350 Mag MPI Horizon Stalls at Idol

At cruising speed a few days ago I bumped the kill switch by mistake. The boat never re-started and had to be towed in. Motor hardly runs at all. Ran perfectly prior to this incident. 300 hrs on it. Fuel checked out OK, plugs are OK, replaced rotor and cap, replaced IAC valve, checked coil, checked fuel pressure, still wont idle for more than a few mins. Two marine techs still cant figure out the problem. I assume its probably something simple, but at $100.oo an hour, id like to find the problem sooner than later . Any ideas would be greatly appreciated. Cheers !!

This is my first boat. I am depressed that something so innocuous has been so difficult to fix.

1999 Doral 250SE Cruiser.
350 Mag MPI Bravo III
300 hrs

Re: 350 Mag MPI Horizon Stalls at Idol

I bumped the kill switch by mistake.

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Please explain, in detail.

Does the engine crank ok and just not start or will it not turn over when you hit the starter?

Re: 350 Mag MPI Horizon Stalls at Idol

The engine cranks fine. It will start and idol ok if it sits for a while. After a few mins of idol, it will start to sputter and run rough. If I give any gas at all, it will stall. After that, it needs to sit for 10 mins before it will start again. Then the entire process repeats itself. Tried starting it with some throttle while in neutral, and it backfired.

Thank you !

Re: 350 Mag MPI Horizon Stalls at Idol

rcgus1 said:

At cruising speed a few days ago I bumped the kill switch by mistake. The boat never re-started and had to be towed in. Motor hardly runs at all. Ran perfectly prior to this incident.

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it happens

rcgus1 said:

I assume its probably something simple

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could be you disturbed the kill switch that may have been on its way out already (corrosion)

did the two techs check the kill switch, or even bypass it to see if this is the problem??

Re: 350 Mag MPI Horizon Stalls at Idol

They have not yet but mentioned that was on the agenda. I guess since it turned over and actually does start, they probably put that on the middle of the list. Thanks for the advise.

Re: 350 Mag MPI Horizon Stalls at Idol

Fuel checked out OK,

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Does that mean that you had plenty of fuel in the tank or you checked the fuel filter for water and found none?

Re: 350 Mag MPI Horizon Stalls at Idol

Immediatly after it quits, check for spark with a spark checker.

Re: 350 Mag MPI Horizon Stalls at Idol

Looks like replacing the Sensor Assembly did the trick. A $60.00 part accompanied by $500 in time. Apparently the sudden loss of power can cause the unit to fail according to a 2000 service bulletin. Oh well .. its fixed. Thanks for help

Re: 350 Mag MPI Horizon Stalls at Idol

Had it out today for the first time since the repairs. It runs a little rough and almost stalled at idol after about 20 mins of cruising. Something still seems a little off. I get 2 beeps immediately after start but no beeps thereafter. Not sure if thats normal. I don't think that was the case prior to the failure. I know that can mean a variety of things ... but I though it would continue to beep at intervals if there was a problem???

Cant seem to find anything refering to 2 beeps only

Re: 350 Mag MPI Horizon Stalls at Idol

Maybe an Idle Air Control valve? But lets see what the experts say.