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2002 Yamaha f115 running rough above 4,000 RPMs

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2002 Yamaha f115 running rough above 4,000 RPMs

Old 03-14-2019, 10:38 PM
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Default 2002 Yamaha f115 running rough above 4,000 RPMs

I have a 2002 Yamaha f115 with only 147 hours. I purchased the boat a year ago and it had been sitting a while according to the owner about 15 months. He claimed to have treated the fuel and that the fuel was 100% ethanol free so I decided to crank it. To my surprise, it fired right up. It ran great for a month or so. Then out of the blue it started running rough at around 4,000 Rpms and fluctuating between 4,000 and 4,500 (running intermittently on 3 cylinders). Knowing the boat had sat up a while, I figured I may as well go through the entire fuel system. I emptied the entire gas tank contents and to my surprise, it was super clean. Like, the cleanest fuel tank I had ever seen. ZERO water since I pumped all the fuel into brand new 5 gallon buckets to watch it separate. The fuel water separator was also flawless. I then started suspecting VST or injectors from reading how common of a problem these are on these engines. I decided I may as well take apart the VST and inspect the filter and I sent my injectors off to be professionally cleaned. They reported back as they looked fine. Again, I wanted to rule out ANY fuel problem. Filter looked brand new in the VST. Then, after talking to a few friends, they informed me of how sensitive the Iridium plugs are to carbon fouling on the 4 strokes. Sure enough, plug 1 looked fouled. As deep as Iím in now, I may as well change them. And it got better! I was about to get into the 5,000 RPMS for the first time! But then, my next trip it started back again. What is interesting is, I pulled plug #1 out and it never looked liked it had ever ďhitĒ before. Didnít have smoke residue on it like the other 3, yet wasnít soaked from fuel. Any ideas on what this could be? I have since swapped my coils around to see if my problem moves to a different cylinder. My compression is also within spec on all cylinders. Iíll report my findings as Iím putting it back in the water soon to do a water test after moving the coils around. Itís really hard to recreate the problem on land without a load. Below are some theories I have kicked around. Am I on target?

Potential Problems:
1) Mechanical Fuel Pump (dried out diaphragm not filling VST tank enough for high pressure pump).
2) Coil? I have the ďbadĒ coil from cylinder 1 switched from away from plug #1 to plug #2. I should see problems on spark plug #2 if this is case.
3) Collapsing fuel line from tank? (Need to check from a different tank/hoses)
4) Fuel Pressure regulator. Should be able to unplug vent line this to see if it resolves the problem?
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Old 03-15-2019, 05:42 AM
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First thing is to verify adequate fuel pressure by having fuel pressure gauge hooked up on injector rail while anomaly is occurring. Should be solid and maintain 38-41 psi all speeds. If fuel pressure remains correct while happening you will have eliminated quite a few things right away. I had an exasperating similar problem at speed on my port motor ('01 LF115TXRZ), ended up being $1300 ECU, ran correctly all demands except at speed after running on plane a while 10-20 miles, would drop 300-500 rpms and no WOT and then come back and run normally very intermittently, felt like a cylinder dropping. Finally flopped ECU's and problem moved to other motor. Also check to make sure all grounds are clean and tight (batteries, ECU harnesses, block, etc.), these motors are notorious for having drivability issues due to poor grounds. Good luck!
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Old 03-15-2019, 03:37 PM
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So my fuel pressure at idle, 3000 RPMs and 4000 rpms is all 36 psi. If I unplug the vacuum line to the fuel pressure regulator the pressure shoots up to 44 pounds and I can tell the motor starts running rich. Do I have a partially obstructed fuel pressure regulator that is dropping my fuel pressure down a little too much, therefor I lose my higher RPMS? Also, we can scratch ECU off too. I swapped it with another ECU and it is doing the same thing.

UPDATE: I replaced my fuel pressure regator with a buddy of mines fuel pressure regulator and my fuel pressure went up from 36 psi to 39 psi. It appears to be the difference I needed. I see a screw at the top, does anyone know if these are adjustable or fixed FPRís?
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Old 03-15-2019, 04:25 PM
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If you checked the rail pressure just at key on, you would see that 44 psi -
because there is no intake manifold vacuum then;
once the engine starts, the regulator drops it to ~36
So sounds like working as intended.
(if you then go WOT the manifold pressure drops, and the rail pressure increases)

As for your running issue,
if you don't see the rail pressure fluctuate/drop at the same time that the engine starts to "act up"
then you don't have a fuel issue.

One long shot possibility comes to mind, since you describe this as
"started running rough at around 4,000 Rpms and fluctuating between 4,000 and 4,500 (running intermittently on 3 cylinders"

there is a microswitch on the shift mechanism in the engine pan -
should be "on" - closed - in neutral
"off" - open - in gear.

I'm not certain on that particular year/model -
but many Yamaha EFI 4-strokes have an ECM function to prevent over-revving in neutral:
if the ECM sees the shift position is "on" it limits rpms to ~4500 rpms -
by causing misfiring to bring the rpms down.

