Mercruiser 350 MPI very rich at idle and fuel mileage off

[wpowell323]

I have a 2007 Mercruiser 350 mpi with a bravo 3. It seems to be very rich at idle. I have noticed some carbon build up on the right through hull exhaust fitting as well. Also I am seeing some carbon in the water when I first crank the boat and it has some fuel smoke as well. It seems to run fine at speed, but my mileage seems to be off. A couple times i have noticed some irradict idleing, but only once or twice. I am just wondering if the IAC could be going bad or should i be looking at something else? Fuel seems good, have seen no water, plugs have about 30 hours on them, although I do plan to change them this week to the new NGK platniums that mercruiser now reccomends for this motor. Any help would be appreciated!

 

[Fun Times]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

For starters Your going to want to check your fuel pressure it should be 43 psi.

If thats ok then your going to need to hook up a scan tool to see what the Manifold Air Pressure/Temperature MAP sensor is reading, And also what the engine Temperature sending unit is reading to the EFI system, Not the one at the dash.

Keep an eye on your engine oil levels while your at it.

Is your boat still under warranty?

 

[TilliamWe]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

IAC is a very common thing to fail. And failing erractically is one way they can go. Temp sending unit is another great point. My father had one (on a 350MAG MPI) that was telling the ECM that the boat was running at 300 degrees! It was constantly injecting extra fuel to try to cool the cylinders. Mileage was way off as a result. His also has trouble restarting because of it.
Changing your plugs will almost certainly not help. Unless one is just broken/not working. What do they look like?
If it's not electrical, I'd check compression.

 

[wpowell323]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

thanks for the suggestions...I guess i will take the DDT home today and see what the temp says...i guess I can check the IAC with it as well, but it isn't going to really help me if it is trying to go out and not actually broken. I did notice idleing back in yesterday that I heard a lot of air trying to be sucked by the motor, which would lead me to think the IAC was stuck open, then all of a sudden the motor almost stalled and the noise went away, after that the noise from the motor was normal and it ran normal..

 

[cappie]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

I am having the same problem. All of my plugs were covered in carbon with black soot on the exhaust and running really rough. I started with a complete tune-up, fuel filter, cap rotor, plugs and wires. I ordered a new IAC sensor and a coolant temp sensor. Should be getting them tomorrow. Going to try mercurys quickleen to help clean the fuel system in case I have a bad injector and do a throttle body service with mercs pro-tune. I am also hooking up the scan tool this weekend but I was told the IAC would not show up as a trouble code. Hope nothing major is wrong.
Erik

 

[cappie]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

After changing the IAC sensor and the Coolant temp sensor, doing a complete tune up and throttle body service, the motor cleared up and runs great.
Erik

 

[wpowell323]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

with the DDT my IAC is showing 26% at idle...i am trying to find somewhere that shows where it should be..I can hear an excessive amount of air, trying to come in so I guess it is sticking....I did change plugs and the richness seems to have gone away some, but I a, still pretty sure the IAC is still sticking..ddt showed motor temp was spot on....guess i am going to take the IAC apart and clean it and see what happens

 

[Fun Times]

Re: Mercruiser 350 MPI very rich at idle and fuel mileage off

with the DDT my IAC is showing 26% at idle...i am trying to find somewhere that shows where it should be..I can hear an excessive amount of air, trying to come in so I guess it is sticking....I did change plugs and the richness seems to have gone away some, but I a, still pretty sure the IAC is still sticking..ddt showed motor temp was spot on....guess i am going to take the IAC apart and clean it and see what happens

Yes go for a clean up on the throttle body, Also look for the IAC "muffler" It will be number 6 in this link, Also look at number 16, That is the idle air valve.

http://www.mercruiserparts.com/Show...1681&bnbr=100&bdesc=Throttle+Body(Mechanical)