2001 50 with 3406E RPM & GPH

Fuel flow on Cat display shows actual burn taking into account what is returned

You can use the fuel burn totalizer to keep a fuel log. It is very precise. On 2000 gallon tank I m usually within 30 to 50 gallons when I refuel

Tachs are very accurate, maybe too accurate and will indeed vary by 20 or 30 rpm depending on turn, list or waves.

Had these engines. Know them well. Your WOT load throttle target is 2325.

Interesting that the WOT no load shows a variance between the engines as well as throwing a code. There may be an issue here with a sensor that has the engine showing an RPM while running at another. These engines have about 50 sensors on each of them. Locate the throttle sensors and swap them between engines, rerun the test.

The tachs on these engines are Cat displays, and they read what the ECM sends to them. Gotta start with settling the basis data by eliminating known errors. See both engines act the same under no load, then work from there. This could turn out to be a series of issues, even beyond a throttle sensor. For the moment I'd trust the props more than the data.

I'm pretty sure on Pre - C series Cats that the fuel burn is calculated off of a base fuel map, that takes into account injector pulse, load, and rpm. I don't think it's as accurate as the C series fuel burn calculations. Don't get me wrong it's close, maybe 95% accurate or better......but not 99% + accurate that the newer C series computer does. Rpms can jump around 20 rpms at cruise with a little wave action as the boat is going up and down and over waves. I see as much as a 60 RPM swing with some engines and boats. A seatrial is a good idea. Also changing the aftercoolers if they haven't been done in 6 years is another good idea.

What's fuel pressure at cruise on each motor?

I have the same setup as well. Generally, at 1850-1900 I see 23 boost, Fuel Press Should be between 65-80 psi for optimal running, and a fuel burn rate of 24-26 gal/hr. per side.

If your fuel press. gets below 50, I would be changing racors, maybe on engines as well. I generally do my racors 3-4x per season, but I do a few hundred hrs. a year of running too.

Sounds like you're running a lot of load at 1850. Is the bottom and props clean? Do the engines hit 2300 WOT?

To be clear, you're running CAT 3406E-800? And if so, why is the discussion that WOT is 2300 when it should be 2325? WOT under no load should be 2415. But certainly under full load the target is 2325.

On these engines, fwiw, if you lose fuel pressure and performance, those manual fuel pumps are typically garbage. Use them once, and their seals are compromised, sucking air. In the final days I replaced the pumps, and vowed to never use them. I installed an electric system and plumbed the engine to allow me to use that pump to prime, etc.

Also, the props should be tuned to provide 2325 WOT with full wet load. If you aren't achieving the RPM your engines are overloaded. Don't tune to a 50-75 percent load.