Yamaha F115 engines are haunted

Scott0301

Cadet
Joined
May 26, 2012
Messages
7
I have a pair of 2005 F115 4-strokes that have been driving me to distraction with an intermittent fuel system problem. Both engines have 1125 hours and run like new across the whole RPM range except once in a while one or the other engine will suddenly bog down, sputter and spit for about 10 seconds and then quit. They will sometimes go for months without doing this, then do it 5 times in one day. It never happens to both engines at the same time. It is obviously a fuel supply issue because if I am quick enough to run back and squeeze the primer bulb when the engine starts to bog, it will rev right back up and run fine again, sometimes for a few hours, sometimes for weeks with no problems. This can happen from any RPM from idle right on up to 5600 RPM with no warning, usually after I've been cruising along fine at a steady speed for quite some time. I have replaced the entire fuel system from the tank up the the low pressure fuel pumps, including anti-siphon valves, fuel lines, primer bulbs, filters, and all fittings, and checked that the tank vent was working. Thought I had found the problem when I pulled the anti-siphon valves and both were leaking, put in new ones and both engines ran flawlessly for 6 months, but this past week the problem came back again. I put in another pair of new anti-siphon valves and a new low pressure fuel pump on one engine but that had no effect either. When I take the boat in to have the problem diagnosed, they can't find anything wrong because we can't make the problem occur on demand.



Seems to me it has to be a systemic problem since both engines are doing exactly the same thing, although the port engine does it about 10x as often as the starboard engine. I inspected the inside of the fuel tank with an endoscope to see if something might be periodically blocking the fuel outlet pipes, but it looks clean as a whistle (welded stainless steel tank). It acts like an air leak, but we can't find any place where air seems to be leaking in, and why would a leak only be a problem on the second Tuesday of each month?



Some of the suggestions I've gotten have been dirty VST filter, clogged injectors, bad fuel, bad HP fuel pump, but none of these passes the logic test for me. If you've got clogged filters or injectors, it for sure will make the engine run poorly, but it will run poorly all the time since these are steady state problems. These engines run like Swiss watches 98% of the time and I don't see how they could be doing that if something in the fuel system was clogged badly enough to cause stalling. I'll probably do the VST and injector cleaning anyway, just to see what happens, but I don't expect it to solve this problem.



Also had someone suggest an intermittently bad connection between the ECM and one of the sensors causing the stall, but I don't see how that could be stopped in its tracks by a few squeezes of the primer bulb.



Did I mention that this only happens on the when we have friends or family out on the boat? Somehow the engines know.



Getting desperate, any suggestions short of finding a real deep spot and pushing these demon engines overboard?



It's awfully hard to diagnose a problem when you can't make it happen.



(I posted about this a couple of years back when it first started happening on one engine but thought I'd take one more shot)



Scott
 

lakensea

Chief Petty Officer
Joined
Jan 30, 2002
Messages
542
Have you checked the fuel pick-ups in the tank? I've seen cracks and also seen the tubes loosen up at the top, both of which will cause an air leak. Also, what type of clamps are you using on the primer bulbs?
 

Scott0301

Cadet
Joined
May 26, 2012
Messages
7
I checked the welds on the pickup tubes and the look solid, no visible cracks. I replaced the spring clamps with stainless hose band clamps. Pretty sure they are not leaking. The bulb stiffens up when pumped and doesn't seem to leak back.
 

Scott0301

Cadet
Joined
May 26, 2012
Messages
7
I keep coming back to the fact that an air leak should affect performance all the time, not just once every 5-10 trips out. That's what's throwing me, the randomness of it.
 
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