5.0 MPI Misfire Under Load

[paoutdoorsman]

I have a 2002 Mercruiser 5.0 MPI in a Crownline 225BR that just started to have a misfire under load. 409 hours. I've owned the boat since 2009 and have personally serviced it regularly. First time out this year it had a slight vibration, and was extremely down on power while trying to take off/plane. Back in the shop I found 1 plug out of 8 that was fouled. I assumed from the fogging oil during the previous winterization. I replaced all 8 with new 41-993 AC Delco Iridium plugs, and lightly cleaned the distributor and rotor contacts, then went back to the water yesterday. Unfortunately it had the exact same behavior.

Running on the muffs it sounds ok. What would be the most effective way to pinpoint the issue out of the water? The lake is 1.5 hrs away, so it would sure be nice to be able to go back to the water confidently.

 

[Bondo]

Ayuh,.... Check the contents of the fuel filter for anything but clean fresh gasoline,.....

 

[Lou C]

#2 that’s black wonder if that injector is sticking open/dripping fuel?

 

[paoutdoorsman]

@Bondo, good thought on the fuel filter. I always keep a new one on hand but only change every other season. I put about 20-25 hours a season on the boat. Just changed it, and everything looked great.

 

[skyman2112]

Seems #2 is not firing so its either an injector needs cleaning or its not getting spark, maybe a plug wire got pinched or pulled out? Put it on the hose in the driveway, pull the #2 spark plug wire and listen for any change. If the RPM does not drop then your not getting spark. Even if the injector is dirty it will still spary fuel. I would change the cap and rotor also they are a high wear item, light cleaning opens the gap even more between the rotor and cap so its not really effective.

 

[paoutdoorsman]

#2 that’s black wonder if that injector is sticking open/dripping fuel?

I'm not sure what exactly is happening, but here are the plugs on the cylinder in question, and the cylinder on either side of it. Not sure what this pattern might indicate to an experienced tech... But it starts well and runs pretty smooth on the muffs...

 

[paoutdoorsman]

@skyman2112 Good point on the distributor/rotor. It's been several years since they were replaced. Ordered new ones, and will put on the muffs and test pulling the #2 plug wire after I get them installed.

 

[Scott06]

@skyman2112 Good point on the distributor/rotor. It's been several years since they were replaced. Ordered new ones, and will put on the muffs and test pulling the #2 plug wire after I get them installed.

This was the first thing i was gonna mention the crab caps on mpis are a known trouble spot. Good place to start

 

[skyman2112]

@skyman2112 Good point on the distributor/rotor. It's been several years since they were replaced. Ordered new ones, and will put on the muffs and test pulling the #2 plug wire after I get them installed.

hope you got the merc OEM part and not the cheap china copys, the price is tempting and they look identical but the materials are super cheap and dont last./

 

[salty3rd]

Agreed, sounds like a cap and rotor, I change mine biannually and keep a spare on board as they are problematic.

 

[alldodge]

What's your serial number?

 

[paoutdoorsman]

What's your serial number?

0M327931

 

[silver_power]

0M327931

I think your problem is the cap...change it with genuine mercury, and rotor as well and phase it.....read my story:

350 mag mpi surging under load

Hello..I have a 350 mag mpi (MY2009) alpha one serial number 1A348183. The engine idles very smooth and with steady 550rpms but as soon as I got the boat on plane and want to keep about 3000rpm , it surges from 2900-3100. If I slow down to about 2800rpm , it surges from 2600 to 2800 and it might...

forums.iboats.com

 

[Scott Danforth]

you have a crab cap on the dizzy

they fail internally.

change it (often)

 

[Searay205]

It is extremely easy to check spark plug wire continuity. Probably 20 seconds. I had a similar issue and changed the fuel filter, the distributor cap with no luck. Changed plugs and no more miss. I assume you got genuine ac Delco spark plugs. Look in you plug wire and ensure the connection isn't corroded to the plug. I had one that was corroded other 7 perfect. caught it on my with my Rinda scan tool which shows misfires by cylinder

 

[paoutdoorsman]

Installed the new cap & rotor Saturday, and hit the water on Sunday. We're back in business! All good! Thanks everyone for the input!