Mercruiser Mag MPI 350 Running Poor Under Load

[caken]

1999 Mag MPI 350 inboard running really poor under load. Starboard engine maxes out at 3200-3400 while the port can get up to around 3600. Normally both can run up to about 4,000 rpms under load.

A few months ago we had a guest fill the front fuel tank with water. Drained that and changed out filter and fuel pump. The engine has been running ok since but this past weekend took her out and noticed this issue while under load. Any ideas of what to check next?

 

[Kevlar7R]

How long did it sit with the water? Any chance any got to the engine? I'm thinking possible corroded/loaded spark plugs, or clogged fuel injectors. I suspect your port engine isn't getting up to speed because its doing work the starboard is usually doing. anything else unusual? Different oil pressures temperatures between the two? Dinged prop?

 

[caken]

It probably sat for about a week with the water. I don't think any got to the engine since we pulled the plugs and no water came out and have had it running well since. I am leaning towards the fuel injectors or spark plugs as well since everything else has either been replaced, changed, or inspected. Is there an easy way to tell which injectors need to be replaced? I was thinking of running a few cans of sea foam in the tank to clean them out but will that help in this situation?

 

[caken]

I have not noticed anything else different. Not sure about the oil temp but the pressure is normal for both of them. And yes I agree about the port, it seems to have the ability to go faster just not when it is supporting the starboard engine. I also do not think it could be a dinged prop, there is not much in our area to hit and I would have noticed if we did since the last time it ran well.

 

[Kevlar7R]

I don't know of a way to check the injectors, but running seafom or Lucas cleaner can't hurt. Not sure how you would go about it on a boat, but what we used to do on race bike was check the exhaust manifold temp on each cylinder. If one was significantly hotter or colder that cylinder was either lean or rich. Might start just pulling the plugs and looking at them. When was the last time plugs and wires were changed?

 

[caken]

I had changed the plugs about a month before the water in the tank but checked them after and they all looked good. I haven't checked them since I noticed it running poorly. I will head over there tonight to see what they look like. If one or more of the injectors were bad would I be able to notice anything on the plugs? I am not sure when the wires were last changed.

 

[alldodge]

Need to see if you have any codes being thrown. The scanner can also advise if the injectors are a problem having a balance test done. If you have a pressure gauge available plug it to the fuel rail shrader valve and see what the pressure is. Need a gauge to read as high as 60PSI, but pressure should be around 38. If you can provide a serial number we could look into it further

 

[Fun Times]

A few months ago we had a guest fill the front fuel tank with water.

Do you know if each engine has it's own fuel tank? Maybe you are starting to show signs of a failing fuel tank anti siphon valve due to the past water causing corrosion restricting fuel flow. Since you suspect a fuel problem, fuel pressure is important to know before moving on to other things. Hope you find the problem soon, good luck.

 

[caken]

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There are 4 tanks on the boat and I have tried while isolating each of the starboard tanks so I don't think its the anti siphon valve. I pulled the plugs last night and all were good except 1 of them. It had a significant amount of buildup on it. I read that this could be caused from burning oil. Any other things it could be or am I looking at a leak somewhere?

 

[caken]

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