This question is for my friends boat. He has a 1997 fountain with original 454 MPI 385hp engines he bough over the winter.. Engines were winterized and today he tried starting them for the first time. Both engines will not idle. One idled very rough. They would rev at 1800 rpms no problem but anything under that they would stall. Is there a sensor that is bad ?

Any help would be appreciated

Just a couple of thoughts from a novice

1. Are both engines behaving that way? If so I would strongly guess that it was old gas or dirt in a filter.
Was the carburetor fogged or was the special elixir run thru the fuel injection system-

If not I would try a clean thru the fuel before I would start messing within anything else.

3pointstar

Fouled plugs from fogging?

engine scan results ? fuel pressure readings at idle is and with just key on ?? Pull some spark plugs are they oil or fuel fouled or hopefully some are not dry as a bone. Is the engines running rich - you can tell from the condition exhaust and smell ?

take a new or very clean remote / portable fuel supply tank and new fuel line with a outboard primer bulb on it, run engine on this new stuff plus new fuel = if engine(s) will not run or idle on this set up - squeeze the primer bulb many times in a row - any change ? if no possible fuel restriction engine side, change all fuel filters, check fuel pressure readings - make sure they are with in spec. Still use the remote fuel set up for this testing.

Possible fuel pump(s) issue and or injectors could be restricted to. Other possible issues if not fuel related of course is determining upon engine scan results.

Determining if your fuel rich or lean - if lean you have some sort of fuel restriction issue ie: fuel pump(s) and or fuel injectors - basket filters could be somewhat contaminated inside the fuel injectors.. Condition of the fuel filters

If it is running rich could be map sensor or fuel pressure regulator or stuck open - leaky injectors. Even in extreme cases a really dirty flame arrestor(s). Spark plugs will show rich conditions as well

if you still have the spin on fuel filter that came with the engine - COMPLETELY dump the gas from it in a clear jar - what does that look like ? Always do this when changing fuel filters no matter how well your engines are running. It can prevent fuel related problems upon any changes in good clean fuel. It is wise to catch this beforehand that too much crap does not run completely or continues to run thru the fuel system.

Testing and the results is part of the processes to eliminate the problem areas. That's where you need to start.

POS batteries. Believe it.

Fuel water sperators have water / bad fuel / or major mix of 2 cycle oil and stabilizer.

That's where I'd start.

I second this. Don't dismiss the batteries. It's easy to check and fix. Then do as BUP suggested. IAC might be boogered up with stuff too.

Usually water passing by the fuel filter upon advancing the throttles will not allow rpm's to pick up. 95 % of the time will shut down the engines. Water is not combustible. Tiny amounts of water passing yes engine will still run but will not clear out until it is gone.

It also needs to be determine it this is a rich condition or lean condition.

Really the poster needs to throw out a lot more info Like running the engines on the boat fuel supply - any fuel filter change outs. Batteries and do not forget battery cables conditions. At first start up - condition of the exhaust and even running.

Who knows what condition this boat fuel supply that has sat around because it needed engines and finally got some. Posters with problems really need post a lot more indepth of things. Its like playing darts - just keep throwing them out there especially not knowing what was done and what was not nor the end user experience with mechanical situations.

I'm kind of unclear will these engines REV past 1800 rpms as well ? if so from 1800 and up how does the engines run? smooth and spot on with throttle advancement ?

I appreciate the answers I got so far and I will pass them onto my friend. As for posting more I can't because we don't know. Like I said. He bought the boat over the winter. This is the first time the ignition key was turned to the start position. They both started, That's a good sign, no engine knocks etc. Nothing overheating, good water flow out of exhaust. They sound smooth, no missing at 1800 rpms. The throttles have a lag delay. Idle gets real rough on 1 engine the other dies immediately. Moving the throttles up it delays before the rpm raises. Isn't there a throttle position sensor on these. The boat has not fully been in the water yet. This was a start up test only.

Each engine has it's own sensors. Since you say both motors have idle issues we are looking for things that the engines share or may share. Fuel / batteries / etc.

He doesn't happen to have the boat pointed downhill when he runs it does he ? I caught someone that did this. Motors where reverting bad because of this. Not good !

As Bup said, without more info all we can do is throw schit against the wall and see what sticks.

Fouled spark plugs from fogging or bad fuel or both.