Dropping RPM's

[Penns Beach]

2003 2700 SCR 5.7 Bravo 3, any thoughts on what is happening??? Engines starts great runs fine for an hour (yesterday ran up bay at 3800 rpm's 22 kts) after an hour the rpm's dropped IE: from 3800 to 3400 and right back up. it did this several times then only dropped IE 3800 to 3600. I arrive at marina and shut down.

[mmwjr]

Penns,
could you detect the engine change in speed by sound and boat speed as well, i.e. did the boat slow down and engine noise decrease? If not this may be a bad signal to tach (lose or dirty wires on tach). Also could be tach itself.

[rgreenwell8962]

Is this the first tank of gas for the season? Is it a fresh tank or last seasons fuel? Did you top off your tank before putting her up for the winter? Do you treat your fuel? How about the water separating filter? Is it new? Could be fuel. Happened to me once even though I take all the precautions to keep the tanks topped of treated and good. I changed the water sep filter, ran the tank really low, filled her with fresh fuel, treated the fresh fuel and used some water additive to burn it off and put another new water sep filter on and it did the trick.

[Penns Beach]

I don't recall a change in boat speed but if it happens again I will be on the look out for it.

I did fill the tank up and treated the fuel prior to winter. And this is the "WINTER GAS" I am running on. I also added more treatment prior to starting up this year. Yes I installed a new fuel water filter. Thank you for your repsonses!

[mmwjr]

Ok let us know what you can find out but it may just be a false reading from the tach and nothing serious.

[Penns Beach]

We took a ride up the bay last night for about 25 minutes and the boat ran great. Tied the boat up and went to dinner. After dinner as soon as we pulled away from the dock the engine started dropping rpm's immediately. And you could feel a drop in speed and the sound of the engine change. After a while I could hear the belt make noise. The gauges (temp, oil, amps) were reading normal. Only the rpm's would drop alng with the speed. Still on the winter tank of gas. Installed a new distributor cap and IAC value two weeks ago.

[mcoffey]

The belt noise could be a clue. It is odd that there is no belt noise at all and then when this RPM drop starts, the belt noise starts. Could be the alternator under sever load (faulty regulator maybe)? Could be the PS under load (though I can't imagine anything that would cause that to be intermittent). Do you notice your Volt gauge showing more than 13-14 volts when this occurs? I had an old Chevy 454 that did something similar to this. After it warmed up, the regulator would crap out, the alternator belt would start making noise and the volt gauge would jump to 18 volts (and all of the lights would get super bright). Checking with a meter, it was indeed jumping to 18 volts and replacing the alternator (with internal regulator) fixed issue.

[mmwjr]

I have seen what mcoffey says on a friends car along time ago. How old is the battery and do you hook up to shore power at your marina and turn the charger on. I have seen a weak battery cause dim headlight and low of power and this will cause the alternator to put out max power. If you are charging the battery at the marina it will hold the charge for awhile but as you are running the boat it's voltage will drop off causing the alternator to try and compensate. Also just for grins since you mentioned a new distributor cap check to ensure the wires are all seated fully.

[shrew]

I doubt it's a dead or bad battery, per se. However, it does sound a but similar to when the AC comes on in a car. I can tell when my fridge turns on, because my volt meter takes a dive and the temp. guage simultaneously goes up. I can tell when the fridge gets turned off because the volt meter goes back to 14 and the temp goes bck down. This is a draw issue somewhere on mine though. I would certainly be looking at any changes in the guages.

[Penns Beach]

The starter battery is a year old and the deep cell is brand new. The volt gauge stays right on 122 volts it doesn't move. I actully took a video of this while it was happening to show a mechanic. All of the gauges don't move only the RPM's drop. I took the new water seperator out yesterday and the gas is fine. I am going to put a pressure meter on the fuel system tonight plus replace the inline fuel filter and maybe even the fuel lines.

[shrew]

Originally Posted by Penns Beach

The volt gauge stays right on 122 volts it

12.2volts? not 14?

[mmwjr]

I would use a hand help voltmeter to verify. 12.2V is what I would expect the battery to be with the engine off and no load. with engine on should be 13 - 14V.

[Penns Beach]

the volt meter when running is 14 I hooked a pressure meter to the fuel system and started the engine It went right to 40 lbs then immediately started dropping off down to 20 lbs. I think it might be the fuel pressure regualtor of the fuel pump. What are your thoughts?

[ss3964spd]

You said it's a 5.7, but which one?

[mmwjr]

Based on the fuel pressure I am assuming you have a mpi engine,as ss3964spd asked what engine package do you have? I did a quick search and it seems that 40 psi is the normal pressure. The fault could be the pump or the fuel regulator on the fuel rail. Question: when you made the measurements (voltage amd pressure) was the RPMs low? I would think that all measurements would be in th acceptable range is engine is running correctly and would only be abnormal when RPMs drop, but that's my thinking.

[Penns Beach]

It's a 350 5.7 MPI with a Bravo 3. Mike "mmwjr" I see your in Kingsville We dock the boat in Havre de Grace at Penns Beach. I am taking it to Tomes Landing tonight. They are going to look at everything you all mentioned. I thank everyone for their input. I will let you know the outcome.

[mmwjr]

Penns, you're welcome and good luck. BTY i checked my merc manual and it says fuel pressure should be 30 psi, turn key to run but don't start to measure is the instructions they give.

look forward to hearing the outcome