Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

I have a Mercruiser 350 MPI Horizon (OM643971) that will run fantastic for over an hour and then start to surge power and go to idle. If power is left up it will continue to surge from idle to high power then settle on idle eventually. Shutting the engine off and letting it rest sometimes helps, but then the problem will come back a short time later. I have always been able to just idle in on the worst cases, but yesterday the motor just wants to die instead of maintain idle. Obviously fuel filter, but what is causing the surge to happen? Could it be the fuel pump or boost pump? This tends to happen more when lower on fuel as well. For example, an hour plus day trip out it runs fine, but on the return trip, ~2hours of fuel burn, when almost home, is usually when this occurs. Thanks for any info.

Jimmy

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

Ok, so boat would not even start when in driveway Monday night after being towed in and adding gas, just in case. Tuesday morning I go out to put it away, and she fires up no problem and runs strong through entire range of throttle. Very responsive and no surging. What could possibly be getting tired throughout the day that causes this to only happen after running for long periods, and then clear itself after rest?

So confused.

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

Bump. Anyone?

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

Not sure what else to write here to get someone to reply. Losing faith in finding an answer on this forum. Come on all you engine bad asses! What's wrong with this thing!!!???

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

A little more info on the engine might help like the year for starters... read the sticky at the top first and then repost your question with all the info needed

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

Mac525 said:

A little more info on the engine might help like the year for starters... read the sticky at the top first and then repost your question with all the info needed

Click to expand...

Engine is a 2003. I reread the sticky and can't seem to find what else I need. Year, engine make, and serial number are all there. Thanks for any insight you might be able to give.

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

I would start with sticky #3 HOW TO: Testing your boats fuel system.
Also might want to do a fuel pressure test --- you can find that test in Merc manual #36 page 2A-15

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

muc said:

I would start with sticky #3 HOW TO: Testing your boats fuel system.
Also might want to do a fuel pressure test --- you can find that test in Merc manual #36 page 2A-15

Click to expand...

+1 Check fuel system.

Re: Mercruiser 350 mpi horizon runs fine then surges and maintains idle.

boatguya1 said:

+1 Check fuel system.

Click to expand...

Ok, in the works, but replaced fuel filter this morning and dumped old one's contents into a jar. Couple hours of settling and only normal sediment at the bottom, but confirmation that it needed replacing.

**New problem** Boat began to overheat at first test run after new fuel filter. Upon inspection, it tends to be using water from the fresh water cooling system. I refilled it and ran it with cap off to check for bubbles and got nada. just normal water overflow. This sounds bad to me and I am still lead to believe there is a blown head gasket maybe. Might be time for a pro. Can these two symptoms be related? Dumb question, but confirmation is nice.

Thank you both very much for the time.

Jimmy