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Monsoon 320 low power

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Hey, everyone new to here, been reading a lot and haven't' found exactly what I'm looking for. I have a boat that is new to me. 1997 Sunsetter with a monsoon 320 EFI with 260 hours. Bought from a dealer that did an oil change and compression test and checked out well (all 180 with one 175) The replaced the plugs and shipped it out to me (1000 miles away). Boat gets to me and I dip it in the lake and get very little power out of it. I've checked the cooling system, replaced the impeller for good measure. Replaced both fuel filters and used some fuel drying agent. Engine starts easily, and runs VERY smooth. Without load the engine will RPM up to almost 4K and sounds like it's hitting on all cylinders. No exhaust blockages I can find. Tried again tonight and same issue with low power. The boat tops out at 2500 RPM and about 28 MPH but still runs smooth. Oddly enough it seems to kick harder in reverse than it does it forward? I'm not certain about this though as it is hard to gauge when the prop starts to cavitate. The throttle seems to move the full range it should both in forward and reverse. No check engine light. I have not checked fuel pump pressure yet, but plan to tomorrow. Could this be a MAP or throttle position sensor issue? any help would be appreciated. Thanks!

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sounds like a fuel system problem , i had a similar problem last week ,couldn t get the boat over 2500 rpm, got some bad fuel and in turn needed to change both fuel filters and clean the fuel pump screen. all seems good now though.

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If the boat sat for any length of time with ethanol (sp) laced fuel you may have some fouling issues because of the fuel. Change your filters and run some good cleaner through it for the next few tanks of gas. Seafoam works well for me keeping the fuel system clean.

If you are useing the boat a good amount then stabil is not required however if it will sit for a month or so you will need to have stabil in your fuel to keep this from hapening again I generally start putting in the stabil near the end of the season to make sure it is in when the bu does get put away.

Keep us posted on how this works out for you.

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Wouldn't the engine be running rough if this were the case as some injectors would be clogged more than others? I would like to drain the gas, but the problem is what to do with 25 gallons of fuel once you get it out! I did change the filters, but I would imagine if there is a contamination issue, they just replugged. Here are some videos:


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Do you hear any screaming noise from your fuel pump?

Check the fuel pressure at key on, idle and WOT. If you need information on how to do this, just ask, it's easy and you can usually borrow the gauge from a local auto parts store.

Siphon out as much as you can of the old gas you can get rid of it by mixing it in with good gas in your truck 5 gallons or so at a time. Add the sea foam or startron to the old gas and as much fresh gas as you can to dilute it.

Bakes has the chart under there resources tab as to what you need to have.

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Wouldn't the engine be running rough if this were the case as some injectors would be clogged more than others?

Not necessarily. If you had, say, two injectiors 100% clogged, those cylindes would still be drawing fuel from the nearby good injectors through the manifolds. These engines are not direct injection. In fact, the ECM fires 4 injectors at a time, not one at a time when each cylinder needs it. It is a batch fire set up, to quote Electricjohn.

So a couple or a few clogged injectors would have 'almost' the same effect as a lower throttle setting. A little more to it than that, but you get the point. Fuel is still getting to the cylinders through the manifold, just not as designed.

I had 2 injectors clogged up pretty good (#7 and #8). Lots of visible crud on top of them. Other injectors were not quite as bad. I was still able to get 4000 RPM out of the engine under load, and it ran pretty smooth up to that point. That would not have been the case if the engine was ruinning on 6 cylinders. It would have been rough, for sure. Cleaned the injectors and was back up to 5K RPM.

It's not like if you have a clogged injector, that cylinder is not firing at all, like you would think.

EDIT: After wathing your video, I would say you definitely have a fuel delivery problem.

Edited by RTS
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What do you mean unhook it from the trailer? it seemed to be strapped on pretty tight so I just left it.......... ok, so seriously: I don't hear the greatest, but the fuel pump does not appear to be whining. Additionally, when I changed the filters, i think I introduced enough air into the system to choke a diesel truck. The system pressured up, and started first crank, no sputtering or hesitation. I would think if this were a sensor, I would be getting a check engine light, but I'm not. Going to the store to get a bunch of gas cans, and a fuel pressure gauge.

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OK, replaced all the fuel and started up again today with the fake lake. Noticed a small fuel leak around the top of the fuel pump near the clip where the electronics connect. Looks like I need a new fuel pump anyway. Anyone find a part number that can be easily found at autozone or napa? Searching has found a Carter P60962 or one at napa that the old threads have as a broken link.

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OK so just checked the fuel pressures. Looks like i'm still within spec even with the fuel leak. Running about 39-40 PSI at idle, 42 at throttle (can't do wide open in neutral). The leak is slow, and the fluid coming out IS fuel, but VERY discolored yellow. Still not safe for regular use in my mind, so I'm planning on replacing it. thought the pump might be my issue but looks like it's not.

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Did you replace the fuel? If you did, one thing I found a little helpful when working on the fuel system was to add Stabil to the fuel. It will tint the fuel red, and help with any leak checking. It will be (more) obvious when you wipe the connections with a rag. I guess the Ethanol Stabil will tint it blue. Still easier to see than just gas.

Sounds like you have good pressure. That really makes me think you have clogged injectors. If you have the ability to change out a fuel pump, removing the injectors should be no problem for you. Pull them and I bet you will find they are your problem. Someone just recently posted a pretty good write up on getting them removed.

With the ethanol gas being so prevalent, I think this is going to be a common problem that we see, especially at the beginning of the season and/or when someone buys a boat that has maybe been sitting for a few months.

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Fuel replaced, injector cleaner added, Pressure checked, Need to order a fuel pump I guess. It will give me time to check the injectors. Any chance the injector cleaner will clear it up? using 44K and sea foam

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I doubt at this point an injector cleaner will do the job by itself. If the injectors are the problem, it is a bad problem if you are only getting 2500 RPM.

EDIT: An injector cleaner run throught the tank, I mean.

Edited by RTS
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The fuel seems to be coming from the c-shaped clip that goes near the top of the pump. It appears this is where the wires enter the pump and seems to be where the fuel is coming from. All fittings above there are clean and tight as far as I can tell.

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Hmmm seems weird to me that you would be getting the right fuel pressure if you had a leak in the pump itself. I had some debris in my intake and it took care of most of my problems, I just have weak hole shot now. I can't find a second fuel filter on my boat to change.

You might want to check the fuel pressure on the lake, while running. That is where mine was really the worst. Are you getting any back firing? or just weak and low rpms?

It could be that you be aren't getting the right fuel pressure under load due to the leak, more of a flow issue than a pressure issue caused by the leak.

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  • 4 weeks later...


So the place I bought the boat from sent a tech up. Troubleshooting showed 3 clogged injectors. Took them in to be cleaned and they could not be recovered. They paid to replace the injectors and I'm getting pretty good power after an hour on the lake. Rpm 4300, 40 mph at 4500 feet with 13x13 prop. I still have fuel seeping out around the top of the fuel pump where the wires are coming in. I'll have to get it replaced ASAP anyway.

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