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  1. #1

    Question 2002 BF225 RPMs and Power dropping off under load

    OK guys one of my twins has decided to be a problem child. It runs fine in neutral up to 6000 + RPMs. In gear it'll only go up to 3000 before it starts coughing and bucking. It has to be under load to do this. If I bring the other engine up to 4000 rpms first I can get the "problem child" to run up to 4000 rpms. If I try to give it any more throttle it'll cough and buck in protest but it will ease up to 4200 but then cough and fall back to 4000. No alarms no lights. New fuel pump, new fuel filters, new fuel pressure regulator, cleaned fuel injectors (replaced one), cleaned the screen at the regulator, cleaned the screen under the fuel pump, new spark plugs (NGK IZFR6F11), primer bulbs function as they should, compression is great in both engines. The other engine "The good son" runs off of the same tank through the same fuel filter and water separator and runs perfectly whether it is running alone or with the "problem child". I'm STUMPED.

    I ran the compression test when I had the plugs out which was early in this process of figuring out this problem. I do not remember the numbers exactly but I was surprised they were as high as they were. The cylinders were really close on the pressures. My memory is saying 110 to 125psi but that is my memory and not recorded numbers. It passed the compression test though with flying colors. The engine only has 500ish hours on it. I swapped ECMs with the "Good Son" and the problem persisted. ?????
    A pair of 2002 Honda BF225s

  2. #2
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Hmmmm! Compression should be up around 175 - 200 psi. The way to test it is: kill switch activated, fresh battery, all plugs out, throttle full forward, crank engine for at least 5 seconds for each test. If you still get low compression, then should consider decarbonizing the engine.

    However, I suspect that's not your real problem. At 3950 rpm the ECU activates the the intake air bypass by opening vacuum to the vacuum diaphragm that sits right in front of the cover to the high pressure fuel filter - that's the one you move aside to get to that filter. I suspect that you are experiencing a vacuum leak when that is activated. First check the vacuum hose to be sure it is connected on both ends and has no holes in it. Next, remove the diaphragm itself, push in the arm all the way, and place you finger over the nipple the vacuum line attaches to. The arm will come back out a little bit but should stay there until you remove your finger. If it doesn't, then you have a bad diaphragm. Unfortunately, that is not a replacement part. It comes integral to the entire IAB. See part 4 in the following link.
    http://www.boats.net/parts/search/Ho...OLD/parts.html

    While you are doing that, pull the lever that the diaphragm arm connects to on the IAB, pull is out a bit and make sure it springs back in place. If it doesn't, send me an e-mail at [email protected] and I will send you a write up on how you MAY be able to fix it.

    If all that checks out, then take a look at the IAB valve assembly (part 9 in the above diagram) and check to see if that is functioning correctly.
    CHawk 25 DLX

  3. #3

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Thanks for the insight CHawk. I had done your procedure on both my engines previously to get those IABs working properly. I'll check for a vacuum leak later today.
    A pair of 2002 Honda BF225s

  4. #4

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    CHawk my compression numbers were in spec. I couldn't remember what they were but I had the specs during the test and my numbers were good. I'll be checking the health of that diaphragm as well.
    A pair of 2002 Honda BF225s

  5. #5

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Oh and I can see how the IAB could be the trouble source at the 4000 rpm issues but when I run that engine solo it won't get near 4000 rpm. Solo that engine starts protesting at 3000-3500 rpm. Would increased load make the engine open the IAB sooner?
    A pair of 2002 Honda BF225s

  6. #6
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Not sure about that. Let's eliminate the IAB and a possible vacuum leak, then we'll go from there.
    CHawk 25 DLX

  7. #7

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Ok. I've been out all day and it'll be dark when I get home. I'll have to get on this tomorrow morning.
    A pair of 2002 Honda BF225s

  8. #8

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    And its raining out. Looks like I won't be fishing tomorrow because this ain't getting fixed in the rain. I'll hold out hope though, maybe it'll clear up with enough time for me to get this done.
    A pair of 2002 Honda BF225s

  9. #9

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    OK found something. Finally got to look into the problem today. Upon removal and inspection it turns out that the IAB diaphragm is leaking. I can't pull a vacuum on it. I blew into it and I get a flow through it and can hear the air escaping inside the housing. Next step is to fix that issue. Thanks for pointing me at that CHawk.
    A pair of 2002 Honda BF225s

  10. #10
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Glad you found it. So it is a vacuum leak as suspected.

    As I mentioned before, that is not a separate part, but integral to the IAB itself. The easy fix is to replace the whole assembly, but that is north of $350. I suspect you might be able to find a replacement from some other source.

    Many, many years ago working in my dad's garage, when the vacuum advance on the old distributors failed, he would rebuild them. I do not recall how he did that.

