oz boater

About 40 hours on the engine.

Spent another $100 replacing plugs -- nothing changed.

Identical fuel flow until about 3000 rpm, then the port starts taking more fuel and throttle to stay at the same RPM.

Not sure Diacom would help since my service guy just confimed that there are no fault codes stored.

Baja_man

Partial Clog in the boost pump or weak boost pump?

Knot 4 Me

Toss a fuel pressure guage on that motor and take it up past 3000 RPM. Baja Man might be onto something here.

oz boater

I assume you are referring to the fuel boost bump? The engine runs perfectly smooth and uses 5 gph more than the starboard at 4000 rpm.

My service guy thought that these were not symptoms of a bad fuel pump. But I can have him check it anyway.

wjb21ndtown

Is it the motor that powers the power steering pump that is using more fuel?

It's weird that the problem seems to come and go throughout the power band, but that might account for some of the fuel irregularity.

oz boater

I believe the starboard one drives the power steering pump. The problem is on the port one (which is even stranger).

The problem really doesn't come and go. It's pretty consistent: identical fuel flow until about 3000 rpm, then the port starts taking more throttle and fuel to stay at the same rpm. This continues up until the port maxes out (using about 5 gph more at same rpm). At that point I have another 300 rpm or so for the starboard. At full throttle they use the same gph (but have approx 300 rpm diffrence).

The crank position sensor was replaced last year. Is it possible that it can somehow retard timing?

TWIN-SPINS

why not swap ecm,s and see if the problem goes with the computer

oceanoutlaw

iTrader: (3)

Check your fuel pressure under load at high RPM. The paint is known to peal in the fuel module. I have seen the paint clog the fuel regulator resulting in high fuel pressure and clog the fuel line resulting in low fuel pressure. Also do a cylinder drop test at high speed with your diagnostic computer. That should tell you if you are running on all eight.
What is your MAP pressure reading at idle,3500,and full throttle

oz boater

I think I will need to buy the Diacom software. At this point, I am relying on my marina service department. And I will be up to the price of the Diacom shortly in diagnosis hours. So I was hoping someone with a 496 would have seen a similar issue.

This may be a dumb question, but can a fuel pump/line clog result in a higher fuel flow (5 gph per smartcraft)?

I guess the basic mechanic part of me thinks that a fuel system problem/clog would not result in higher fuel flow. But I am no expert at these fuel systems.

patyacht

Just went throught the same thing. Props looked great but I had them checked and they were way out. Straight from Mercury. Not unusual from what I hear now. Get the props trued and balanced. Fixed my same issue.