Fluctuation in RPM's

[Robertjohn]

Hi everyone. This one has me, and the mechanic, scratching our heads. I have a 2000 268 Islander with a 2000 Yamaha 250 2 stroke. When I run the boat trimmed out at say 5000 RPM's for about 10-13 miles the rpm's will all of a sudden start to fluctuate from 5000 down to 4700, back to 5000 and drop to 4800, drop to 4600 and then go back up. When this happens she is unresponsive to me giving more throttle and the rpm's will just drop. This will continue until I drop into neutral and sit at idle for about five minutes, then get it back up on plane for another similar run and the same thing happens. For that 10-13 miles she runs perfect with no fluctuation at all. Here is what we have done so far trying to rectify the problem. I should mention that over the winter all the injectors were pulled and sent out to be professionally cleaned and the engine has been professionally maintained since I have owned it. It has approximately 800 hours on it. All fuel is fresh and treated.

Checked all cylinders at dock, all were firing but replaced one questionable coil
took mechanic out on the boat and when the issue came up, shut down and hooked up a 6 gallon portable tank. Ran great till it emptied. Connected main tank and it bogged down again.
Replaced all fuel lines from the tank to the engine. Thinking we had here was that maybe they were deteriorated from the inside and collapsing and pinching the fuel supply.
After replacing all fuel lines I took it out today and ran it 40 miles and still had the same issue three times.

I am not a mechanic so I am reaching out to those of you that are more experienced with issues than I am, I would welcome suggestions and thoughts form members. Thanks

 

[Jfchampl]

I am not a mechanic and just my 2 cents from experience...what is the condition of the fuel in the boat main tank? Has it been sitting for awhile? Rec 90 or ethanol? What type of fuel was in the portable tank during the test and how fresh was it? Have you checked or replaced the fuel/water separator filter and engine fuel filter? Even though you replaced all the fuel lines, the items above could be impacting performance compared to the portable tank direct hook up test. Also what is the condition of the fuel tank vent?

 

[Robertjohn]

Thanks for the response. only one tank and it holds about 150 gals, all fresh within the last 3 weeks. All filters have been replaced at the beginning of the season and have been checked this past week, fuel is 10% ethanol as was the fuel in the portable tank,

 

[DennisG01]

Not a mechanic here, either, but I'd probably start with cleaning the VST out - at least check it out - clean the VST filter. If it's still happening, put a fuel pressure gauge on the pump and run it. Observe when the issue happens. Ask your mechanic buddy - this is a pretty easy and straight forward thing and he would (should) know what to do.

 

[Fishermanbb]

Could be one bad spark plug. Happens all the time. They will look/act fine at the dock and even run fine most of the time but when they get hot, etc.....They start to fail....cool them down...work fine. If you haven't already....Try replacing them - You can even do it one by one. If you have replaced them.....Doesn't mean the new ones weren't bad.....That happened to me. You can also check those by replacing one by one or seeing if the computer is giving you a misfire code.

 

[seasick]

First place to check as mentioned is VST screen.

 

[Andrew93]

I was fighting some of the same issues as you last year. I did all of the cheaper and easy stuff first like the VST and all of the filters. What mine ended up being was the O2 sensor. It was the most expensive so I did all of the filters first.

 

[Harpoon]

You mentioned you couldn't re create the problem on the auxiliary tank. It sure sounds like you've isolated it to bad or wet fuel, clogged pick up or anti siphon.

Routine cleaning of 02 sensor and VST are an important part of maintenance on that motor also, so there is no downside.

 

[DennisG01]

Along the lines of what Harpoon said... when it was on the 6-gallon tank, did it run long enough to where you would have expected to see the issue pop up? When I read your initial post, I took it to mean that you didn't run in long enough on the portable tank to where you might expect to see the issue. If that's not the case, it could be as simple as a clogged vent or anti-siphon valve. When you idle down (less draw) and stop for a bit, it allows enough air in to get you going again. If these were the case, the primer bulb would likely be going flat. But you could try running with the fuel cap off (protect from water spray, obviously) and/or cleaning out the anti-siphon valve.

 

[Parthery]

You mentioned you couldn't re create the problem on the auxiliary tank. It sure sounds like you've isolated it to bad or wet fuel, clogged pick up or anti siphon.

Routine cleaning of 02 sensor and VST are an important part of maintenance on that motor also, so there is no downside.

X2. Sounds a lot like an anti-siphon valve.

 

[seasick]

You mentioned you couldn't re create the problem on the auxiliary tank. It sure sounds like you've isolated it to bad or wet fuel, clogged pick up or anti siphon.

Routine cleaning of 02 sensor and VST are an important part of maintenance on that motor also, so there is no downside.

X2. Sounds a lot like an anti-siphon valve.

If it is a restricted anti-siphon valve, restricted hose or a blocked vent, I would expect the primer bulb will get soft or collapse when the rev drop occurs (assuming it is related to fuel flow).

If the problem happens more often in rougher seas it is probably the VST filter. I don't fully understand why that is but I have seen it first hand.

I also don't fully understand how a bad O2 sensor can cause the rev drop. The O2 sensor is uses as one of many data inputs to the the ECU. A clogged O2 sensor will result in an overly rich fuel mix. An open O2 sensor can cause a lean fuel mix, a more serious situation.
So if the O2 is not working, I would expect poorer or rough operation but not the rev drop and rise as mentioned.
Then again, I have been wrong so many times. another one isn't a tragedy

 

[Robertjohn]

Thanks everyone for your responses. Update...I ran over 90 miles since Sunday and had minimal issues which showed significant improvement since replacing all fuel lines. Yesterday I ran her 60 miles at a steady 4800 rpms and did not see a drop for about 45 miles then she dropped 400 rpms and stayed at 4400 until I stopped and idled for 5 minutes then she was back up to steady 4800 for the remaining trip. Mechanic thinks there maybe something in the bottom of the tank getting picked up and then falling off when idling.

 

[Robertjohn]

I know this is a little late after the original post but I wanted to let you all know the problem ultimately was resolved. I had the o2 sensor replaced and in the process they found a small section of fuel line that was deteriorate. Since replacing that and the 02sensor I ran the boat for 30 hours with no issue in rpms or speed.