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View Full Version : Lost rpm on top.l


fishnfireman
02-11-2004, 11:37 AM
2.5 280ss,
wont turn much past 6800 with 28 pitch or 26 pitch prop.
any guick thoughts?
I did check the intake butterfly and it was going a little past level.
Could this be enough to hurt RPM.
pirogue
02-11-2004, 01:14 PM
check plugs, could have fouled a plug or two, very common. also check for fuel problems. give us some more info!

the butterfly thing aint gonna hurt.
fishnfireman
02-11-2004, 01:54 PM
It is bad to foul plugs-so I keep a spare set in the boat.
After plug switch it was still down on RPM

This is a 99 model that was bought after a ring/pin failure.
We sent pistons to John to be top pinned.
Block back to Merc.
It was not ran on this boat (2003 Allison) before rebuild.
The idle stop was not set right after rebuild so the TPI could be off just a little, but that should not effect the top end.
We did not reset break in switch - so that should not limit Rs. It has plenty of hrs on it so that would not be a factor.
I had a 28 pitch prop for the first few outings after break in with a heavy load. It would only turn 68-7000. So i put the 26 on to turn it on up--- still would only turn 68-7000,

If it is a fuel problem where would be a good place to start looking.
O/B Dude
02-12-2004, 02:00 AM
Are you saying it ran over 7000 and suddenly now it won't, or are you saying it ran over 7000, you rebuilt it, and first high rpm run on the new rebuild, it won't go past 7000?
fishnfireman
02-12-2004, 11:23 AM
Will not go past 7000 after rebuild.
Would hit limiter on a different Allison before rebuild.
The only difference between my boat and the one it came off is mine is 1 year newer. Set up is the same.
ON THE HUNT
02-12-2004, 06:10 PM
The PCM uses the input from the map for fuel curve from 0 to around 1500 rpm then it relies on the input from the TPS to WFO as the main info device to set the fuel curve.. A TPI will "flip" if it rotates far enough, i.e. the PCU will see peak voltage(wfo) and as it continues to travel it will "flip" back to low voltage(idle). One other thing that caused this same problem with an engine on a triple rig was the block psi sensor. There is a service bulletin reguarding this. Certian PCU #'s and coffe or black colored bodies.Unfortunately the fix in this case was replacement of both the PCU and block psi sensor.Hope this helps...