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How steady should RPM be on a 225? (with video)

M

metal-chicken

Regular Contributor
Our season is over now so I trailered the boat back home this week where it will be stored inside and I can carry out basic maintenance. Today I flushed the engine using a big tub for the water so no muffs.
While it was running I noticed some fluctuation of the RPM (about 150 in total) when running at about 1500-2000rpm. I didn't notice this during the summer while on the water but it may have been there - in fact on its last run in the sea a few days ago it got what was probably its longest cruise of the year and I remember thinking how steady it was running at about 3800rpm.
Today I also looked at the LP fuel filter and I saw that it was only about a quarter full when running, if I squeezed the primer bulb it would fill up but then slowly drop back to the same level - is this normal? Primer bulb would go hard after a couple of squeezes.
All filters where replaced in May this year and the engine has probably done a max of 30 hours since then. Engine had done a total of about 720 hours - ignore the hour meter on the tach.

Video showing rpm variation at a few different speeds:
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I'm sort of speculating here, but it has been my observation that until the engine is fully up to operating temperature throughout, you'll get those types of fluctuations at the lower rpm range. Not sure why, but suspect that the ECM is getting varying signals from the sensors and is making adjustments to timing and fuel flow accordingly. And as Johnny stated, under load, it's not noticeable.

As for the low fuel in the LP filter bowl, several folks report that there's is consistently below 1/2. However, when I have observed my own, it's usually above half. I doesn't seem to be a problem if it's low. As you know, the LP fuel pump is mechanically run off the cam, so as the engine accelerates, the more it pumps and the VST holds enough fuel to compensate in any initial lag of fuel feed.
 
BTW - If you have the Helm Shop Manual, I found it very informative to read Chapter 18 thoroughly, concerning the PGM-FI system. and it shows you fairly well what the ECM is reading and adjusting constantly. Pages 18-4 through 18-7 are especially informative for your situation.
 
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