Misfire/Backfire.... Dealer is killing me $$ (So is the Malibu Crew)

1998 Supra Legacy, Indmar Assault 325mpi

At around 3000 RPMs and up I am experiencing a misfire/backfire and they are inconsistent. They do not occur at regular intervals. The “interesting” part is when the problem occurs and you go to neutral the RPMs stay around 3000 and the motor winds down very slowly to idle. It does not kill. The boat can be turned off and starts like normal.

The problem seems to be worse after the boat has warmed up and we sit for a while. We have gone out several times thinking the problem is solved, only to pull up a skier and have to stop.

Boat starts great, Idles great, everything is good until we ski or cruise around the lake.

I have replaced:
Plugs
Fuel filters
Cap and Rotor
Throttle Position Sensor (Dealer)
Gas is good/fresh
Coil (dealer)
Fuel Pump (Last Year)

I am now at the mercy of a dealer who is calling Indmar. $500 the first trip (Throttle Position Sensor), now 7Hrs of labor We are hoping to salvage what is left of a short summer.

Does it actually backfire? You hear a loud thump? If not, I would do the ignition sensor and maybe wires. I had one go bad and it would just lose power (sounds like a misfire). I think they are less than $100.

Mike

Sounds like a computer problem is it setting any codes?

I hope that my dealer would check that (ignition sensor). As far as codes I was just told that every time it misfires the computer disconnects.

There is a thump.... Would that be a first step from the dealer?

I wouldn't put anything passed a dealer. With 7 hrs labor they should have gone through the whole motor.

Or is the computer disconnecting and causing a backfire. Try tugging on the wire looms.

The super high idle has me thinking vacuum leak. That will also cause a backfire out the intake rather than the exhaust. Intake backfires sound louder than exhaust backfires on a boat.