You might look into that switch - try to see if its 'loose", out of place,
or if the shift slide mechanism is out of adjustment -
something that would make that switch close when in gear
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Old 03-15-2019, 07:33 PM
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If you can eliminate the fuel issues then I would look at the electrical systems.

Can you get plugged in with a YDIS system? Maybe it is throwing a code or a sensor is bad. I THINK those engines have knock sensors in the cylinders and it may be causing the engine to cut back at high RPMS.

Other low hanging fruit would be to conduct a spark check, check for loose wires, etc. After that I would go straight to the main wire harness. Seen it before and it took weeks to finally bite the bullet and change them out. Good luck though because it is a pain to remove and install.

If you didn't have that weird spark plug I would say to check the fuel system harder as there may be a clogged filter you didn't check, but from what I heard it may be an electrical issue.

On another note, it is very impressive on how preserved the fuel system was after so many years of not being used. Did the owner empty the VST while in storage? Do regular maintenance yearly even though it wasn't being used? If not, then what the hell was he putting in his fuel to keep things so clean?
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Old 03-15-2019, 08:41 PM
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​​​​​I am going to chase this fuel pressure regulator rabbit for now. The boat runs fine on land now with my friends FPR installed and the additional 3-4 psi it is providing over mine. With my FPR installed, I could hear is sputtering ever so slightly at higher RPMs. Iíll learn more putting a load on the prop with a water test.

In regards to how how he treated and stored his fuel I actually asked him about this recently so I could follow his same footsteps. He was obviously doing something right having never serviced the VST and tank and everything looked great. The boat was seldom used over the years, but never sat for more than 2-3 months without being taken out. It was a freshwater only boat here in the Deep South portion of Alabama where there is no such thing as winterizing and you can boat 9-10 months out of the year. He would only use a tank or 2 of gas (48 gal tank) each year and was the original owner. So, at the end of each season, he would fill his tank to the brim and treat it as 50 gallons with stabil. Less air = less condensation. He also changed his Fuel/water separator every 2 years (only like 10-20 hours of runtime in his case, lol). He even told me that the boat has never even been rained on. Itís lived a very G rated life as far as boats go. It lived under a boat house on a freshwater creek.
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Old 03-16-2019, 04:32 AM
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Originally Posted by Yamaha_Rida View Post
I THINK those engines have knock sensors in the cylinders
Nope.
Not until 2014 with the F115B.
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Old 03-16-2019, 12:07 PM
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Originally Posted by Fwpratt View Post
Nope.
Not until 2014 with the F115B.
So they added the knock sensor and lost the thermo switch? Brilliant
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Old 04-06-2019, 04:39 AM
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Did you ever load test this after swapping the FPR? Iím having a similar issue but believe dirty fuel is clogging up my VST/injectors. I think there is a large amount of sediment in the tank. I removed my fuel/water separator and dumped into a bucket to find a large amount of debris. This was a brand new filter changed at the end of last season. Iíve only done short test runs on the boat this season so minimal amount of fuel actually had passed through the filter at this point. The filter housing on the motor had a dirt in it as well. The filter itself seemed ok.



i plan to clean out my tank, add a second fuel/water separator. My injectors are out for cleaning Iím checking the VST next week.
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Old 04-06-2019, 05:14 AM
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You sure do know your way around an outboard. Iíve been chasing low rpms on my 2003 115s and have one running well but the other is still losing toonend rpms

one thing I see you hadnít addressed is fuel lines. I replaced mine as I was seeing bits of the inner lining accumulate in my engine filter bowls. That does tend to restrict fuel flow. And have you checked the in line filter before the vst tank?
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Old 04-06-2019, 05:20 AM
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Originally Posted by stobra34 View Post
Did you ever load test this after swapping the FPR? Iím having a similar issue but believe dirty fuel is clogging up my VST/injectors. I think there is a large amount of sediment in the tank. I removed my fuel/water separator and dumped into a bucket to find a large amount of debris. This was a brand new filter changed at the end of last season. Iíve only done short test runs on the boat this season so minimal amount of fuel actually had passed through the filter at this point. The filter housing on the motor had a dirt in it as well. The filter itself seemed ok.



i plan to clean out my tank, add a second fuel/water separator. My injectors are out for cleaning Iím checking the VST next week.
Look at the black plastic mount that the fuel/water seperator screws to. Mine had a tiny crack that was only noticed when I was pumping the primer bulb with the cowl off.
At high RPMs it was sucking some air.
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Old 04-06-2019, 05:31 AM
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Originally Posted by muskrattown View Post
Look at the black plastic mount that the fuel/water seperator screws to. Mine had a tiny crack that was only noticed when I was pumping the primer bulb with the cowl off.
At high RPMs it was sucking some air.
Iíll look into it, but I was able to reproduce the problem on portable tank. I ran about 4 miles at 3800-4000 rpms so I should gotten new fuel through the VST. It didnít run any better while pumping the bulb while running so Iím ruling out low pressure fuel pump.
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