2004 5.0 MPI mercruiser losing power, looking for help

Darwaypen

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hello, new poster here.

I have recently purchased a 2004 5.0 MPI Monterey. On first time out it ran smooth but seemed lacking on power. I am a pretty good auto mechanic so I performed a tune up on the old 305. Plugs, rotor, cap, wires changed fuel separator, even changed the coil and module assembly. Took boat back out, ran great, until I ran full out for about an hour, then it suddenly dropped about 1000 rpms and ran like it did previous to the tune up, lacking power. idle remains very smooth, acceleration is very smooth, just lacked power. After putting around for about 15-20 minutes I took off and all was good again for about 20 minutes at wide open, then the sudden drop again. I have now looked in the fuel tank, it is very clean, checked the anti siphon valve, it is not stuck and appears to be working fine. any suggestions would be much appreciated. thanks
 

tpenfield

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:welcome: to iBoats . . .

. . . and . . . welcome to MPI marine engines. It's all about the computer (ECM), and without the diagnostic software and a computer to connect to the engine, you won't be able to tell much. One thing you do know is that all the stuff you replaced is not the problem.

Wondering if you know anyone who has the diagnostic software/computer :noidea:
 

alldodge

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Do you hear 2 beeps and the fuel pump turn ON when you first go to start the motor?

When it runs out of power, remove the gas cap and listen for air being sucked in. If you hear air the vent line is clogged, if no air is heard and it still won't run, need to get the motor scanned as Ted mentioned

Being a 2004 might also check fuel pressure at the rail
 

Darwaypen

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Thanks for the responses. I do not have anyone that can run a scan, but I will take it in to have that done after I have negated all the easy stuff.

I do not hear the "two beeps" but I do hear the fuel pump come on when I turn on the ignition, it pressures up for a few seconds then shuts off. When I inspected the fuel tank I checked the vent line it was clear. I have a new fuel pressure test gauge coming in tomorrow, will check the psi on the fuel rail. from what I've read it should be about 43 psi?
could heat be a factor playing in? when I run full out for awhile the temp gets up to 175-180, not sure if that would affect the (cool fuel) system or not....
 

alldodge

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I do not hear the "two beeps"

This could mean that the motor is beeping letting you know what is wrong but the beep either doesn't work or has been disconnected.

I've read it should be about 43 psi?

Yes that is correct, at 1800 rpm and should stay about the same throughout the rpm range

could heat be a factor playing in? when I run full out for awhile the temp gets up to 175-180, not sure if that would affect the (cool fuel) system or not....

Don't think its heat related at this time
 

achris

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ECM555 does not issue a double beep on power-up, just a single, and that's to confirm the alarm, associated wiring and internal circuitry, are working.

If you are not hearing any beeps, then check you have the alarm module and it's wired in correctly....

Chris......

Alarm module in the circle, with a purple wire (which should be powered whenever the key is ON), and a tan/blue wire (which will sound the alarm when grounded).

Click image for larger version  Name:	Alarm.JPG Views:	1 Size:	152.0 KB ID:	10723319
 

QBhoy

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that’s running hot for that engine. It may be that your beeper isn’t working but your guardian mode is ? Mine runs at 152-158 all the time. Recently been told from others that theirs do too.
 

achris

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that’s running hot for that engine. It may be that your beeper isn’t working but your guardian mode is ? Mine runs at 152-158 all the time. Recently been told from others that theirs do too.

180 is a tad warm for a sea-water cooled engine, but not anywhere near hot enough to invoke an alarm or Guardian mode.It could just be that a 160 thermostat is in instead of a 143....

Chris.......
 

QBhoy

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180 is a tad warm for a sea-water cooled engine, but not anywhere near hot enough to invoke an alarm or Guardian mode.It could just be that a 160 thermostat is in instead of a 143....

Chris.......

