350 MAG MPI rough run...no throttle

[Oliaar]

Hey all. I’ve got a 350 Mag MPI that intermittently will run rough at idle with no throttle response. If I cycle the key and start it again it runs great. Often it will also stall when coming back down to idle after cruising or pulling. Replaced IAC as recommended by someone else and it made no difference. Also installed new cap and rotor with no improvement. Am I chasing more of an ignition type problem or fuel? Could water in fuel be the cause? If so how do I check it? Is there anyway to read trouble codes in a DIY way? Suppose to head to the lake for 3 weeks soon and would love to get this figured out.

Engine SN# 0M660390

 

[tpenfield]

:welcome: to iBoats . . .

Sound like you started playing what I call sensor and component roulette. More good IAC's are probably replaced than bad ones. No good way to check for codes without code reader or the computer software.

Since the problem is intermittent, I would suspect that you may not even be getting a fault code. You could try checking the electrical connections to all of the sensor to see if there is something loose or not making good contact.

MPI engines run great until they don't. Then it can be quite difficult to troubleshoot, because the methods and tools are all different than what is used on carbureted engines.

 

[Oliaar]

Thanks Ted. I’ll start there and see if I find anything obvious.

If if it was a water in fuel concern would it be happening all the time or intermittently?

 

[tpenfield]

Sort of depends if you have 'water in the fuel', or 'fuel in the water'.

Meaning how much water. If you have a water separating fuel filter, you can always check that.

 

[alldodge]

There is no DIY made tool for reading codes with the 555 ECM, would need a Rinda scan tool or Diacom software.
With it coming and going agree could be electrical, but might also have a fuel pressure issues. Could rent a gauge from the local auto store and get some readings

 

[Oliaar]

tpenfield:
There is a spin on Luberfiner filter on the lower starboard side. Is it suppose to “catch” the water?


AllDodge:
Do you know the spec for fuel pressure on the rail at idle?

Great info guys thanks! Another tidbit of info is sometimes it’ll start fine and then idle away from the marina ok but then once I try to throttle up it falls on its face with no throttle response.

 

[alldodge]

The filter is a fuel water separator filter. Pull it off and pour contents into a clear container to see if there is anything but clean gas..

Fuel pressure spec is 43psi at 1800 rpm, and should stay there as rpms increase

 

[Oliaar]

So also had a vibration intermittently coming from the out drive. Have found drive bellows had a crack in them and was full of water. This has taken out gimbal and u-joints are very rough as well. Any chance this could cause enough resistance to cause an idling concern? Or should I still be looking for an engine concern?

 

[alldodge]

So also had a vibration intermittently coming from the out drive. Have found drive bellows had a crack in them and was full of water. This has taken out gimbal and u-joints are very rough as well. Any chance this could cause enough resistance to cause an idling concern? Or should I still be looking for an engine concern?

If the gimbal and u joints are still rough then yes this will cause some major issues

 

[Oliaar]

I’m thinking I will address all of that then first and then re-evaluate how it runs. Wish me luck on these repairs.

Thanks AllDodge