marine engines running rich

98220swift

I have noticed it seems marine engines run very rich. You can normally smell them running at  idle and underway. This is even with fuel injected engines. Why is this? Also is there a way to flash the ecm for better performance? Mine is a 99 5.0 tbi. I would think the newer ones have more advanced ecm design over the older ones. I would think that making them run so rich used more fuel and is not as good on ring seal??  I have noticed this on several marine engines not just mine.  I had two Honda pwcs that where port injected and they would make them selves black on the transom area because they ran so rich.

212rowboat

Rich engine is less likely to predetonate. predetonation is harder on pistons, rings, bearings, cranks, ect... Than a rich engine is.. a lean engine is hotter, and it's easier to ignite... Fuel is just the catalyst, its air that goes boom... timing on marine engines is Re tar ded to reduce the likelyhood of predetonation... It's not uncommon for a predetonating engine to throw a part... Throwing parts through the hull is a bad notion. first you're stranded (and more likely so with a lean engine) and then you're sinking... That would suck, no? 

Now the newer ones that have o2 sensors, they can safely advance timing and trim fuel.  You'll see improvement with those... The controls on engines in marine environments has been limited until the advent of pcm controls and being able to reconcile trim and spark with knock sensors and o2 sensors to sniff exhaust... Erroring on the side of caution (rich) is mucho better than risking lean. 

98220swift

That's what I was thinking. But still a 5.0 tbi motor has low compression and runs a low operating temp. I would think predetonation would be very unlikely. Also in 1999 (when mine was made) all fuel inj car and truck motors (the 5.0 is very similar to a 90s gm truck motor) had 02 sensors and a lot had knock sensors. I still don't understand why they would need to be so rich. Is there a way to flash the ecm like a truck or car ecm?

Black_Diamond

You won't have an O2 sensor on a '99, only the newer catalyzed engines have those if I am not mistaken.

I'm sure you could flash the ECM..but a boat motor runs far different than any car/truck engine.  I would avoid it and I doubt you would get much performance change out of it anyways.

Michael T

One of the first rules an engine builder learns for any type of high performance engine is to not let it run too lean. It's 100% as DI said.  IMO: you run too lean you run too hot, you run too hot and you ruin your engine - fast. I used specialized fuel pumps and Gaffrig fuel pressure gauges with loud audible alarms if fuel presure dropped below preset levels. Expensive high performance  engine owners that I knew had fuel pressure gauges linked to rev limiters. Fuel drop triggered an rpm drop. WAY too rich isn't good either but a little rich is far safer than a little lean. MT

98220swift

I agree but this is not a high performance engine. I know about running lean I have built several nitrous engines for drag cars. This 5.0 is running too rich at idle and at cruise. There is very low risk of detination at those engine loads. I know a different tune will probably not give any real performance gain but could help with fuel economy. Which I know will never be good its a boat. It just seems to me to be sloppy tuning by mercruiser. Hopefully the newer ones are better.

Michael T

Hey Swift - There are a number of companies that can provide you with relatively inexpensive reprogramming flashes that should nicely alter your A/F curve and correct the "generic" programming that we all have to deal with in our vehicles. MT

212rowboat

throttle body injection is always tricky, no?

you can dial back your fuel pressure as the easiest way to reduce delivery.  we're just talking a nudge, now... this is a scary prospect to me, though.

you can advance your timing from the 8~10* OE request to around 20*, and just fuddle around until the engine is full temp before hammering on her.. you may even get by with 30ish degrees.. this will allow more time for combustion and burn the excess, or what's being NOT burned right now...

I would do one or the other, not both.

you could also advance the timing at the crank for a mechanical advance by aligning a few teeth off of dots, but, I'd rec you not do that and just use the light, instead.

on a PCM controlled engine, the only way you can advance the timing is either with a programmer/flash or by doing it at the crank... you can also, though I don't rec it, index your ckps to do it...  Does your TBI engine have a PCM?  Some of the early models didn't.  If it does, you could address your fuel delivery there with precision, which makes altering fuel trim a less scary prospect and something I wouldn't hesitate to do...

reducing fuel delivery will do nothing but save you some fuel, though- and will actually reduce torque for the same RPM's as you're seeing now... adjusting the timing, however, will absolutely power your ride up- maybe as much as ten ponies over stock at any given engine speed... with the low compression, and the low operating temperature of the engine, I'm willing to wager breaking something is not likely to happen... I further wager that Merc, along with every other marine manufacturer, lighten the spark timing simply for added engine longevity from catastrophic failure, and that alone.

98220swift

I have been thinking about messing with the timing some. The only problem is I don't know how to get the engine to base timing. The gm ecm has a connector to unhook so you can set the base timing. I don't know much about boat electronics although I am ASE master auto tech for almost 20 years. The fuel injected boats are a new thing to me.

212rowboat

google just what you said...

on my wee lil 4.3 with the thunderbolt ignition, base timing is set by connecting two wires- one from the dizzy, the other in a harness that lays on the intake manifold... wham bam... base timing... you just have to kill the engine and cycle the key to remove it from base timing..

I don't know how it works on a TBI engine.  What kind of ignition controls do you have? is is PCM/ECM controlled or a thunderbolt?  Do you have mechanical fuel pump or an electric fuel pump?

98220swift

I would think it is ecm controlled. It also has a electric fuel pump.