Hi Chris. Good to hear from you. Do you reckon my Engime will be a 143 when she runs at about 152 usually ? I was worried that’s a bit cool for it ?
 

achris

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Temperatures in marine engine is a balance. Engines like running warm, it's where they're most efficient (some car engines run up close to 200 F).. But at higher temperatures, seawater disassociates into an acid, and quite a strong one. Metals don't like acid, especially aluminium. So marine engine designers try to find the maximum temperature to run the engine at before the salt does its thing. Which is why close cooled engines have a much higher temperature thermostat. The engine can run in the more efficient range, while the seawater remains cool enough not to disassociate.

The other thing to always keep in mind is that dash temperature gauges are rarely accurate, they are just an indicator. When the engine is new, you see where the gauge sits, and that's your reference... If the gauge says 173 degrees, it could be as much as 10, or even 15, degrees out.

The temperature rating of the thermostat is when it will START to open, not when it is fully open, so an engine running at 152 on a 143 'stat sounds pretty much right.

Hope this helps.

Chris.........
 

alldodge

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MPI motors have a 160 stat or should have from the factory. Prior to re-power mine would get above 160 running hard, now it still gets up to 180 to 190 running WOT with 140 stat
 

QBhoy

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Temperatures in marine engine is a balance. Engines like running warm, it's where they're most efficient (some car engines run up close to 200 F).. But at higher temperatures, seawater disassociates into an acid, and quite a strong one. Metals don't like acid, especially aluminium. So marine engine designers try to find the maximum temperature to run the engine at before the salt does its thing. Which is why close cooled engines have a much higher temperature thermostat. The engine can run in the more efficient range, while the seawater remains cool enough not to disassociate.

The other thing to always keep in mind is that dash temperature gauges are rarely accurate, they are just an indicator. When the engine is new, you see where the gauge sits, and that's your reference... If the gauge says 173 degrees, it could be as much as 10, or even 15, degrees out.

The temperature rating of the thermostat is when it will START to open, not when it is fully open, so an engine running at 152 on a 143 'stat sounds pretty much right.

Hope this helps.

Chris.........

Appreciated.
 

Darwaypen

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Apr 7, 2019
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Found the Alarm Module, one of the wires was loose, fixed it up. it does beep once when the ignition is turned on.
 

alldodge

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Found the Alarm Module, one of the wires was loose, fixed it up. it does beep once when the ignition is turned on.

Good, now if there is another alarm you will hear it.
Guess we need to see what the fuel pressure is now
 

Darwaypen

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Ok, fuel pressure 42 psi @ appx 1800 rpm. Took the boat out yesterday, added some Seafoam to the fuel, don't know when the last time injectors were cleaned. Boat ran great until I ran it full out for about 40 minutes, power loss again, then one of my passengers noticed a "popping" sound coming from the engine compartment that I couldn't hear from the drivers seat, I went back and listened, it sounded life a backfire, multiple ,pop,pop,pop , go away then pop,pop,pop again. I pulled back on the throttle a bit and it completely went away. Could not feel a miss during the popping and boat still accelerates smoothly and idles perfectly. Any ideas???
Still need to get it to marine shop and hook up to computer. Any recommendations on a hand held unit I could use myself?
 

alldodge

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The pop is the motor running lean, and the ECM should be picking up a knock and retarding timing IF the knock sensor is connected and working. In any case, did you keep the fuel pressure gauge on the motor and watch it as the rpm's increased and when it started to pop?
 

Darwaypen

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No, I did not keep the pressure gauge on while I was running, will have to do that next time out. Is the knock sensor on a marine engine like an automotive where it screws into a water galley on the block?
 

alldodge

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Yes, it screws into a boss on the side of the block..Will have a dark blue wire attached
 

Darwaypen

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Ok, got bad info from son in law on fuel psi. Sorry.

checked it myself with new OTC gauge, ignition on pump pressures up to 40 psi and shuts off. Motor running at 1800 rpm, 37 psi. Pulled vac hose off of regulator, pressure comes up to 41/42 psi. I have read about a low psi pump prior to cool fuel system, do not see one on my boat.
serial# is om669798 if that helps. Could I install a 3/4 psi pump prior to spin on fuel filter as a booster?
 
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