F115 Runs Rough at High RPM

[Cascade Cruiser]

I'm a relatively novice boat owner in SE Alaska and appreciate any help that the forum can provide. I have a 22' Hewescraft Searunner with a 2006 Yamaha F115 outboard at ~1000 hrs. The motor runs rough above about 4300 rpm. It accelerates and cruises just fine up to this rpm, but attempting to accelerate beyond this rpm results in the engine sounding like it's hesitating or misfiring and fluctuating between about 4400 and 4800 rpm. Before this issue cropped up the motor would run fine up to and exceeding 5000 rpm.

Over the past few months I've replaced the following items in an attempt to troubleshoot:

• Racor fuel filter
• Primary engine fuel filter
• Inline fuel filter between low pressure pump and VST
• VST screen
• Fuel injectors (cleaned and tested)
• Pressure Regulator Valve on fuel rail
• High pressure fuel lines
• Spark plugs

Fuel filters were replaced starting with the Racor and ending with the fuel injector service. The VST screen and primary fuel filter looked clean and the person who serviced the fuel injectors said they were in good shape even before the cleaning. I've pumped the priming bulb while the engine was running/hesitating and it did not have any effect. Prop does not appear to be slipping.

Next up I plan to test the fuel pressure at the fuel rail and possibly run the motor on a spare fuel tank to see if that changes anything. May also disconnect spark plugs and fuel injectors one at a time to see it I can isolate a cylinder-specific issue. Thanks for any suggestions!

[marchmadness]

Seems like you covered a lot . Just my opinion but I would do a compression test to see the health of the motor. Also check all fuel lines and clamps for air leaks. Is the oil level correct, simple but to much oil can cause problems.

[99yam40]

monitoring fuel pressure would be the 1st thing I would do to make sure that stays in spec

[Cascade Cruiser]

Thanks for the replies. I will plan to do the fuel pressure test, I just need to track down a gauge with hose long enough to keep the cowling on when I'm running at speed. Agreed with the compression test as well. The high pressure fuel lines were replaced and some of the low pressure lines when I installed a new fitting for the Racor last year, but I will take another look at the lines, particularly the low pressure side. I have not opened the fuel tank either, I should probably take a look at the pickup to see if it is restricted.

More info on the symptoms that I was thinking about last night....the engine ran fine for 2-3 minutes at 4800 rpm when I first took it out on the water after the VST and fuel injector service (long enough to make me celebrate fixing the problem!) Then the issues started happening and it wouldn't really turn faster than 4300 or 4400 rpm. I'm wondering if the low pressure fuel pump is going bad (or there is a restriction in the lines from the tank) that is not keeping the VST full at high rpms. I'll try shutting the engine down, or at least to idle, after the symptoms occur and pumping the priming bulb until firm to see if I can get another 2-3 minutes of smooth running before the problem comes back.

Thanks again for the insight.

[99yam40]

Sucking air is a possibility.
just enough to make the VST to run low only at high RPM

[Cascade Cruiser]

Well, I ran it today and have more questions than answers. Slow cruise out out of the marina at about 1000 rpm then accelerated smoothly and ran at 5400 rpm for 5 minutes with no problems. Brought it back to idle, turned around to run in the other direction and accelerated again...initially it ran up above 5000 rpm but the hesitation/misfire started to creep in again and within a few minutes it would not run above about 4300-4500 rpm. I shut the engine off and checked the primer bulb, still relatively firm and multiple squeezes only made it slightly firmer. Started and accelerated and immediately hit the "limit" at around 4300 rpm. So I'm thinking my theory that the VST was not keeping full is probably incorrect. Next step will be to hook up a fuel pressure gauge while running.

The baffling thing is that it ran great at 5400 rpm for 5 minutes without any signs of malfunction. It seems like if I had any fuel flow restrictions or issues with the fuel pumps that this would not be possible.

[Cascade Cruiser]

UPDATE...still stumped! After two months of winter weather I was finally able to get back out on the water today. New parts include ignition coils/plug wires and new fuel lines from the fuel tank to the motor including a new primer bulb. The boat ran great at 4800 rpm for 5 minutes then the misfire began creeping in just as it did before...almost at an identical spot on the water away from the marina. I hooked up a fuel pressure gauge to the fuel rail and it showed 37 psi at idle and remained steady at about 42 psi while running at speed, including while the engine was running rough. I'm starting to doubt a fuel issue since I've replaced/cleaned about everything on the fuel system and the fuel pressure holds steady.

The identical onset of the symptoms between the last two sea trials (5 min idle out of the marina, 5 min high rpm cruise before symptoms appear) now has me thinking this is temperature related. The engine didn't seem overly hot when I put the fuel px gauge on and I have not received any overtemp indications. Thermostat was replaced in December.

Any ideas? Still haven't checked the compression and don't have the test leads to check any of the engine sensors. Probably time to take it to the Yamaha mechanic.

[MoDaddy71]

I would love to know the solution. I have a 1997 Yamaha 150 that has very similar problems and I am baffled.

[Cascade Cruiser]

Ran the motor boat today with YDS 1.33 hooked up. Ran rough as usual above about 4300 rpm. Water temp sensor showed about 120 F for the duration of the testing, which looks like it aligns with the temp at which the thermostat starts to open. Throttle position sensor voltage was within range and appeared to correspond correctly with throttle application and throttle valve position. IAC valve commanded position ranged from 40-50% at lower rpms to 90%+ above 4000 rpm. Timing seemed to fluctuate a bit, not sure if this is normal. At 750 rpm, timing ranged from TDC to 5 ATDC. At 1400 rpm it was TDC +/- 1. While the engine was running rough between about 4300 rpm and 4600 rpm, timing fluctuated between 19-21 BTDC. Fuel injector duration ranged from about 2-2.5 ms at lower rpm to 5-6 ms above 4000 rpm. I also did the active IAC test at idle, rpm increased when IAC opened to 100% and I could hear the clatter when it closed and rpm came down.

Appreciate any insight if these numbers appear abnormal.

[Cascade Cruiser]

Problem solved...see this thread:

https://www.yamahaoutboardparts.com/...eshooting-help

[TownsendsFJR1300]

Problem solved...see this thread:

https://www.yamahaoutboardparts.com/...eshooting-help

Congrates, glad it's fixed!

For others:

I believe I have solved my problem. Last month I discovered my batteries were no longer charging. Voltage rectifier/regulator was fried along with a few pins at its connector. I replaced the rectifier and connector...it fixed my charging problem but not the overall misfire. However I kept at the electrical troubleshooting and decided to swap in a borrowed ECU. The motor ran great on the borrowed ECU for 15 to 20 minutes with no sign of misfire at any rpm. Today I ran it again just to be sure. Ran great on the borrowed ECU, then swapped in my old ECU and the misfire came back after 2.5 minutes...I assume because it heats up to a temperature where it starts to fail. As to the overall cause of the bad ECU, I suppose it may have been related to the fried rectifier/regulator and perhaps some battery wiring that I cleaned up over a year ago. As an aside, I've read about stuck ISC valves causing ECU's to go bad, but I think my ISC is fine as I can hear it clatter every time the motor shuts down and rpm's at startup and idle behave appropriately. Anyhow, nice to have closure on this one.