496 Mag HO RPM issue
 

Hi everyone. New poster to this forum but have been reading for years. Tons of great knowledge! I am stumped on an issue and posting here is my Hail Mary. I have read many posts and tried everything others have done and I can think of.

2009 Formula 370 SS, 496 Mag HO. DTS 1A306986 (STB) 1A306867 (Port)

The STB engine lags about 200-400 RPM vs port on most everything over 1000 RPM. Up to 1000 they are dead on. At WOT and trim in I can get about 5000 on my port engine and only 4600 on STB. It is not a drive problem as the split occurs in throttle only mode as well as while underway. Issue started late last year and is still present even after the following attempts to fix:

-Both gen III cool fuel modules removed, insides sand blasted, cleaned and new filters installed
-New fuel pressure regulators on both
-New low and high pressure fuel pumps
-Injectors removed and professionally cleaned and tested
-Fuel regulator vacuum hoses swapped
-Ran boat off remote tank, eliminated fuel/fuel tank issues
-New spark plugs
-New wires
-Swapped MAP sensors
-Swapped cam position sensors
-Swapped throttle bodies
-Rinda scan tool/diagnostics show no unusual codes
-Cylinder misfire test shows even RPM drop for all cylinders under load
-Fuel pressure at idle, under normal load and at WOT all match on both engines
-Vacuum on fuel regulator hose match at idle and higher RPM on both engines
-Compression test checks all cylinders

The only thing I'm not sure of if there is some change in the PCM fuel trim or if that is even possible. I also find it odd that the RPM are totally even up to 1000 and then it's very apparent that something occurs right above 1000 to cause the split. Is there something opening/closing at that RPM that might be getting stuck?

Looking forward to thoughts on how to attack this next. Thanks in advance!

Alex

Have you tried swapping ECMs? I'm not familiar with the DTS system as mine is cable on my 496HO so I don't know if that is possible.

Thanks for the reply. I have tried to swap PCMs in the past for another issue and the system went haywire...probably due to DTS and Axius. I can try again...but would like further advice...maybe someone can chime in on this.

These numbers coming from the boat tach or scanner?

The dash Vessleview tach and Rinda rpm match. You can actually hear how the engines are not in sync when both throttle levers are at the same position with sync on or while in one lever mode. In order to have rpm sync up I have to have the STB throttle cracked higher and sync mode off.

As for the opening closing question, the butterflies in the TB would be doing that but you said you swapped them with no change

Suggest a compression test
If compression checks out, then misfire test to see if loss of rpms are the same per cylinder

Yes...I swapped them and saw no change. TPS values on the Rinda were fine as well.

Thanks for the reply. I did a compression test and misfire test (buried in the long list of things I tried in my original post LOL) . Both tests were normal.

Have you checked the props for same pitch?