1999 2100i 350 MPI huge backfires at WOT only

[bbethke]

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?

[boatwakes]

Ignition sensor inside distributor. Mercruiser part number 87-8929510Q02 about $60. Sierra part number 18-5116-1 about $45. The new unit comes with potted circuits and an extra ground wire that needs to be bolted to the engine block but instructions come with them.

[NICKYPOO]

x2. I'm just gonna ride your coattails this morning, boatwakes.

[bbethke]

Thanks, BoatWakes, for the suggestion. Could this part be bad and still have the engine start easily, idle smoothly, and have full power, smooth acceleration up to almost WOT and only fail at high RPMs? I replaced this ignition sensor about 3 years (which was sadly only 30 engine hours ago) because it was totally dead and would not start.

Also, the part number I see listed for my SN of OL305452 is 87-892150A02 and I could not find a Sierra cross for that. I could not find your number of 87-8929510Q02.

Do you still think the sensor is the problem?
Which Merc part number is correct?
Does anyone know of a Sierra cross for the correct part?
Thanks.

[Joeprunc]

I can't help you, but Boatwakes knows boats very well, I would trust his recommendation. Plus he has the same boat you have.
How does your distributor and rotor look?

[boatwakes]

Sorry, gotta lay off the crack pipe. Your number of 87-892150A02 is correct, musta been dyslexidrunk. I still think it's the sensor as backfiring is only one of two things: 1) timing is off, or 2) intake valve is floating/stuck open. To test the sensor, unplug both leads and sing an Ohm meter, hold a tester on each wire. If the reading is less than 100 Ohms, replace it. If it tests okay, do a compression test. Let us know what you find.

[ddmracing05]

Mine was a dirty throttle body.

[bbethke]

Solution Fixed It!

As the ignition module was only two years (and few hours) old and it wasn't a catastrophic failure (as it was the last time the module went bad) I wasn't convinced the ignition module was the problem. Being an Electrical Engineer I imagined if the circuitry worked at idle, at acceleration and at 3000 rpm that it would certainly work at 4000 rpm instead of the total breakdown of no spark then. However, I took Boatwakes advice and he was right on! Put in the new ignition module sensor in the distributor, cleaned the ground connection well, and off it went with no more backfire! Thanks Boatwakes!

[boatwakes]

Word.