2008 5.7 mpi running rich and low idle, under 600rpm

[stang32]

i have no codes, scanner #'s look good, the IAC was tested ok, swapped it out anyway and still low idle, engine runs smooth but at such a low idle a slight stumble can be heard from the exhaust. the engine has 41, yes 41 hours on it. the boat was in storage for many years, put back in service 2 years ago and ran great, its just now that this issue has come up. cap/rotor clean.

 

[achris]

Check your in-gear switch. It's a normal closed switch, that tells the engine if it's in neutral. The reason is because in the 'info' section of the ECU program is where the idle speed parameters are. The idle speed for neutral is different to in gear. If the ECU thinks you're in gear, the idle speed will be lower...

 

[Fun Times]

Starting right around 2007 model year Merc lowered the neutral gear idle speed to 540 rpm and some engines may bump itself up to 600 RPM when put in gear under load according to what the scan tool should mention...
Somewhere in the scan tool either it was when first powering up (communicating) or inside the "read ECM" info area of the tool options, it should mention the idle RPM idle specifications which the last I saw recently read 540 & 600... As far as idle, you look to be fine... Watch your engine Temp though, 168 degrees at idle "could be" an early sign to come.... Usually it hangs closer to 163 is why I mentioned it.

Running rich? have you checked the spark plugs to confirm? Do you see a rich layer of fuel in the exhaust water?

 

[stang32]

Starting right around 2007 model year Merc lowered the neutral gear idle speed to 540 rpm and some engines may bump itself up to 600 RPM when put in gear under load according to what the scan tool should mention...
Somewhere in the scan tool either it was when first powering up (communicating) or inside the "read ECM" info area of the tool options, it should mention the idle RPM idle specifications which the last I saw recently read 540 & 600... As far as idle, you look to be fine... Watch your engine Temp though, 168 degrees at idle "could be" an early sign to come.... Usually it hangs closer to 163 is why I mentioned it.

Running rich? have you checked the spark plugs to confirm? Do you see a rich layer of fuel in the exhaust water?

good information to know, thanks Achris and Fun times.

the boat hasn't been in the water yet this season, i am getting it ready when this came up. the reason i am flagging it as running rich is because of the smell of the exhaust, and it is rather heavy wich i am sure when in the water will show up .

 

[achris]

Also look at the crap cap (on the distributor). If it's cracked then you could be getting a weak (or no) spark to a cylinder allowing un-combusted fuel into the exhaust...

Chris.......

 

[QBhoy]

Hi. Cleaning these caps…mostly doesn’t do much. Get a new one for sure. Especially given your situation. I’d also say that all mpi run rich until they warm up. Very obviously too.