7.4mpi just started running rough and with gas coming out exhaust

[Ozshadow]

1999 7.4mpi 0L854***
270 Sundancer single engine B3

Just before I was about to pull into the marina on plane my boat started sputtering and missing like it had water in the gas. I dropped it down to about 1k. It idled in running but a little rough. It smelled rich. It wasn’t until I got to the dock that I noticed a fuel sheen from the exhaust. You could smell it.

inside the bilge is fine. The plugs were new. I have yet to pull them since this just happened but they are still all connected. The water was cold. The oil is new and clear with no fuel smell.

Thoughts on what would cause this? Particularly the gas in the exhaust?

Could this be a bad cool fuel module? I think the regulator is part of the assembly. Is that something that can be replaced in the water at the dock? It’s a single but still looks difficult to reach. Gen 2 type.

 

[jimmbo]

Does it have spark on all the Cylinders? Just because the Wire is connected, doesn't mean the Plug is Firing. The Spark must be good enough to jump at least a 5/16" gap

 

[dubs283]

Agree on checking spark at all cylinders especially since plugs were just changed.

Will state too that the 7.4 MPI Mefi controlled efi fuel system went through a few changes in design over the years. First check would be that all the vaccum lines related to the fuel presure regulator(s) are connected and intact. Then check to see if fuel is present in the vacuum lines. This is a sign that one (or more) of the regulators has failed. Thrid would be to verify fuel pressure at various RPM and loads. If too high, definite possibility a regulator has failed.

On some fuel rails GM installed a regulator that Mercury left in place as a damper for pulse width which helps even out fuel pressure in the rail. One style the damper/regulator is replaceable, the other is not and requires a new fuel rail assy

 

[Ozshadow]

UPDATE - I’m not sure it’s just an injector. It runs rough and weak. The entire passenger side plugs are fouled. I’m sure they were wet. The ceramics are black. The 4 on the other side are white. Each cylinder has its own injector in this MPI setup. Can one bad injector foul every single plug? Would this mean a bad ECM? It believe it is a MEFI2 system.

 

[dubs283]

Mefi system uses a batch fire for injector pulse meaning an odd/even fire for injectors. It does not pulse each injector individually during the timing/pulse/firing cycle of the engine. 7.4L engines really like fuel, plus fuel works great to cool the combustion chamber so merc decided to map a batch fire pulse width during engine operation to ensure enough fuel reached the cylinders. However if one or multiple parts of the system fail the engine runs rich/poorly. It's possible you have a ground issue in the harness or the ecm is failed/failing causing injectors to stay open. Focus on the rich/fouled plug side of the motor, do you hear/feel the injectors clicking/pulsing?

 

[alldodge]

0L854***

Need a better serial number

Only way it may have a MEFI -2 is maybe if its a L29 model

Fouled plugs one side and white on the other, double check all the plugs

 

[achris]

1999 7.4mpi 0L854***

Need a better serial number

Yep, like all of it. Not sure why people post partial serial numbers, makes it impossible to look up and see exactly what ECU is installed (or anything else for that matter)...

@Ozshadow if you think posting only the first part of the serial number is somehow protecting something, it's not. All anyone can do with the full serial number is look up parts and a description, which is precisely what we need to do. Please post the full serial number...

Chris...

 

[Ozshadow]

Need a better serial number

Only way it may have a MEFI -2 is maybe if its a L29 model

Fouled plugs one side and white on the other, double check all the plugs

0L354842

That should be it

The injectors are difficult to access with the boat running. The upper intake plenum covers everything

 

[dubs283]

The injectors are difficult to access with the boat running. The upper intake plenum covers everything

Understood, however imo this is, aside from a live data reading/fault status check, the best way to troubleshoot your issue based on info provided.

FYI, all merc mefi controlled 7.4MPI engines use batch fire injector control

 

[Ozshadow]

Thanks. Is it possible for one bad injector to make the entire side run very rich?

it’s this style of intake. I hope there are no issues removing the upper plenum in the water. I can pinch the intake hose I guess.

 

[alldodge]

Thanks, the SN does show L29
No way one injector will cause all cylinders to go black on one side.

Check your fuel pressure = 43 psi

The MAP sensor or vacuum leak can cause on over rich mix but with this just happening, I'm leaning more toward fuel pressure

Can find out if there are any codes with a 12V resistor LED or using a voltmeter that has a tone on the ohm side. Use a wire/paper clip to connect pins A/B. Place LED or meter on pins E/F.

Turn key ON but don't start the motor, counting flashes/tones,
Exp: For 14 will see, flash, short pause, flash, flash, flash, flash longer pause then it will go to next code or repeat

 

[Ozshadow]

The engine had two possible fuel pressure specs I’ve attached.
when the key is turned on it would jump to 45 and settle at 41. Turning the key off, sometimes it would hold pressure solid for many minutes. Occasionally it would plummet right to zero.

Running, the pressure was 36-37. I did not test it by removing the vacuum from the cool fuel regulator.

 

[dubs283]

The issue you are having ime has never been a fuel pump issue, pressure yes. In most cases the solution is a regulator. There have been a few changes to the design of the fuel supply system for 7.4L MPI engines. Most have at least two regulators. You need to check fuel pressure with and without the vacuum line(s) connected. This will inform you the regulator(s) are working or not.

My WAG rn is based on info provided you have either an injector issue (control or mechanical) or a damper/regulator issue. Damper being located somewhere on the fuel rail

 

[Ozshadow]

Sounds like tearing it apart first is unavoidable. I was hoping I could narrow it down and be ready to tear it apart and reassemble in the same day.

Would an auto OBD2 scanner be useful or not? I wasn’t sure it there’s a connector.

Hiring it out is not an option - summer would be over.

 

[alldodge]

Auto OBD2 will not work

 

[Ozshadow]

Thanks, the SN does show L29
No way one injector will cause all cylinders to go black on one side.

Check your fuel pressure = 43 psi

The MAP sensor or vacuum leak can cause on over rich mix but with this just happening, I'm leaning more toward fuel pressure

Can find out if there are any codes with a 12V resistor LED or using a voltmeter that has a tone on the ohm side. Use a wire/paper clip to connect pins A/B. Place LED or meter on pins E/F.

Turn key ON but don't start the motor, counting flashes/tones,
Exp: For 14 will see, flash, short pause, flash, flash, flash, flash longer pause then it will go to next code or repeat

View attachment 362036

Should this tell me if the problem is with the ECM or it’s injector driver?

 

[alldodge]

Don't think its either

Check fuel pressure. The mefi 1 and 2 has very little smarts. It can show if there is a code, but usually does not provide enough info to designate what the actual problem is

Of fuel pressure is high. This can indicate that maybe some if injectors are leaking. Testing those will indicate if they show a issue if they can be fixed. Right now we just don't know enough to advise

 

[Ozshadow]

Don't think its either

Check fuel pressure. The mefi 1 and 2 has very little smarts. It can show if there is a code, but usually does not provide enough info to designate what the actual problem is

Of fuel pressure is high. This can indicate that maybe some if injectors are leaking. Testing those will indicate if they show a issue if they can be fixed. Right now we just don't know enough to advise

I put the fuel pressure test results in post 12

 

[dubs283]

I did not test it by removing the vacuum from the cool fuel regulator.

Keep in mind this is a requirement for accurate results. Most likely a regulator/damper on the fuel rail as well

 

[alldodge]

I put the fuel pressure test results in post 12

yep it was right there
The issue of black plugs on one side and white on the other is just weird, unless someone has moved the injector connectors so this would happen. If that's the case, then I would expect this is a ECM issue or a bad connection