Wisdom of the Forum needed:

Fellow Forum members, I come to you this day to garner the wisdom of the Forum.
My ST has had this nagging problem for going on about a year now, I believe I know what the problem is but I would like the wisdom of the forum to confirm or deny my thought process before I spend a bunch of money on parts.
The problem is that over the last year when my ST riding gets HOT, and the fuel tank is down to two bars, or less, the bike wants to miss and buck and hesitate, as if it were out of gas. When I fill it up to half or more then she runs just fine and pulls strong. I know at two bars on the fuel gauge I have plenty of gas. So it is not a matter of running out of gas. This only happens when it is warm outside or if I have been doing a lot of stop and go riding. During the winter I have not had this problem. It seems that today after experiencing this phenomenon again, even after filling the tank back up, I could feel a very slight hesitation upon acceleration. Bike is a 03 ST1300 with Almost 92000 miles on it. I have my suspicions of the cause but would like to hear your ideas. Thanks. Need to get it fixed before WESTOC. The ST gets the call for the WESTOC trip.

If it were an ST1100, I'd say it was the fuel pump. If the pump is in the bottom of the tank like the ST1100, heat could be causing it to fail when the fuel is low because the fuel is providing cooling.

Uncle Phil said:

If it were an ST1100, I'd say it was the fuel pump. If the pump is in the bottom of the tank like the ST1100, heat could be causing it to fail when the fuel is low because the fuel is providing cooling.

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Yes the fuel pump is the same set up as the ST1100.

Pinched vent on the tank?

Have you tried opening the gas cap and seeing if it will start when it's low on fuel?

<D>

I was thinking the fuel pump was getting hot and acting up. Don't you just love a consensus?

kckbiker said:

I was thinking the fuel pump was getting hot and acting up. Don't you just love a consensus?

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Could also be some water in the tank, which would be at the bottom, but I agree that it's probably the fuel pump overheating.

Chuck - My 'opinion' ;-) is that this ethanol mixture is causing premature fuel pump failures. I've lost two so far in two different ST1100s. The first notice is they started surging when I was using electronic cruise control (I initially thought it was a cruise control problem until it happened the second time on a different ST1100). Then, when the fuel got low, they just quit pumping and the bike acted like it was running out of fuel. If they cooled off by sitting a while or fuel was added, they would spring back to life. It was always when it was hot and the fuel level was pretty low.

A friend had a similar issue and it turned out to be the vent line.
I hope this helps,
Rich

Chuck, let's get together and comiserate. Maybe better yet, let's swap bikes on a ride and see what the other feels like.


Mine has been doing similar for several years--at elevation or at low fuel level, when it is heat soaked, it is downright embarrasing as the engine sputters and surges, rattling the whole drive train.

Here's what I have done over the years, to no avail: changed fuel filter, changed MAP, changed FPR, changed fuel pump, changed plugs, checked for vacuum leaks, ... and yet the problem remains and even is getting more prominent.

I have net another set of plugs (assuming one is shorted when hot) and a backup set of coils to try next. It is frustrating.

Fuel pump issue has been mentioned before so possible. vacuum lock is possible, just open the tank cap and see if it straightens out...

BakerBoy said:

Chuck, let's get together and comiserate. Maybe better yet, let's swap bikes on a ride and see what the other feels like.


Mine has been doing similar for several years--at elevation or at low fuel level, when it is heat soaked, it is downright embarrasing as the engine sputters and surges, rattling the whole drive train.

Here's what I have done over the years, to no avail: changed fuel filter, changed MAP, changed FPR, changed fuel pump, changed plugs, checked for vacuum leaks, ... and yet the problem remains and even is getting more prominent.

I have net another set of plugs (assuming one is shorted when hot) and a backup set of coils to try next. It is frustrating.

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Sure when do you want to get togeather and trade off.

Fergie has been trying to figure this one out on his project '03 bike.. Kries was thinking it's the airbox temp sensor.. he replaced the fuel pump with a used one and while it 'seemed' to decrease the issue we were riding in cool temps so that may have masked it.... a new 'used' fuel pump did not eliminate it.

Fergie's bike has been sputtering. I think he learned from H2Orider (Kries) that it may be the MAP sensor.

Discussed here: MAP

Ah, the simplicity of the ST1100 :lol:

Uncle Phil said:

Ah, the simplicity of the ST1100 :lol:

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ok, I guess you and George are tag teaming now.. LOL

Mellow said:

Fergie has been trying to figure this one out on his project '03 bike.. Kries was thinking it's the airbox temp sensor.. he replaced the fuel pump with a used one and while it 'seemed' to decrease the issue we were riding in cool temps so that may have masked it.... a new 'used' fuel pump did not eliminate it.

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Chuck an easy fix,but it will run rich acording to Kreis is to run a papper clip across both contacts.I thought maybe conect wires to both contacts put them on a switch on the dash and turn on when it acts up?
Then I just ordered SENSOR, AIR TEMP $14.13 from Ron Ayers Oh ya $5 shipping,there a used one on ebay $9.99 & & $4.98 shipping

My first thought is also a hot fuel pump issue but if not, the only thing different
from a full tank to a low one is where the fuel gets picked up from.
Just guessing here, but there is a rubber boot at the bottom of the fuel pump that holds a pre-filter .

When gas is plentiful, the gas enters the pre-filter from the top of the rubber boot.

When gas is low, it enters through a few holes at the bottom of the rubber boot.

Maybe the bottom of pre-filter or the lower holes are clogged causing the fuel starvation issue .

There is also a rubber fuel line on the main fuel filter that I would check .

I would pull the fuel pump, and see if anything looks out of whack.

I would check the obvious for venting, fuel pressure,and look into the air temperature sensor, etc..... but, IIRC, prior to '05, ST's didn't have any insulation under the tank to prevent engine heat from potentially causing these issues. You may want to add some?

RaYzerman said:

I would check the obvious for venting, fuel pressure,and look into the air temperature sensor, etc..... but, IIRC, prior to '05, ST's didn't have any insulation under the tank to prevent engine heat from potentially causing these issues. You may want to add some?

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Good call Ray. It begs the question though, how long has this been going on? Has this been happening only during the last year? It would seem to me that the problem would always have been there.

I had vent problems, but it did not matter what the fuel level was.