letting it get warm off choke and it did it again.
I was thinking maybe it was just bad clamps or hoses, but wanted to see what other issues there may be.
 
91st11rookie said:
... I had puddles front and rear...
Click to expand...
Depending on the inclination of your driveway...
- front: possible the w/pump seal leaking, which is dripping down aside the clutch cover...
- rear: likely the 'internal passages' leaking into the cylinder valley, dripping down between gearbox and alternator...

In any case: time to dig in to investigate...
 
St-Gerard said:
Did the electric fan come on?
Click to expand...
Bush said:
Lots of reasons why an engine may overheat. You have named a couple. but if it was blown head gasket, you would probably see a lot of air burping out the rad filler with the cap off, along with a possible oily sheen in the coolant, or you will find your oil contaminated with coolant and looking milky. Not likely the head gasket from what you say so far.

If that bike sat with either a weak watery coolant or even just coolant that was unchanged for that entire time, the water pump impeller may have corroded so much it doesn't move much coolant efficiently. The fan not coming on is already mentioned. A defective rad cap is a possibility, as is a defective thermostat not opening fully.

I would suggest you should plan to go through the entire cooling system and replace parts that will be well past their normal life. Hoses and metal elbows in the V of the engine for sure. The rad could be flow tested at a repair shop, if you can find one of those these days. If you also consider looking at the timing belt, which should be inspected at least, you could also pull the water pump to see the condition of that impeller.

Answering a few of the questions already asked may help move this along.
Click to expand...
OK, since I wrote last I have removed the radiator and found a whole bunch of brown slimy goo which I immediately recognized as Bars Leaks which I had added when I first got it running and had a small leak from beneath the carbs. That leak got fixed with a new metal elbow in the cooling circuit. I flushed out the radiator with a garden hose and got some more brown slime out. Blew through the radiator with my mouth and felt like it was not clean enough. Filled the rad. with vinegar and left it overnight. Flushed again and blew through it. Better but still a little resistance to airflow. Filled rad. with hot water and dishwasher detergent and left overnight. Flushed and blew through it again. It seemed like it was as clean as it should or could be. While I had the rad. off I connected a hose to the inlet to the engine and attached a large funnel to the hose. Filled the funnel and hose with water and started the engine. Rad. cap was off and water gushed out the filler neck like crazy. Figured I had the problem solved. Reinstalled the rad. and topped up the coolant. Started the engine.
Grounded the fan so that it was running the whole time of the test. Not as quickly as before but the temp went into the red again. And this time I had three coolant leaks: one around the center of the radiator, one from the left side off the transmission, and one from around the rear wheel on the right side. I couldn't see the origin of any of the leaks. The coolant surge tank was fuller than usual but not overflowing. I've been working on this POS for a year and am ready to give up. Anybody need any parts? The bike runs and drives well and has brand new tires (Avon). It is also in great cosmetic condition.
 
Sorry to hear, but that Bars Leak stuff can really muck up the works- when I was 17 or 18, I had an old Celica (a 1974) with a radiator leak driving down to Florida and used a similar product- caused the heater core to get blocked up and could not be unblocked (blew the head asset due to overheating), so instead of swapping it, due to lack of funds then, I bypassed the heater core (and had no heat for the rest of the time I used the car), my brother helped me swap out the gasket (and milled the head), drove it another year like that, then got my first new car in 1986. I had to tow that car back to NJ at the time with a U Haul and a car trailer. If I had to do it again then, I would have abandoned the car in Florida and took a plane home.

As for the bike, any chance of sourcing a replacement (even used) radiator and trying again, or are you just done trying to repair it?
 
Erdoc48 said:
Sorry to hear, but that Bars Leak stuff can really muck up the works- when I was 17 or 18, I had an old Celica (a 1974) with a radiator leak driving down to Florida and used a similar product- caused the heater core to get blocked up and could not be unblocked (blew the head asset due to overheating), so instead of swapping it, due to lack of funds then, I bypassed the heater core (and had no heat for the rest of the time I used the car), my brother helped me swap out the gasket (and milled the head), drove it another year like that, then got my first new car in 1986. I had to tow that car back to NJ at the time with a U Haul and a car trailer. If I had to do it again then, I would have abandoned the car in Florida and took a plane home.

As for the bike, any chance of sourcing a replacement (even used) radiator and trying again, or are you just done trying to repair it?
Click to expand...
If I can't find someone to buy it I guess I will keep working on it, but I'm sick of working on it. I've had the carbs off three times. I might try attaching a water hose to the engine inlet for constant water flow and see what happens. Pretty sure that would keep it cool no matter what, but what would it tell me?
 
The cooling circuit is pretty simple on the ST1100.
If the radiator had Bars Leak in it, it is probably toast as you will not get the flow back you need.
If the 'vanes' on the water pump are worn out then that would also cause your problem.
Even if there are leaks, unless they are gushing out water, it should still cool.
Normal behavior sitting still is the gauge will get about a 1/4-1/8 Inch from the red overheat and stay there until you get rolling again.
The fan should be kicking in and bringing the temps down somewhat to keep it out of the danger zone.
I had one that would not cool down and it was the radiator not getting enough flow - even running down the road at a good clip.
It flushed fine but there were issues with it internally I could not see.
If you feel like doing it, I would take the thermostat out just to eliminate it from the equation.
I have had brand new ones be bad and watching the flow at the neck may or may not tell you if the flow is adequate.
FWIW, a blown head gasket on a ST1100 is pretty rare unless they have been ridden a long way overheated.
 
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