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BF90D running issues, surging or stumbling at mid range

M

MATPOC

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New to me 2017 BF90DK4, starts and runs fine, runs great at WOT around 6,000 rpm, accelerates fine from idle or near idle and from above 4,000 rpm, no hesitation, however at around 2,000 rpm it starts surging, about 10 second intervals it drops about 300 rpm and slowly picks up, holds RPM for 5-10 seconds and drops again, slowly accelerates and repeats, no problem at 3,000 rpm, then same problem comes back around 4,000 rpm, like someone is playing with a throttle, goes away above 4,500 and no issues all the way to full throttle.
Another issue is the acceleration, same window, above 2,000 below 4,000 if I try to accelerate there is a hick up, feels almost like a bang, no problem below 2k or above 4k, I can accelerate from stand still and it's smooth, but if I'm running 3k and give it gas it has a very noticeable jump, it bucks, and then accelerates hard no issues.
No waning lights (this one has 3 red and 1 green bulb) I got new plugs coming an will do compression test as well. New fuel filter, pretty sure it's not fuel related because of the narrow range where the problems occur, if it was starving it wouldn't be running that well above 4k.
 
Hi,
Please know I have no experience with this outboard. Just speculating on the symptoms you have listed.

If at all possible, I think you would be best off getting a scan with the Dr. H software at a competent shop. But I know circumstances don't always allow for perfection so I'll offer some guesses.

Based on what you've said, it sounds like there could be a bad spot or two in the Throttle Position Sensor. Often referred to as a "glitch". This often appears as a reliably repeating issue such as you've described.

The TPS can have bad spots in it, cause problems and not trigger an engine warning or log a code.

A GOOD mechanic that KNOWS what he's doing, can interrogate the TPS with a labscope. Ocasionally faults can be seen with an ohmmeter but an oscillocope is much faster and more likely to capture what is actually going on. Even the engine ECU, hooked to Dr.H, might not catch what the O'scope can see.

This might also be caused by a dirty throttle body. Cleaning it is something that can be done as part of a process of elimination but specific precautions need to be observed when doing that.

It apperars that the MAP sensor is mounted directly to the TB so its not unreasonable to think that dirt in that area could be interferring with the sensor's ability to provide accurate load information. That can "confuse" the ECU and adversely effect fuel delivery.

It's never good to have preconcieved ideas or get "tunnel vision" about what's happening when trying to diagnose a problem like this. These outboards can create some intense vibrations at various rpms while running and a poor connection or comprimised wire could be affected. If this occured in the fuel pump circuit, for instance, it COULD create a fuel delivery issue at varying speeds. I don't know. Again, just speculation. But testing fuel pressure during the occurance should not be ruled out.

Just a couple of thoughts.
I hope you can get it sorted without too much pain.

Good luck.
 
I agree, A scan is likely the best option to start with.

Do the plugs look clean? How many hours on it? Do you spend a lot of time at low rpm or idle?

What is the quality of your fuel?

My first thought from a common sense point of view is dirty fuel system (varnish from running at low rpm) or valves needing adjustment. Or bad fuel- do you have a large internal tank?

You could run a fresh portable tank of fuel, high test and ethanol free if available, with a high dose of something like Sea Foam in it. Burn a tank of that at high rpm, 4000 or above, see if anything changes.
 
First of all, appreciate the replies, really miss old days when online forums were a place for exchange of information and ideas, unlike current social media that is mostly clickbait.
Little more info as I'm starting to figure this out:
Engine is 2017 BF90DK4 which is NMEA2000 compatible, however the harness and ECU have been replaced with 2016 or earlier non-NMEA model, so they modified the harness. It's a proven method that had worked in the past and this engine ran fine on the stand, so seller being an honest guy, is working with me to solve this issue.
I spent last night on my laptop, well past midnight, searching for info related to those two systems. Best I can tell there are minor differences, mostly backwards compatible, the one big issue that that I have a single oil pressure "sensor" on the engine block for a NMEA system so it can monitor the actual oil pressure, it's a 3 wire sensor with ground, 5V reference and signal to ECU.
Meanwhile the harness has provisions for 2 oil switches, both are on/off type, one grounds out to the engine when it looses pressure (idiot light) and the other is for ECU limp mode, also simply grounds out to ECU itself. Not a straight swap as one is 1/8 NBPT thread (tapered so it seals with teflon tape type) and the other is M8x1.25 with an o-ring, so I would need an adaptor.
Why am I looking at oil switches? Wondering if at a certain RPM the "sensor" wired to older ECU can trigger the limp mode as the principle of operation is vastly different.
Prior to buying this motor I have never seen one up close, new to boats in general, and now I'm almost an expert, especially since I do have lot's of professional experience with cars, and this BF90 powerhead is basically the motor that is used in a Honda Fit (Jazz in Europe) although peripherals are vastly different.
Fuel system is OK, I had another engine on this boat and I kept a brand new tank and water separator that was installed on the boat, it's clean and fuel is new non-ethanol that I was using 2 weeks ago with a Yamaha that was on there without any issues.
I downloaded a manual, although it's for NMEA engine, and have been checking sensors for resistance, so far so good, will have to get it running to do the TPS and MAP as they require 5V reference, might try to hunt down a scope as I only have a multimeter. Both of my parents are electronic and communications engineers so I'm very familiar with oscilloscopes and smell of rosin solder :) but they both retired long ago and don't have any of their old equipment, also my dad's old scope was a size of a microwave (I'm 52 they are almost 80)
I found several iffy connections where the wires were simply twisted together, and although they all look good I am going to solder and heatshrink everything and hope it fixes the issue...
Plugs look good, might be a touch lean, got new ones anyway
Coils are clean, no burn marks
Will do compression check for good measure
I also checked the dreaded VST grommet, not an issue on this one.
Throttle body and intake silencer are kinda dirty, could be a result of me trailering the engine with cover removed down the dirt road on the way home, I'll clean it for good measure anyway.
 
Yes, you know way more than I do about this. NMEA is something I've never dealt with.

I have 2 or three of those "microwave sized" analog scopes though and they are probably too slow to be of much use these days unless you're looking at tube amps or something. I'd preobably be electrocuted if I tried using one :unsure:

Hope you NAIL it! Let us know.
 
Most of what I learned was last night, I can't leave things half finished so I was up late researching and now I'm paying for it, exhausted...
NMEA is basically a CAN BUS that is compatible with many new gadgets, I just bought a chartplotter specifically non-NMEA because it was half price of a newer "N" (network) model because ultimately I just want to go from point A to point B without hitting anything, I don't even have a radio on my boat, removed and tossed it in the trash... I want to unplug when I'm on the lake, use phone only for taking pictures, like the one below from 2 nights ago ))
Got to admit it's a neat system because you can just plug compatible devices along the wire with a use of a "T" and IN THEORY they seamlessly mesh together using a universal communication protocol, so for example if I had N chartplotter and NMEA ECU I could buy a $100 network starter kit from Honda and my chartplotter would show RPM, oil pressure, trim, engine hours, etc. I never tried it, so I can't say for sure if it's that easy, but in theory it should be plug and play.
CAN (controlled area network) is quite brilliant, it was invented in early 80's by Bosch and released on the late 80's license free, first car with CAN BUS was a 1991 Merc S class that cost more than my house did at the time (NO JOKE! they sold for $70-140k depending on model) and now I can piggyback a CAN on any 2017+ Honda engine for about $100.

PXL_20260726_000247701.jpg
 
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