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2006 bf225 falling/surging rpm’s.

J

Jsas1

New member
2006 225 with about 1800hrs. From approx 1400 - 4400 rpm’s, it will drop 200-400 rpm’s and then come back up. Doesn’t do it after 4600. Currently only using the main tank. New fuel lines, water separator filter, new HP and LP filters. Also just had a valve adjustment done. Drained the vst, but I don’t think I drained it enough. Pulling the boat tomorrow and will drain again.
 
..should add that the reason we are only using the main tank is we found water in both tanks, so drained the main tank completely before replacing the filters, fuel lines etc. Have not replaced the filler hose or vent hose (doing that tomorrow)
 
Check your IAB (Intake Air Bypass) for proper operation.

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. The details of why that works gets very complicated – whole books are written about it.

After I had been running my BF 225 for about two years (2008 - 2010) I started noticing a surging of about 100 to 150 rpm in the 4000 to 4500 rpm range. I was told by several folks, including two maintenance shops, that this was the result of the VTEC system trying to decide whether to get into VTEC mode or not. However, once I replaced the IAB the surging problem totally disappeared.

To test, proceed as follows:
  • Open up both sides of the upper cowling by first removing the two bolts under the cover at the rear of the engine – those two below the exhaust outlet.
  • Remove the six or so bolts on each half of the top of the cowling.
  • Peel back each side of the cowling so it clears the exhaust outlets.
  • (This is the same as you would do to get access to the HP fuel filter to change it out.)
There sitting in front of the HP fuel filter you will see the IAB control diaphragm. Disconnect the vacuum hose to the diaphragm. The diaphragm is held in place by two Phillips head screws that you typically remove to get to the HP fuel filter. Remove those two screws, swinging the diaphragm aside. Grab the lever coming out of the diaphragm with needle-nosed pliers and pull on it as if the diaphragm was activating the IAB baffles. It should move until the spring arm engages (or nearly engages) the “stop” adjustment screw. Then let go and it should pop right back in place. If so, the IAB is operating as intended. If it sticks open or is slow to return, you will need to remove the entire IAB and lubricate the shaft that connects to the vacuum diaphragm arm, all the way up into the IAB itself. See the attached writeup for more detail and pictures.

Before you re-install the diaphragm, hold it in your hand, gently push the arm back into the diaphragm, and hold your finger over the opening in the tube that the vacuum hose attaches too. When you let go of the arm, it should come out a little bit and stay there until you remove your finger. If so, then the diaphragm is operating correctly. If not, the diaphragm is leaking air and must be replaced. Unfortunately, it looks like a replacement is not available, since that diaphragm is part of the entire IAB assembly. However, I suspect that if you search around a bit, you should be able to find a replacement for this fairly standard part.

And do yourself a favor. Replace the two Phillips-head screws that hold the IAB diaphragm in place with either six-sided SS bolts, or better yet, SS Allen-headed bolts, if you can find them.
 

Attachments

  • Honda 225 Intake Air Bypass.pdf
    451.4 KB · Views: 137
..I noticed that it seems the IAB has more affect on higher RPM’s. Do you think if it was bad, it would cause the rpm fluctuations I’m having between 1400 and 4600? I’m going check it out as you described regardless.
 
My IAB used to be stuck but I managed to get it moving freely now but it took some soaking over a number of days/weeks in various lubricants.
That was a few years ago and it's been perfect since but, thanks to the two Phillips head screws being stuck, I do have to take the whole IAB off every year to change the high pressure filter - so checking and lubing the baffle is easy.

You might also take off and clean out the IAC valve with some carb cleaner. I also do this every year and it's another easy one (two bolts and one electrical connector).

Not sure if it's related but I used to get some fluctuation at idle and no longer do.
 
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