Fuelling issue

[Flipfly]

Let's see what the collective think.

fullntank of fuel and the bikenwillnsitbhappilynandncruise steadily at 80-85 no issues. When I get down to 3 bars that all changes and the bike will still sit at 80-85 and accelerate slowly to past 95, but it feels like it's starved of fuel.

The fuel filter has been changed, the breather is free breathing but it is an ongoing issue from last year.

obviously the answer is to fill it up more often But with an impending trip to Almeria in a couple of weeks, a solution would be better.

My thinking is possible water in the fuel or a tangle of the fuel pick up line (when I replaced the fuel filter) meaning it doesn't pick up the fuel cleanly when the tank is low.

Any other suggestions please?

 

[MattW]

As you say, sounds like fuel starvation when there is less head over the pump. To me this indicates a partial blockage on the intake side of the pump or a failing fuel pump.

I'd have the tank off and the internals out for a look. If all the fuel lines appear ok then I'd suspect either a faulty filter or the pump (is the pump any noisier than normal?).

In your position, I might get one of those cheap Audi pumps that people have written about on here and take it with me...

 

[topcat2006]

Split in fuel filter housing or hose which is uncovered when the tank level drops?

 

[Slipperyeel]

I had this on my ST many moons ago. Eventually I had to replace the fuel cap and all was sorted. I'm not exactly sure how the fuel cap works on the GS but before you dive into the tank internals maybe take the filter cap apart (quite straightforward iIRC). Yes there is a breather hose but the cap is often also involved in the pressure equalisation as well. Someone may correct me of course. Just sharing my thoughts! Let us know if you find a fix.

 

[Timolgra]

I had a single spark that did something like that, changing a coil and HT lead sorted it.

 

[MattW]

Wouldn't ignition issues manifest all the time, not only when the tank runs low?...

 

[Flipfly]

Wouldn't ignition issues manifest all the time, not only when the tank runs low?...

That's what lead me to believe it was fuel related.

 

[Timolgra]

Wouldn't ignition issues manifest all the time, not only when the tank runs low?...

Not on mine obviously, HT lead was breaking down at high revs. Not saying it is that but worth considering ....... before you fit a new fuel pump I have a spare pump if you want to try it.

 

[Steptoe]

Split in fuel filter housing or hose which is uncovered when the tank level drops?

So how does having standing fuel covering a split in a high pressure line keep the internal fuel pressure at 2.5 bar ??

 

[Steptoe]

My thinking is possible water in the fuel or a tangle of the fuel pick up line (when I replaced the fuel filter) meaning it doesn't pick up the fuel cleanly when the tank is low.

Yes, possibly water in the tank. Usually felt most after being parked on the side stand causing the water to flow to the L/H side of the tank, and once under way the water slowly creeps to the R/H side as it levels out, causing the problem you have as the pump starts to pick up the water.
Because of the delay before the symptoms start, i.e. can happen once the temp reaches three bars, people assume it's to do the engine heating up...... I've had a few bikes in over the years and have been told by the owners it only happens once it warms up. First question i ask is "do you park it on the side stand" If they say "yes", i then tell them the i'll have your problem fixed in 10 mins ;D




Also if the fuel line has twisted causing a restriction the fuel would be very aerated causing the effect you're suffering from, but i suspect that would be all the time rather than only when the fuel is low.

 

[earthmover]

My thinking is possible water in the fuel

After Saturday, most likely!
Mark

 

[topcat2006]

So how does having standing fuel covering a split in a high pressure line keep the internal fuel pressure at 2.5 bar ??

Good point, well made and why you have a business repairing BMW's and I don't.....

 

[Steptoe]

Good point, well made and why you have a business repairing BMW's and I don't.....

Glad to see you've taken it in the spirit is was posted. Rather than miss the point and get all stroppy like many

 

[Flipfly]

Yes, possibly water in the tank. Usually felt most after being parked on the side stand causing the water to flow to the L/H side of the tank, and once under way the water slowly creeps to the R/H side as it levels out, causing the problem you have as the pump starts to pick up the water.
Because of the delay before the symptoms start, i.e. can happen once the temp reaches three bars, people assume it's to do the engine heating up...... I've had a few bikes in over the years and have been told by the owners it only happens once it warms up. First question i ask is "do you park it on the side stand" If they say "yes", i then tell them the i'll have your problem fixed in 10 mins ;D




Also if the fuel line has twisted causing a restriction the fuel would be very aerated causing the effect you're suffering from, but i suspect that would be all the time rather than only when the fuel is low.

I only ever park on the centre stand cause I'm all manly like

Thinking about it it has done this for a while because it did it before the fuel filter change which was done on an empty tank so the water theory may be wrong. Maybe a lazy pump but the other week I accelerated way past 80 mph after a long ride without issue,so it seems to deliver for short periods but constant speed is what causes the issue. Mind I had a decent 1/2 tank of fuel then.

Would a poor connection between the fuel pick up and pump possibly draw air on low tank levels causing poor fuel pressure?

 

[MikeO]

Easy to check if it's an issue with the fuel cap -put a spare key in it and, when the symptoms appear, release the cap. If they clear, you have the culprit - if not you've eliminated a possibility...

 

[Slipperyeel]

Easy to check if it's an issue with the fuel cap -put a spare key in it and, when the symptoms appear, release the cap. If they clear, you have the culprit - if not you've eliminated a possibility...

Good idea. And reminds me that I had to ride with the fuel cap undone on long journeys to prevent a big vacuum being created. It was confirmation the fuel cap was the culprit...