454 MPI WOT = 3600

[davidl]

According to the manual my recent acquisition (99 Bayliner 2855) should operate at 42-4600rpm. Trouble is it only gets to 3600. Its a 7.4ltr (454ci for non-metric types, mpi). Hull is clean,leg has been antifouled, props (Bravo 3) are in good cond. The engine is 140hrs old and according to the the diagnostics, in excellent shape. In neutral it revs to the 4700 limiter no problems. Why wont it go above 3600 (29 knots) underway? Any help appreciated.

 

[Trent]

Re: 454 MPI WOT = 3600

First check your prop size or for prop damage. You could be over propped. Next NEVER run your engine above 1500 RPM in neutral or you might see what a rod looks like going thru the engine block.<br /><br />Is your throttle plate opening all the way?

 

[davidl]

Re: 454 MPI WOT = 3600

Thanks for the reply. The prop is fine though trying to establish what size/pitch it should be is not as easy as I thought. How do I check the throttle plate opening? BTW, it was the Mercruiser mechanic who revved it to the limiter, I pray (hope) he knew what he was doing.

 

[johnkc]

Re: 454 MPI WOT = 3600

he didn't !(couldn't resist making a remark}i will check tomorrow what bliner recomends for props for you

 

[DJ]

Re: 454 MPI WOT = 3600

Sanity,<br /><br />You've got alot of boat to push even for a 7.4L.<br /><br />Is this rig NEW to you?<br /><br />I cringed when I heard a marine tech. revved it to 4700 with no load. FIRE THAT GUY!!!<br /><br />Common sense would tell us that the boat is over propped. But, there are other considerations:<br /><br />1. Can the 7.4L carry a 4000 # boat?<br />2. Is the tach. correct?<br /><br />Check #2 first, if the tach. is correct, you are over propped.<br /><br /><br />Note: The 7.4L GM engine does NOT like alot of unloaded rpm's. There are alot of heavy parts in those engines.

 

[davidl]

Re: 454 MPI WOT = 3600

Thanks now I'm really scared. The Bayliner comes as standard, even now I think, with this engine config. IE new 2855s have a 454mpi. I assume they have powered this thing correctly. The boat is new to me, having just purchased it second hand. The tach agrees with the diagnostic unit carried by my (new) Merc mechanic. He is now saying that the knock sensor is always on and is investigating this angle. He is talking about HT leads being too near the sensor and causing an error. Comments anyone

 

[johnkc]

Re: 454 MPI WOT = 3600

sanity, the drive ratio should be 2.00:1 and the props should be 24 pitch, witch sounds a bit high, compared to other lighter boats that we have running the same drive ratio and props and weighing 4000 lbs. with a knock sensor sensing a knock all the time it will retard timing witch equals less h.p. and less rpm, he may be on to some thing there. check timing at wide open throttle full load and see if it matches the spec in manual.if the timing is low then the knock sensor is probably pulling the timing down

 

[davidl]

Re: 454 MPI WOT = 3600

John,<br />Many thanks for the information, will compare it with what is installed and await a report re the knock sensor.<br />Regards,<br /><br />Sanity

 

[johnkc]

Re: 454 MPI WOT = 3600

sanity,your boat weighs between7&8 thousand lbs.

 

[ODDD1]

Re: 454 MPI WOT = 3600

Sanity, find yourself a new tech, this guy likes sellin motors, LOL ....even though this is a newer mpi motor, dont forget the basic troubleshooting that you would use on any motor...the engine management is soo nice on these engines you can pull a plug wire at idle an not notice it...hence FOULED PLUGS slip past even seasoned people....any motor in your situation should be placed in 'service mode' and the plugs shorted one by one just to make sure you have 8 working cylinders/plugs/injectors....then a fuel pressure test [by the way, you did check for a fuel filter full of water?] and johnkc's WOT timing check.....if you have time, let us know how you come out...