Yamaha engine unstable RPMs

[Boatproblems99]

I have a 2019 C302 Coupe with twin Yamaha 300 outboards. Recently I've been having issues with the starboard engine losing RPM's. Cruising along at 4,500 rpm on plane and anywhere between 5 mins and 30 mins the starboard engine starts dropping rpms. i.e. cruising at 4,500 rpms, it will all of a sudden drop to 4,000 then back to 4,500 then down to 4,100 then back to 4,500 then down to 3,800, then back to 4,500 then down to 3,900 etc. This continues until I slow the rpms down to say 4,000 where it remains stable (at least for an extended period of time) then the problem sometimes reoccurs at 4,000 rpm.

The issue also seems to occur when throttling up after running at low rpm for an extended period of time. i.e. rough water and slow crossing (close to 2 hours) from Vancouver Harbour to the gulf islands then throttle up in smooth protected waters.

I've had the injectors and fuel rails cleaned. Checked/replace fuel pump. No luck. But really seems like a fuel starvation issue.

Any ideas?

 

[Ralf H]

Hi there,

I agree that you may have some fuel delivery issues. Considering what has already been done, I would then check the fuel lines, especially the pickup tube in the starboard tank. Check to make sure that the pickup is not clogged and that the screen (or anything else, for that matter) hasn't fallen into the tank. Check all the fuel lines for kinks, or any hose clamps that could potentially not be correctly on the barb and restricting fuel flow. Check the fuel filter and then continue to follow the lines aft to the rigging tube and engine to ensure that nothing has somehow become kinked or crushed. Hope that gets you started but let us know if you have any more questions.

Cheers!

Ralf

 

[Boatproblems99]

Hi there,

I agree that you may have some fuel delivery issues. Considering what has already been done, I would then check the fuel lines, especially the pickup tube in the starboard tank. Check to make sure that the pickup is not clogged and that the screen (or anything else, for that matter) hasn't fallen into the tank. Check all the fuel lines for kinks, or any hose clamps that could potentially not be correctly on the barb and restricting fuel flow. Check the fuel filter and then continue to follow the lines aft to the rigging tube and engine to ensure that nothing has somehow become kinked or crushed. Hope that gets you started but let us know if you have any more questions.

Cheers!

Ralf

Hi There,

Thanks. Ok, tried that. No luck. Ran the engine on day tanks, replace fuel filter, replaced fuel pump. No luck.

Had the boat out on Friday (over the weekend). Friday, the apparent starvation happened within 15 mins and had to back down to 3,800 rpm (approx. 15 kts). Spent all day Saturday on the water (granted fishing at a low rpm of 700) but no issues. Spent most of Sunday fishing (again low RPM) and then ran in to my marina (approx. 1 hour at 4,100 rpm) and had one hiccup at approx. 40 minute mark. I backed down to 3,900 for about a minute then throttled up again to 4,100 and it ran fine. That said, the starboard rpms are far more unstable than the port rpms. i.e. Port motor will run at 4,100 and occasionally flicker up to 4,200 or down to 4,000 but the starboard motor is in almost constant flux between 3,900 and 4,400.

Small fluctuations, I don't care about, but when the starboard motor drops off substantially you can definitely feel it, the speed drops and the flex on the motor mounts is huge.

It seems to be temperature and/or stress related. i.e. happens more frequently in warm weather. Happens when warmed up and generally not when initially started. Once it starts happening, it is nearly impossible to stop unless I shut the boat off for a while. I initially thought overheating fuel pump but changing it doesn't seem to help.

Any other thoughts?

Thanks

 

[Ralf H]

Hi again,

I'm starting to wonder if the issue isn't with fuel quality. You may have had some water get into the fuel on one tank at some point. If that's not the issue it would defnitely be time to give Yamaha a ring and see what they think. Other than what you had done, have any of their techs done a ruanbility diagnosis?

Thanks,

Ralf

 

[Boatproblems99]

Hi again,

I'm starting to wonder if the issue isn't with fuel quality. You may have had some water get into the fuel on one tank at some point. If that's not the issue it would defnitely be time to give Yamaha a ring and see what they think. Other than what you had done, have any of their techs done a ruanbility diagnosis?

Thanks,

Ralf

Thanks Ralf,

Yes. In fact I've had it at a certified Yamaha tech and out of the water for a month while they worked on it. They did all the replacements and cleaning and everything as I wanted a certified person working on it. They have called Yamaha and replaced pieces based on Yamaha recommendations. The issue is random other than seems to occur when something heats up (i.e. I can run at low RPM for an hour or so and as soon as I throttle up, it happens).

The timeframe is strange too. Sometimes it is within 15 mins. Other times it runs for hours before happening. The one consistent is that it seems to happen when the RPM is over 4,300 - 4,500. The other trend that I'm seeing is if (regardless of warm up) I throttle up right away it happens sooner. If I ease in over say 5 mins, I generally get some grace from it happening. My tech keeps thinking it is electronics and/or wiring, but short of stripping it, it is tough to test.

I'm at my wits end. I've spent a fortune trying to fix it.

 

[Ralf H]

Hi,

Ok, one thing I would like to explore more, just to make sure that we can eliminate it as a potential issue, is the fuel pickup. I re-read the thread just to be sure it wasn't already covered, but didn't see any mention of whether the pickup was checked. It should be pulled and made sure that the screen at the bottom is firmly in place and that there is nothing inside the tube blocking it. It may be electrical in the end, as in your suspicion, but I really want to make sure that this is eliminated from the equation. Good news is that it isn't difficult to do. The pickup can be easily unbolted from the tank and pulled for inspection. If you feel up to the task, I'm sure you can do it in a few minutes yourself. Let me know and I'll provide support to help you get it done. Once we know where that stands, we can move forward.

Also, one other thing that we should also do is to look at the check valve on top of the tank to make sure that it is not failing intermittently. Let's look at the pickup tibe first and we'll discuss this more afterward if necessary.

Cheers!

Ralf