Yamaha F70 dies while on step

[CountryRN]

I have a problem with a 2013 yamaha F70. The motor starts and idles fine it will push the boat slowly without any issues. The problem kicks in when running on step.
The motor will shut down while at speed but will crank immediately back up once in neutral. The process will repeat. I do notice that the ball is not as tight as it could be when it shuts down.
I had the controls open a couple of months ago, so I reopened them and actually removed the kill switch to remove the possibility that it was shorting out. All other connections in there look good. I replaced the OLD fuel line from the tank incase it was separating internally causing a blockage. All filters including the VST have been changed, and the fuel is good quality non ethanol gas. But the problem still remains the same. Also there are no computer codes being logged with this problem.
I feel like it may be a fuel pump issue ,but I don't know which I should change first in this troubleshooting process. Both the low pressure pump and the high pressure pump are sporty in price. I would rather buy one that should fix the issue rather than both.
Any suggestions would be greatly appreciated. Also are there any sensors that could cause this issue if they were going bad.
Thanks

 

[GYPSY400]

I was going to recommend replacing the fuel line.. but since you did that, I would recommend testing the fuel pressure.. its starving for fuel.

 

[Pappy]

One is easier than the other.
Pump the bulb at the RPM the problem occurs at. If no change you can try a 6-gallon remote tank to eliminate the boat system as a possibility.
If no change then you will have to bite the bullet and check fuel pressures.
You have checked the gas for water.....right?
You did not state the history on this issue? Was the engine sitting up for a long time? Bad fuel that you changed out or diluted with fresh gas? Water in the gas?

 

[turbotodd]

there was a run of prematurely failing fuel pumps around the tail end of that timeframe. The complaint was exactly as you describe. Dies with load applied, and sometimes intermittently. In order to diagnose, you would need a fuel pressure gauge attached to the rail or better yet a way to datalog fuel pressure.

Barring a supply side (fuel tank pickup, line, filter/separator) issue, I'm leaning toward a fuel pump.

Call a dealer with your serial number and ask them to check if there's any bulletins or recalls for your particular motor's serial number. You might get a surprise.

 

[ktoelke54]

Did you replace the connectors between the fuel line and motor, and line and tank? I had this happen and the problem was a loose connection between the fuel line and motor. Worn seals allowed the motor to suck air instead of fuel and caused it to cutout at full throttle.


Sent from my iPad using Tapatalk

 

[CountryRN]

Thank you guys for the replies. As for the history, the motor does not sit up for long. Typically runs 10 to 11months of the year, mostly in fresh water. It has roughly 225 hours on it total. The problem started up shortly before November of this past year and has progressively gotten worse.
The fuel in the tank is fresh and I run a stabilizer in each tank. There is a new water separator in line, along with a new fuel filter and a new VST filter. As of today, there is a new low pressure fuel pump on there , but the problem still occurred when running at mid throttle. Just as a test, I dropped the hammer on it from a dead stop. She jumped up on plane and ran strong at WOT, bit as soon as I drop it back to mid throttle, she died again. I would assume (probably incorrectly) that if the high pressure pump was going out , that it would not run all out like it did. I did not have the lowrance with NMEA data hooked up today, so as for rpm range when the issue occurs, I don’t know.

Just sitting here scratching my head.

 

[turbotodd]

vst pump failure was random and there was no real pattern. Some would quit at idle, some would always lose power at full throttle and some would run for an hour and then lose fuel pressure. Completely random.

The pump impeller is plastic and is pressed onto a metal shaft. The bond between the two was not good on the original pumps, and the impeller would slip sometimes. Someday, they were GOING to die completely (guaranteed) which is why the bulletin was issued.

I didn't want to mention it because your issue sure sounds like a VST pump, but I did run into one particular F70 that had two separate problems causing similar customer complaint. One, the engine temp sensor was bad. Replaced, sent it out. Back a day or three later with what he called "same problem". Loaded up the laptop and headed to the river. Soon as I get off the trailer and opened 'er up, I noticed the problem immediately. It was severely under propped. It was running over 6500 RPM actually probably would run higher than that but I pulled back to keep from overrev. Underpropping causes overheating. Called customer from the river and asked him how long it's been like that, he said since the day he bought it (about 4 years prior) new. Called the other tech at the shop and said bring a fishing rod and the one 4 blade prop we had in stock. Showed up about 45 min later and we swapped the prop, and headed upriver. Caught a few nice catfish, motor was FLAWLESS at that point. YDIS showed nothing abnormal. Top RPM 6140 which was perfect given the conditions. Now that we narrowed it down, I called the guy back and he said that the people that make the boat told him specifically to run the prop he had on it and that it wasn't the problem. So I told him to come on out to the river and we'd go for a ride. He shows up, brings a "spare" prop with EIGHT inches more pitch and said lets' try this one. What a slug. Wouldn't come out of the hole, and really struggled to maintain 30mph, and within 5 min run time it was showing sign of overheating again. Pulled to a sandbar stuck our 4 blade back on, went for a ride. He was astounded and agreed that the prop was the whole problem (although his temp sensor WAS in fact bad). Problem solved. Still a good customer too, or was, I left the dealer and we haven't stayed in contact. We probably need to get together & go catch a few crappies soon.

so yes, you still could easily have a VST pump issue. Or perhaps a temp sensor. Or prop problem. What's your max RPM at full throttle? Ideally you would need YDIS to properly diagnose but that is a dealer-only deal unfortunately. If you have access to a fuel pressure gauge, you could rig that up.

OH one last thought that just crossed my pea brain. Are your battery cables tight and clean? I've seen that become an issue on motorcycles, specifically EFI bikes....even if they're just a little bit loose or dirty. Use nylock nuts on them not wingnuts. Wingnuts rarely stay tight.