View Full Version : EFI Rich fuel mixture

08-14-2009, 04:38 PM
I have a 2003 Mobius LSV. I have the 325hp with throttle body injection. When I start the boat I get black exhaust. Sometimes it will flood. I know its not burning oil because my oil level hasn't changed since I changed the oil at the beginning of the season. Any way, I have no O2 sensors that I can see so how does Indmar adjust the fuel mixture? Can I adjust the fuel mixture or do I need to replace sensors? Which sensors and where are they located? It runs well but seems to be a little fuel hungry. Any suggestions?


08-14-2009, 06:20 PM
All the EFI's i've seen are regulated by the brain and sensors and are hands off that i know of. Sounds like a dealer issue. You might try injector cleaner too.

08-14-2009, 10:39 PM
Being a Bosch Fuel Injection guy (they invented it anyway) I can probably answer a couple of questions or at least give you something to think about.
Bosch EFI has had 2 types of systems--open loop and closed loop. The original Bosch L-Jetronice which came out in 1975 (VW, Opel, Fiat) and had an injector per cylinder (I laugh when I see "Tuned Port Injection" because all Bosch systems were always one cylinder one injector) and an airflow meter, it was OPEN loop. There was NO oxy sensor in the exhaust so it couldn't check the output of the system hence it is an open loop. It measured the airflow, rpm and the water temperature of the engine. This went to the computer and the system ran off a map. You could NOT adjust the mixture unless you fooled the water temperature sensor (which we did somewhat) but it still would not run black smoke. The current Bosch systems are closed loop (1983ish) and get feedback from a few more sensors, rpm, engine water, airtemp and the newly added O2 (oxy) sensor. The computer has more control now because it can read the output of the system in the exhaust, hence closed loop. It was pretty much driven by the EPA and fuel economy.
Even then you could not fool it to make it run black smoke. When an injector shoots it makes a FINE mist and when one goes bad the engine will run rough. If the fuel pressure regulator goes bad it will most likely not run. I honestly don't know if Delco has a closed loop system in these boat motors as it isn't needed in a NON EPA mandated boat engine and would be more expensive to manufacture and I haven't seen any O2 sensors but I haven't really looked that hard. But I going to bet not. And even if an O2 sensor goes bad it usually won't make black smoke.
I would first start looking at the engine ignition system, check the plugs to see if they are all light grey and that you don't have a dark one. Check your plug wire resistance too. ALWAYS start with the ignition system when checking out a gasoline powered engine!
Or we could get SC to offer the V10 VW diesel marine engine like MasterCraft does and won't have to worry about no stinkin spark plugs or wires! Oh and NO SMOKE EITHER!!!

Engine Nut
08-15-2009, 09:35 AM
The three most likely causes of a rich running EFI engine are:

1. A faulty coolant temp sensor that is telling the ECM that the engine is cold when it is hot.

2. A MAP (Manifold Air Pressure) sensor thet is telling the ECM thet the manifold pressure in the engine is high when it is low.

3. High fuel pressure caused by a faulty fuel pressure regulator or by a restricted fuel return line.

08-15-2009, 11:39 PM
Thanks Larry!

Again I want to stress you must always check your ignition system before checking the fuel system!!!
Inspecting spark plugs can tell you a lot of what is going on in an motor!!!

08-16-2009, 03:07 PM
I had looked at my plugs earlier in the year and they looked like they should. They had the tan color to them. No fouling. The temp sensor seems like a candidate for problems. The reason I think this is when the engine is warm it still seems like it is being choked. When the engine is cold it fires right up and runs. But later after it is warmed up if we stop and start up again after sitting for about 20 minutes, it acts like it is flooding. Where is this sensor at? I will take this to the dealer as a last resort. When I bought this boat from the previous owner I was told all service was done. When I actually started looking at the engine I realized they did a really crappy job and actually cut hoses instead of removing them for winterization. I don't trust my dealer.

08-20-2009, 08:59 PM
Found the problem. I took out the boat and was running around. I never saw my temperature go past 140F. Moomba states they run around 160-180F. I never even get close to that. I pulled my thermostat and notice it is rated for 140F:mad:. Any question why I don't trust anyone to do the work on this? Any way thanks for the help guys.


08-20-2009, 11:30 PM

So your computer noticed that the engine coolant was still cold and was making the fuel ratio richer.
We used to race Bosch Fuel injected cars and we would pull the temperature sensor out, it would have the Ohm range printed on it (thats how you would test it in a pot of boiling water with an Ohm meter). We would go to radio shack and buy a variable resistor with a knob, plug it inline with the sensor and install it on the dash. Now the engine was slightly modified but not much but this would allow us to fine tune the mixture until we got the highest idle speed. In this way would could compensate for altitude or weather conditions and know we had the optimum fuel ratio. You just went about it in a another way! :D

08-21-2009, 10:18 AM
I take it the tempsensor is a positive temperature coefficient (PTC) thermistor then? I'm an EE and I would like to understand ECM's a little better for some stuff I would like to build....for fun.:twisted: I don't plan on messing with the boat though because from what I understand Indmar had done alot to get these motors to perform the way they do and modifications usually mess things up. Again thanks for the help I wouldn't have figured it out without it.