    Jimmy - any suggestions?
    CHawk 25 DLX

  11. #11
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Nasty Wendy - if you have access to a Honda auto dealership anywhere nearby, why not remove the diaphragm and take it to their parts department and see if they can find a match. Honda autos have had vacuum diaphragms in just about every engine they ever made. So there is a good chance you could find a match. For example, see this picture from an older Honda Prelude....

    Click image for larger version. 

Name:	Honda Diaphragm.jpg 
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    CHawk 25 DLX

  12. #12

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Well I've disassembled the entire IAB cover and the vacuum diaphragm works manually....yes I used my mouth. I'm thinking that "Leak" could be built in relief because even with it there the diaphragm still works. I'm going to repeat the process to clean up the crud from the rest of the assembly. It was harder to move than just the pressure the spring provided. Maybe I didn't have enough grease in there after I did it the first time. ?????? Here is to giving that a try. Should I need to replace the diaphragm I will be searching every source I can to find an equivalent before I cave into paying $350 because a piece of butyl rubber failed.
    A pair of 2002 Honda BF225s

  13. #13
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Guys,
    I have been lurking and am interested in seeing you work through this.
    Chawk_man, they probably had rebuild kits your dad could buy. I seem to remember them back in the day but the whole assembly was usually so cheap, by the time I came along, I probably went that way because I don't recall rebuilding one myself.

    Nasty Wendy,
    I agree with the both of you. Find a substitute or cut your own diaphragm. That's what I would do to save the $$yenom$$ !!

    You might try taking it to a Honda car dismantler to see if they could come up with something that would work for you. Just a thought.

    Good luck.
    Last edited by jgmo; 12-17-2017 at 03:02 AM.

  14. #14

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Well after disassembling the entire IAB assembly I have it clean as a whistle. I used brass whool to clean up the insides of the "bearings" that the shaft rides in. I lightly polished the shaft sections that rides in the "bearing" with 600 grit sandpaper. I gooped in some industrial "never seize" and reassembled. It is as smooth as butter again. The diaphragm works if I help it a little to start. I used a "Mighty Vac" to pull vacuum on it. Once it starts to move it works but I cant get it to start on it's own. Maybe the Mighty Vac can't pull enough vacuum to get it going. I'll start my car's engine and use a vacuum line from it to see if a running engine will make it work. If not I'll have to cut it open and operate.
    A pair of 2002 Honda BF225s

  15. #15
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Sounds like a good plan. BTW - the "stop" screw the determines how far the baffles open should not be touched. It is factory set.

    Also, when you reassemble, it's probably a good idea to replace the whole vacuum line. It seems to be standard auto-supply vacuum line.
    CHawk 25 DLX

  16. #16

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Is the IAB something you can switch from your running engine to confirm this is the fix?

  17. #17
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    It's not necessary to swap them over and it's hard to tell if it's working or not just from the engine performance.

    The IAB has a set of baffles that mount on top of the engine air intake. At low rpm's, the baffles remain parallel to the base, allowing for what engineers call resonance charging of the intake manifold and combustion chambers. However, when the engine reaches 3950 rpm, the ECU activates the IAB Control solenoid that opens intake manifold vacuum coming from the vacuum tank to the IAB diaphragm, which in turn, moves the baffles to a 45-degree angle. When rpm's drop below 3750 rpm, the ECU turns off the vacuum, and the baffles (hopefully) return to their parallel state. Technically, what the IAB does when activated at 3950 rpm is initiate an inertia charging effect which draws a higher volume of air under more pressure into the combustion chambers under higher rpm's.

    If, after you have determined that the manifold is working properly by manually pulling the arm to the baffles out and observing it to snap back in place, and tested the diaphragm that it holds a vacuum, then the way to test the whole system is in a test tank or in the water with the cover off and prop removed. You may also need to peel back the right side of the upper cowling so you can observe the control arm coming off the diaphragm.

    Rev the engine to 4000 rpm. At about 3950 rpm the ecu should open a valve on the IAB solenoid that introduces vacuum to the diaphragm that sits right in front of the HP fuel filter. When that happens, the arm on the diaphragm should retract and pull open the lever on the intake manifold (which changes the angle of the baffles in the manifold.) When the engine drops back to 3700 rpm, the vacuum is released and the baffles should spring back to their normal position.

    Item # 4 at: http://www.boats.net/parts/search/Ho...OLD/parts.html
    CHawk 25 DLX

  18. #18

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Yeah I'll be replacing the vacuum line when it goes back on. I'm about ready to develop a quick snap system for attaching the engine cowling panels. I refuse to leave them off overnight. Nothing screams eyesore as loud as a car on blocks in your yard or a boat with no engine or torn apart engine in your yard. Even though my boat is in my fenced in back yard I can't stomach it. I might be a wee bit strange but down here rain can come from nowhere overnight it's saved me on occasion from having a washed out intake track. Ok it's time to get on with this day.
    A pair of 2002 Honda BF225s

  19. #19

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    And I have had both in my yard before.

