Backfire/Loss of power at WOT

[dwheeler7676]

1999 2100i with Mercruiser 350 MAG MPI Ski Tow Sports engine and Velvet Drive Transmission. Boat starts right up and idles smoothly. Accelerates great and gets on plane easily. However, when putting throttle to full the engine will take off with full power but in a minute or two will start to backfire and almost stall. Backfire continues, even with less throttle, unless throttle is dropped way back, then it will run normal again. If I keep it down to about 3500 rpm the backfire problem does not happen. When it backfires the exhaust turns very black (so rich fuel mixture). However, when it runs normal, or during acceleration (before the backfire starts) the exhaust is clean and pure. So this is a sudden problem, that also can go away suddenly (but only if throttle pulled way back). But it always happens when going full throttle (around 4000 rpm) under load. Only 250 hours on engine and it has always run great before. Engine water temp solid at 185 and oil pressure at least 40-60 psi. Ideas? Thanks in advance!

 

[Bondo]

Re: Backfire/Loss of power at WOT

1999 2100i with Mercruiser 350 MAG MPI Ski Tow Sports engine and Velvet Drive Transmission. Boat starts right up and idles smoothly. Accelerates great and gets on plane easily. However, when putting throttle to full the engine will take off with full power but in a minute or two will start to backfire and almost stall. Backfire continues, even with less throttle, unless throttle is dropped way back, then it will run normal again. If I keep it down to about 3500 rpm the backfire problem does not happen. When it backfires the exhaust turns very black (so rich fuel mixture). However, when it runs normal, or during acceleration (before the backfire starts) the exhaust is clean and pure. So this is a sudden problem, that also can go away suddenly (but only if throttle pulled way back). But it always happens when going full throttle (around 4000 rpm) under load. Only 250 hours on engine and it has always run great before. Engine water temp solid at 185 and oil pressure at least 40-60 psi. Ideas? Thanks in advance!

Ayuh,... Welcome Aboard,... Change the fuel filters, 'n Try it...

 

[bb_aes]

Re: Backfire/Loss of power at WOT

If he has black exhaust (telling me rich fuel mixture) at high RPM when the problem occurs that tells me it is not a fuel starvation problem, but an electrical problem where the fuel is not being burned and backfiring (perhaps in the exhaust). Maybe distributor sensor, cap, rotor or plugs?

 

[dwheeler7676]

Re: Backfire/Loss of power at WOT

If he has black exhaust (telling me rich fuel mixture) at high RPM when the problem occurs that tells me it is not a fuel starvation problem, but an electrical problem where the fuel is not being burned and backfiring (perhaps in the exhaust). Maybe distributor sensor, cap, rotor or plugs?

I'm liking the sensor over cap/rotor/plugs since it's an all-of-a-sudden issue, not gradual (coming-on and going-away), and I cannot explain why a fuel starvation problem would cause the (rich) black exhaust condition... Filter/separator was changed out late last season (& hardly used this season), dunno about the in-line filter or gas tank anti-siphon fitting (if there is one); ran engine at night in dark w/o any evidence of arcing. So, will approach by the cheapest route leading to the more expensive one (first asking q's -> then filters-> then cap/rotor/plugs -> finally sensor - before asking more q's). ;^}

Thanks, all!

 

[kd7isf]

Re: Backfire/Loss of power at WOT

Sounds like ignition to me.. Check the ignition module and the shift interupt switch (if it has one) and the ignition fuse and cableing.. Black exhaust is unburnt fuel Yur either dumping in too much (fuel pump making too much pressure, fuel pressure regulator shot, or injectors not sealing properly) or yur just not burning it (ignition problem).
Check all sensor plugs and electrical connections..
This sounds like an electrical issue to me just simply based on the random intermittiancy you describe...

Hope this helps..

 

[dwheeler7676]

Re: Backfire/Loss of power at WOT

Well, after reeling in a few red herring (tightening a loose exhaust clamp which allowed water in the bilge, followed by replacing the oil pan which had rusted out at the oil drain plug), I got back to the backfire issue. I opted to replace the (~2yr old) ignition module and, "Voila!" Runs just like new!!!

Thanks for all inputs!