arrover

Hey guys! I bought a Formula 26br with a 496 mag in december and have been sorting out problems ever since. Now, from initial start-up, I can run the boat for about 5 minutes and then the engine will suddenly stumble and die back down to idle. It usually doesn't die completely. Just comes back to idle. If I don't throttle back when the engine dies, I hear what sounds like a muffled backfire from the engine compartment. I do not have open exhaust on the boat so I'm not sure if its a backfire. After the engine comes back to idle I can throttle back up onto plain then it will stumble back to idle again. Each time this happens, the amount of time the boat will run on plain gets shorter and shorter. It seems to idle in gear for as long as I want so thankfully I can get back to the dock.

I have a Rinda code reader and the boat doesn't throw any codes. The warning buzzer does not sound. I just put in new plugs and new plug wires (thanks Raylar!) but the problem is still the same. I have changed the fuel filter twice and that didn't do anything either. Also, when the boat dies back to idle, the code reader still shows that 100% of power is available.

Here are a couple more points from the code reader:
-iac duty cycle - idle out of gear or in gear low throttle=25-30%, high idle in gear=0% rest is dependant upon throttle position up to about 25%

-throttle position stays constant when engine dies to idle

-exhaust temps are fine

-engine temp is fine

-fuel flow rate drops when engine drops to idle

-spark advance at idle is -9 to +14 but stabilizes when throttling up. Advance drops when engine starts to die back to idle.

Do you have any ideas on what the problem could be? Could it be water in the fuel? The dealer filled the boat up when I bought it and I immediately stabilled the gas. I don't know what was in the tank before I bought it. This problem didn't happen on the test drive.....


THANKS SO MUCH!!!!

MacGyver

I'd check fuel pressure.

Raylar

My simple guess at this point is that something in the fuel supply system is cutting fuel delivery. I would inspect he fuel inlet screens on the fuel pumps or the connections and output of the fuel pumps. Remember there are two, a primary that gets fuel to the coolfuel and another that builds the fuel pressure from the supply to the coolfuel and gets the fuel to the injectors at pressure.
Cutting off a high quantity of fuel is just like pulling back the throttle and if the throttle is not closed quickly enough when the fuel cutoff occurs the engine will tend to spit, cough and backfire until the throttle is closed and the throttle position sensor is at idle postion where the limited supply of fuel can sustain a decent idle still. The fact that it happens with use more quickly kinda of indicates that the blockage or restriction is pulled tighter or closer to the point where it totally restricts fuel flow with use.
Just my thoughts here, but check the complete fuel feed system, tank inlet to fuel rail pressure, also make sure the fuel tank vent is not plugged, loosen the fuel tank fill while under way and see if the condition disappears.
These type of issues are sometimes hard to find, but there is a cause, its just got to be found. Good Luck!

Best Regarfds,
Ray @ Raylar

Pismo10

Ahhh the beauty of EFI....Have fun.

arrover

Thanks for the help guys! I think I solved it with your help. It ran fine but I need to run it for a bit longer to make 100% sure. There was a bunch of grit in the screen of the first fuel pump. I left my service manual in the boat but I think it says not to add any addtional fuel filters into the system. Does anyone do it anyway? I think its kind of strange that Merc put the fuel filter after the first pump.