    As as far as swapping out the IABs go. I would do that if it were an easily replaceable part. I'd do that to confirm the problem and buy the part but with the part being "not available" it has to be fixed anyway so it is likely that I'll end up figuring a way to get this one serviceable if a replacement can't be sourced. I'll be removing the other one and recleaning it as well to make sure it keeps working properly.
    Last edited by Nasty Wendy; 12-17-2017 at 11:12 AM.
    A pair of 2002 Honda BF225s

  20. #20

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    And it's raining....
    A pair of 2002 Honda BF225s

  21. #21

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    OK so I tested the IAB assembly cover with a vacuum line from my car 1987 Buick Grand National. It is a 3.8 v6 so displacement is similar and the car is cammed but makes a 16-17 inches of vacuum at idle. The IAB solenoid is performing flawlessly with my car's engine as its source of vacuum. I could not get it to not operate flawlessly. So now when the weather clears I'll see if I can find a vacuum leak in the Honda engine. I've always checked by idling the engine and using a butane torch to release butane gas around the suspected areas of leakage and if the idle changes then voila there is your vacuum leak. I'll keep ya'll posted of course. It also maybe that my initial cleaning of the IAB assembly went south and the shaft became to hard for the diaphragm to turn. I'll be redoing the IAB cleaning on the other motor as soon as I get through this issue.
    A pair of 2002 Honda BF225s

  22. #22
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Roger that. Keep us posted on your progress.
    CHawk 25 DLX

  23. #23
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Chawk_man....nice explanation of the IAB and "flaps".
    Nasty Wendy....1987 Buick Grand National.....nice car!
    This has been an interesting thread so far.

  24. #24

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Nasty Wendy. my 63 Pontiac Parisienne with 409 has about the same vacuum. The Buick is a nice source of vacuum too.

  25. #25

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Quote Originally Posted by jgmo View Post
    Chawk_man....nice explanation of the IAB and "flaps".
    Nasty Wendy....1987 Buick Grand National.....nice car!
    This has been an interesting thread so far.
    Thanks. I'll post some pics later. The car has an amazing story.

    Quote Originally Posted by NSDON View Post
    Nasty Wendy. my 63 Pontiac Parisienne with 409 has about the same vacuum. The Buick is a nice source of vacuum too.
    You sir have one big vacuum pump there.

    Ok guys I'm working around tasks I have to do for these holidays namely butchering a couple pork loins into a Crown Roast for our Christmas Party/ dinner. Once I get that roast put together and prepped I'll get back on this engine. Hopefully I'll have an update later in the day.
    A pair of 2002 Honda BF225s

  26. #26

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Well I put the IAB cover back on and the engine is running perfectly in my yard. I'll try to splash the boat and give it a test out in the water soon.
    A pair of 2002 Honda BF225s

  27. #27
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Let us know how it goes. Also, anyone reading this thread may get confused, so it would be nice if you posted a summary of the problem, describe exactly what it was, and how you fixed it. Maybe in a new post.
    CHawk 25 DLX

  28. #28

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    There is an outside chance that I splash the boat today. If the problem has been solved I will do exactly that Chawk.
    A pair of 2002 Honda BF225s

  29. #29

    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Sea Trial'd it and it was not the problem. The IAB system is working flawlessly but the boat is still suffering the same symptoms as the original post states. I'm thinking my next move is to test the fuel pump. I'll hot wire it run and measure the amount of fuel it pumps out. Anyone know what the flow specs are for our fuel pumps? I'll go through all the filters and screens again as well. Just to make sure I'm not forgetting any I'll check: Low Pressure fuel filter, High Pressure Fuel Filter, Fuel Pump screen, Screen below the Fuel Pressure Regulator. Am I missing any?

    Wouldn't the O2 sensor give a Code/Light if it were causing this?
    A pair of 2002 Honda BF225s

  30. #30
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    Default Re: 2002 BF225 RPMs and Power dropping off under load

    Sorry to hear that. Thought we may have solved the problem. A failed HO2 sensor should not cause the problem you are having. It will just make the engine run rich and your plugs will show the black soot rich running will generate. A failed HO2 sensor will not necessarily trigger an alarm, depending on what ECU you have.

    A filter you did not mention is the fuel filter mesh 16614-P5G-003 (on fuel rail). There is one on each fuel rail. See item 4 at:

    http://www.boats.net/parts/search/Ho...TOR/parts.html

    Fuel pressure at the relief bolt on top of the HP fuel filter should be between 42 and 49 psi.

    According to the Helm manual, if you disconnect the fuel return hose from the fuel pressure regulator and route it into a jar, then activate the fuel pump for 2 seconds, you should get at least two ounces of fuel discharged into the jar.
    CHawk 25 DLX

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