HELP - Developed a bad Miss-Fire

[RedDog]

This weekend I developed a bad miss-fire over about all RPM ranges. Boat had been running good for maybe 8 tp 10miles of running family around that had never been in a Donzi. Stopped at a marina for a while - fixed problem mentioned below about the trim pump and then ran 2 or 3 miles at 80% WOT. cut it back, turned around, and the miss started. Thought it might be water in fuel related so I let it cool down some and pulled the water separator. No water in it. Ran again and it was strong again - - but then the miss came back.Pulled some folks skiing but the miss was there. Let it sit over night and the miss was there from the get-go.

Probably unrelated, but before it happened, my Bennett trim tab pump/reservoir fell off the transom and was hanging upside down by the wires. The mounting bracket came loose. Put the pump in place. It ran great, and then the miss started for the first time.

Bone stock 1998 502 Mag MPI, 300 - 400 hours, AC R43TS plugs put in Spring 2009. Put about 25 gallons of fuel in it to top off the tank the day before this.

[mrfixxall]

First start with pulling the plugs,look real close at them and keep them in order per cylinder..if you find a dark one its probably a fouled plug..also if you find a dark plug pull that wire off the cap and look inside the wire boot for rust and corrosion on the cap terminal..

it could be a few different things,,if the plugs are really clean/white color it could be an injector,,it being efi and if you had a scanner it would be the easiest way to find out if the injectors or injector is bad by comparing the integrated and the map sensor hg differences..

[gcarter]

I don't have any experience w/that engine package, but if you can get a long, thin screwdriver on the injectors while it's running, you can listen for a clicking sound from each injector, if it's working ( get your ear close to the screwdriver handle).
This is a very basic way to see which direction to go w/o a scanner.

[Tidbart]

Like Fixx said, start with basics. Plugs, wires, cap and rotor. Then move onto the fuel.

Unfortunately, you really can't get to the injectors as George suggested as they are buried under that plenum.

Bob

[DonziJon]

When you pull the plugs, you may find one or more plugs..probably on the same side of the engine..fowled/flooded with WATER. Your symptoms indicate a rusted out exhaust elbow/riser. When you pull the riser off you will see water down inside the manifold. "Sometimes" a very close inspection of the elbow OFF the engine will reveal the leak...but just as often you won't imediately find it. All you need is a pinhole to cause a problem.

After finding water flooded spark plugs..pull the elbow/riser on that side and look for the puddle in the manifold. Fourteen years seems about right for a rusted out elbow/riser. My experience has been the Manifolds last Twice as long (in years) as a riser.

[RedDog]

Thanks for the input. I'll start looking into it this Saturday - I'm on travel until Friday night.

PS - I put new plugs, rotor, cap, and wires on Spring of 2009.

I pulled 2 plugs (starboard side front 2) Sunday morning and they were like brand new with little / no color to them

[MOP]

I have my $$ on a set of wires, the carbon wets out and deteriorates in our apps. I had a real good set on from the get go they lasted three seasons, they get tired and do not carry the load.

Phil

[Tidbart]

Thanks for the input. I'll start looking into it this Saturday - I'm on travel until Friday night.

PS - I put new plugs, rotor, cap, and wires on Spring of 2009.

I pulled 2 plugs (starboard side front 2) Sunday morning and they were like brand new with little / no color to them

I would still check them all. On the wires, do a visual check for damage, cuts etc, and then do a resistance check. You will need a good ohmmeter and you will need to know the ohms/ft in order to get an idea of an acceptable resistance.

B

[RedDog]

Running great again. In fact, the 200 or so RPMs I had lost over the past couple of years are back! Roadtrip suggested running injector cleaner through it. Yesterday I filled the fuel filter/water separator half full of concentrated Techtron (sp?) and then put a 1 and 1/2 20 oz bottles in the tank. Ran it in the drive on the hose for about 1/2 hour and then water tested. No miss. Today I ran it again, 4 people on board, and it ran the best it has in a couple of seasons.

[mrfixxall]

Running great again. In fact, the 200 or so RPMs I had lost over the past couple of years are back! Roadtrip suggested running injector cleaner through it. Yesterday I filled the fuel filter/water separator half full of concentrated Techtron (sp?) and then put a 1 and 1/2 20 oz bottles in the tank. Ran it in the drive on the hose for about 1/2 hour and then water tested. No miss. Today I ran it again, 4 people on board, and it ran the best it has in a couple of seasons.

Didn't know you were down 200 rpm's, if it happens again i would pull them and send them to buiz and have him clean and back flush them and replace the screens in the injectors..

[Cuda]

That's the problem with fuel injection. You can't fix it yourself, or even be sure that's the problem. Give me a carbuerator and a distributor everytime. Then if I can't fix it, it ain't broke.