Oddball Fuel Issues

[lgusto]

Hi All,

We're off to a fast start this season in Maine but I have an engine dilemma. My 2005 Yamaha F250 has 1,600 hours, has always been well maintained, and until now performed flawlessly. But.....twice this year I've left the boat for 3-4 days and when I've come back it won't start or run at idle. It runs fine at higher RPMs but dies when pulling back to idle. This goes on for about 20-30 minutes and then everything goes back to normal, i.e. starts fine at idle and runs fine at idle.

Theories? My only guess is that there's water in the fuel (despite regular overdosing on Startron) and condensation builds up over the course of a few days. The water then "burns off" after 20-30 minutes.

I know there's a ton of physicists and engineers out there way smarter than me. Any ideas?

Thanks in advance,

Larry

 

[seasick]

Hi All,

We're off to a fast start this season in Maine but I have an engine dilemma. My 2005 Yamaha F250 has 1,600 hours, has always been well maintained, and until now performed flawlessly. But.....twice this year I've left the boat for 3-4 days and when I've come back it won't start or run at idle. It runs fine at higher RPMs but dies when pulling back to idle. This goes on for about 20-30 minutes and then everything goes back to normal, i.e. starts fine at idle and runs fine at idle.

Theories? My only guess is that there's water in the fuel (despite regular overdosing on Startron) and condensation builds up over the course of a few days. The water then "burns off" after 20-30 minutes.

I know there's a ton of physicists and engineers out there way smarter than me. Any ideas?

Thanks in advance,

Larry

It's not condensation. I doubt it has anything to do with water. It sounds like the motor needs to warm up to run correctly and that would hint at a fuel mixture issue or a fuel air leak that seals up as things get warm and expand.
Try pumping the primer while starting and initially running and see what happens.
If it is a mixture issue, that would indicate a possible bad sensor

 

[lgusto]

Seasick,

Thanks for the reply. Pumping the primer bulb doesn't seem to help it run any better. But the bulb is a bit softer than I remember. Would that suggest a fuel air leak? Alternatively, if it was a bad sensor would it come and go, or just fail and stay bad? Many mornings the engine starts and runs fine.

thanks again.

 

[seasick]

Seasick,

Thanks for the reply. Pumping the primer bulb doesn't seem to help it run any better. But the bulb is a bit softer than I remember. Would that suggest a fuel air leak? Alternatively, if it was a bad sensor would it come and go, or just fail and stay bad? Many mornings the engine starts and runs fine.

thanks again.

You might want to take note of the outside temperature on the days it starts and doesn't start to see if there is a correlation with temp.
The sensor could be bad and think the motor(or air) is warmer than it really is. That would cause the ECU to make the mixture leaner than needed for a cold start.
Of course, the comment about the softer bulb could point to a few things: Bad fuel pumps are a possibility. You may need to get a high pressure fuel pressure test done. An air leak at the water separator or another upstream fitting is also a possibility. The soft bulb means that either the motor is not holding pressure or air is getting sucked into the fuel system before the bulb.
Go easy with that Startron. Too much is not necessarily a good thing.

 

[Doc Stressor]

You have to first suspect the fuel system whenever you have that kind of problem.

Things to consider:

1) Dump your external fuel filter/water separator and the first internal filter in the engine and look for water. Startron doesn't do much for water in the fuel regardless of what they advertise. No additive can prevent phase separation except those that contain alcohol or glycol and those only have a marginal effect. Condensation is always a problem, especially if you don't keep your tank full.

2) Clean the VST and its internal filter. That's a common problem. Ethanol fuel can cause corrosion and salt precipitation inside the tank itself downstream of the fuel filters. This can partially clog the VST filter (and also your injectors) and can cause intermittent problems before you see a major lose of power.

3. Replace the fuel hose and primer bulb. They are supposed to be ethanol resistant, but nothing lasts forever. The polyurethane liner of some fuel hoses tends to flake off and clog filters over time.

Doing these things is just general maintenance for engines over 5 years old these days. If that doesn't fix your problem, you should have a tec check the engine out for injector or electrical problems.

 

[NikM820]

X2 on the vst.

 

[seasick]

X2 on the vst.

VST issue usually don't go away when the motor warms up. Neither do water issues. Since his problem happens only with a cold motor, I would suspect something else. The soft bulb is probably the best clue at this time. If he disconnects the output of the bulb and plugs the hose, he can test if there is a leak somewhere. If with the output (motor side) plugged, the bulb stays soft when pumped, there is an issue behind the bulb. It could be as simple as a poorly seated water separator.
Ig the bub gets firm but doesn't when reconnected to the motor, the problem is upstream. Poorly seated motor gas filter cups are a common problem on the F model engines. If the clear bowl fills all the way with fuel, it has a leak. Normally it contains gas and an air pocket.

 

[family affair]

Intermittent issues can be a nightmare to track down. Seasick might be onto something. Before I would go crazy testing theories, I would start from the plugs when it isn't running and check for spark and gas.

 

[seasick]

X2 on the vst.

VST issue usually don't go away when the motor warms up. Neither do water issues. Since his problem happens only with a cold motor, I would suspect something else. The soft bulb is probably the best clue at this time. If he disconnects the output of the bulb and plugs the hose, he can test if there is a leak somewhere. If with the output (motor side) plugged, the bulb stays soft when pumped, there is an issue behind the bulb. It could be as simple as a poorly seated water separator.
Ig the bub gets firm but doesn't when reconnected to the motor, the problem is upstream. Poorly seated motor gas filter cups are a common problem on the F model engines. If the clear bowl fills all the way with fuel, it has a leak. Normally it contains gas and an air pocket.

I left out another condition. Of course if the clear bowl is empty, you probably also have an air leak or a fuel blockage. If the blockage is behind the primer, the bulb will collapse.

 

[Tashmoo]

I would consider the fuel pumps in this equation. I replaced mine at 1,200 hours and it got rid of a number of borderline issues, the engine ran smoother, idled better and responded better on acceleration. Just a thought.

 

[lgusto]

Thank you guys. We'll be exploring all the suggestions, in descending cost order, and report back.

 

[lgusto]

Update.......have not had any more trouble with stalling at idle. Boat seemed to fix itself. Can only surmise that it was bad fuel but never found any water in the separators.