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BF225 rough running

M

metal-chicken

Regular Contributor
My 225 didn't get used at all last year and I'm having a couple of little issues this year:
As per an earlier post the tilt/trim indicator is reading much lower than it should - it was fine when the boat went into the water in July but suddenly started mis-reading.
At the same time the rev counter is occasionally reading high by about 400rpm - When I turn the engine off it drops to about 400rpm but when I switch on the ignition it drops to zero. I will then sometimes over-read while running, but not always.
Sometimes when running steady at around 3900rpm the engine note will suddenly change and accelerate a few 100rpm without me touching the throttle - kinda like a cylinder has kicked in (but it doesn't feel like it's missing when idling).
Yesterday once when I accelerated from standstill it wouldn't go above 3500rpm and was surging - I stopped the engine, restarted and it was fine. I must have run about 15 miles at 4000rpm after that with no issues

There's plenty of fuel, the main fuel filter/water separator is new but one thing I have done this year is to use 3rd party low and high pressure fuel filters - I won't be doing that again!
The o2 sensor is dead but I'm not replacing it until next spring as I thing they just 'go off' over time with our usage here.

I can't really test much as the boat is on a mooring for the next week or two and then it comes out of the water. We've not much use this year thanks the the shocking weather in July and August here in the UK.

Any thoughts? Could a poor earth somewhere cause these things?
 
Yesterday once when I accelerated from standstill it wouldn't go above 3500rpm and was surging - I stopped the engine, restarted and it was fine.
Click to expand...
Just remembered....... my immediate thought was 'those bloody filters', but then I wondered about the kill cord as that can occasionally stop the engine due to a poor connection somewhere. So I pulled it, put it back in and the engine then ran fine - which it probably wouldn't have done if the filters were to blame.
Maybe a connection was breaking/making.

I'll replace it over the winter.
 
Glad you found the problem. Recall that the IAB is activated at 3950 RPM, so check to make sure that the diaphragm is working and grab the lever coming out of the diaphragm and pull it in to make sure that the baffles in the IAB move.

Attached is a more complete writeup I did several years ago when I had to replace my IAB and tore the old one apart.
 

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  • Honda 225 Intake Air Bypass.pdf
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Recall that the IAB is activated at 3950 RPM, so check to make sure that the diaphragm is working and grab the lever coming out of the diaphragm and pull it in to make sure that the baffles in the IAB move
Click to expand...
Yep I do that every year as I have to remove it to get to the high pressure fuel filter, so I know that's working OK.
Out of interest does the IAB activating cause a sudden, marginal, increase in revs?
 
I've never experienced that when the IAB kicks in. However, occasionally, I do get a minor surging (~100 rpm) between 4200 and 4600 rpm. At one time I thought that was due to the IAB because it ceased for a while after I changed it out. But it came back. Still haven't figured out that issue.
 
The VTEC activates at 4500 rpm. Do you guys think that system might cause symptoms like this?

Just wondering out loud.
 
Not sure...... maybe, but it's definitely around 4k rpm and it really is sudden change.
I don't remember it doing it previously.
Duff spark plug maybe? 🤷‍♂️
 
Anything is possible I guess but that rpm is so specific to the IAB it's very suspicious. I would think a coil or plug would act up over a wider speed range.

I am always suspicious though of harmonics acting on wire connectors. But....very difficult to find/prove and really not very common. However, in this case, you made a difference with the kill switch so.....???
 
So here's another theory - I wonder if it's got legs:

When an O2 sensor fails the engine defaults to settings that are slightly rich and I think that sometimes it's just the heater element of the sensor that fails.
Whenever I get a failed O2 sensor alarm I can hear the engine revs drop as the mixture is set richer.

So, this being the case....
Would it be possible for the engine to be running on the default settings [because my heater element is dead] - BUT after running at around 4k rpm for a while the exhaust has heated up the sensor enough for it to start working and the ECM then changes the fuel map to a more efficient one that suddenly causes the revs to rise and the engine to produce more power?

Sounds plausible?
(Obvs the way to test this is to chuck a new O2 sensor in but I'm not sure I'm going to get the chance to do that before the boats comes out of the water)
 
Yes, plausible, IMO. The heater element is supposed to get the sensor up to around 600 degrees F.

If you pull fault codes, a failed heater circuit in the HO2 sensor will throw a code 41.
 
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