Sooty exhaust after starting?

[txtaz]

Hello everyone,
Since this is my first inboard engine boat I am being very carefull with any indications it gives. After running for an hour or so at moderate speeds, we stop for lunch on the water. When we come back to the boat and fire it up, there is black soot in the water behind the boat. I've noticed it the last 2 times out. Is this normal?
It is a 350 MAG MPI with 60 hours on it. I pulled the exhaust headers to replace the plugs a week ago and had some build up in the headers that came loose but should have been blown out quickly.
If this is an indication of a problem, I would rather catch it early before other damage occurs. BTW, I have ordered a service manual so I don't have to ask these questions all the time, but iboats is taking their time sending it. On the othe hand, Bart's Water Sports sends their products out quickly. Regular ground arrives 2-3 days.
Thank you,
Wes

[olredalert]

Wes,

------Would have said it was normal for a fat jetted carburator engine, but will have to look toward some of the injected guys for help here. Maybe these injected boats are programmed a little fat to keep combustion temperatures down. Randy, you out there anywhere? If anybody has experience with soot it will be him..............Bill S

[Ranman]

I don't know what year your motor is, or if it is even fuel injected. I thought some of the older MAG 350's were carbed (Harbormasters comes to mind). My moter is a 1997 MAG 350 MPI Gen+. This flavor was only produced from 1996 to 1997. The only other people I know with the same motor is Tidbart and Don's Mile High 18.

Anyway, even if you are fuel injected, a fair amount of soot is normal. Over the course of a day on the water, my transom will get pretty dirty. This is easily wiped away or cleaned. If you're spitting soot into the water (making a rainbow or sheen), it sounds excessive, especially with an MPI motor. If your carbed and this is happening, it makes more sense. If your carb's running righ, you'll gety a lot of soot out back. LEt us know some more specifics and we'll find you an answer.

[txtaz]

Ranman,
It is a 1997 350 Mag MPI (fuel injected). I don't know if it's a Gen+ or not. No sheen i.e. gas on the water, just carbon dust on water. So it sounds as if this is normal. I hadn't noticed it before, but the boat is new to me. I guess after having a $2k outdrive repair, I'm being a little...uhmmm...you know.
Thanks for the help.
Wes

[Ranman]

Wes,

Is this the same as your motor?

Motor Pic (http://www.donzi.net/hull/DNAC8015J697/EngineCompartment.jpg)

BTW, did your letters come yet???

[txtaz]

Scott, The parts just came in last night so I haven't had a chance to put them in. Also, the boat is torn apart (i.e. no seats) upgrading the stereo. I won't get a chance to run it for a few days. BTW, I will put a check in the mail this morning.
Randy, Here is a pic of my motor. It looks different than your pic. I am still waiting for the letters to arrive.
Thanks for the help guys.
Wes
http://www.donzi.net/forums/attachment.php?attachmentid=2076

[MOP]

Wes had a confab with one of my local Merc guys, he says they are all kind of dirty no matter what you do. I brought up what Scott said about them needing the cooler T stat, he sort of wonders why they do that just like I do. All late model autos engines run high temp stats for efficiency and more power, heat is power. Wonder if anyone has any viable info on the marine EFI's having to be run cool? Carbed marine engine gain some power with heat.

Phil

[Greg K]

You both have different engines.
Txtaz, your's is an EFI with a throttle body.
Ranman's is a MPI with a different intake plenum and fuel rails.
They made both of these engines at the same time or at least in the same model year.

[txtaz]

Hi Phil, Thanks for the info. I agree, you would think they would run at the same temp as an auto engine. I have a feeling that my engine doesn't have a T stat or it is stuck open since it never gets above 130F. That probably adds to the soot coming out the exhaust. I just got a new T stat, cap and rotor from Poodle and will put them in later today. I'll get a chance to run it later in the week and post the results. Hopefully, I can get it done and save some bucks over paying someone.
Wes

[txtaz]

Greg,
Thanks for the input. I have wondered about the MPI thing. My cowl says MPI and I have an injector at each cylinder. From what I know, that is an oxymoron. Correct me if I'm wrong, but I thought MPI was multiport fuel injected where there are injectors in the intake plenum right above the intake manifold. EFI has injectors at each cylinder and a throttle body.
Ted can you shed some light on this for me?
Thanks,
Wes

[txtaz]

Randy, I didn't look at your picture close enough. What's up with the Budwiser in the engine compartment? Is it a speed trick? Do you like warm beer? Emergency gas to get home?
:wink: :beer: :shades:
Wes

[Ranman]

Good guesses. :shades: I keep the beer box back there so I have a place for the empty cans to go. Cold ones in the floor cooler, empty ones in the engine compartment. :wink:

[Greg K]

After looking a little more at it. There is a difference. You don't have an EFI afterall. It is MPI.
Your's is an MPI-MIE 350 mag.
Attached is two pics to see the differences in the MPI's
Serial numbers are a better way to try to distinguish these things. :banghead:

[txtaz]

Greg you are right. The schematic looks exactly like my setup. The engine serial number is 0L092705. If you have links to look up the serial, I would appreciate them.
Thanks,
Wes

[Greg K]

This should help you finding all the parts.
Link to your model (http://www.mercruiserparts.com/selectDocsThumbs.asp?doc_nbr=809488%2098)

[txtaz]

Greg, Thanks for the link. I will definately use it for parts.
Wes

[Morgan's Cloud]

Coming in a bit late here but was looking to see what developed.
When I put the 502 mag in the Magnum it was my first personal experience with the new generation MPI engines.Up to that point only had run a number of carb'd engines. I was shocked how dirty it was.. sooty transom after an hour and half running, carbon soot and oily film on the water etc,even smelled like a carb engine with the choke stuck on. I thought we had a major problem.I worked with our dealer on this and with NO help from Merc I found out from 'that other board' that it was completely normal and exacerbated by the fact that our ($6 a gallon) fuel is a higher octane than the motor wanted.
Even changing to stainless manifolds did'nt make that much of a difference because you have to raise the fuel pressure as part of the process.
With all the hoopla about 21st century technology I would have thought that these engines would have been much cleaner.
Steve

[MOP]

I can see the S*#@ eating grin from here, now leave it alone and go PLAY! Happy Donzi Daze!

Phil