Good morning. I have a 1998 40...

[Rakkus]

Good morning. I have a 1998 40 DA w/twin 454 Magnums. 700 hrs. Last tuned in spring of 2018.  Out last weekend and my port engine tach surges 10 to 20 rpms at idle and when I goose it she runs like it is missing. Has anyone experienced this issue? If so how did you correct it? Thx

[Jet_Rooster]

Water in the tank

[Rakkus]

You know, I thought of that. I had to put the boat away with only about 50 gallons a side in the tanks. 7 months on the hard. How should I proceed. Change the fuel separators? Load up on dry gas? Your thoughts?

[Jet_Rooster]

Drain the fuel from the sumps until you get only fuel, I would just drain and dry the tanks for good measure. Replace the fuel filters and clean/replace the water seperators. Fill the tanks with fresh fuel

[wingless]

Is the fuel pressure regulator working properly, when tested according to the FSM?

That is a poor design, with a small / skinny vacuum tube connected to the bottom of the intake plenum.

Oil vapor collects in the plenum, then runs down that tube, blocking the fuel pressure diaphragm, preventing proper operation.

On my boat I yank both plenums annually, to clean the flame arrestor and to clean the plenum interior. I also clean that skinny fuel pressure vacuum hose.

If the problem is poor fuel in the tanks, then run them down low, clean the interior, either professionally (preferred), or DIY, following all safety procedures.

Also change the fuel filters.

Cleaning and testing the injectors is another way to getting the engines to run properly. I used a professional service for mine. It helped BIG TIME.

[wingless]

Rakkus:

Thank you for your comments. It isn't contaminated fuel because there wasn't any water in the fuel filters/water seperators, (2 on each engine). Have delivered the boat to the local Sea Ray dealer, Colony Marine in St. Clair Shores, MI for their expert care. Will advise what the issue was.

wingless:

Please post updates in the original topic so everything remains together.

Thanks for the update.

[Jet_Rooster]

Sorry it didn’t fix it. Please post the results.

[Rakkus]

FYI Follow up. The engine was gone over. Changed ignition wires and plugs, distributor cap. Nothing changed. Then the mechanic noticed that the MAP (Manifold Absolute Pressure) sensor was DISCONNECTED! Sharp eye 'cuz I never saw it. A previous mechanic must have inadvertantly disconnected it by accident. Plugged it in and, Wallah, runs perfect.