496 Mag HO RPM issue

[abvip]

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

 

[Scott Danforth]

first, welcome aboard

second, have you verified the tachometer? have you verified throttle travel?

 

[alldodge]

Welcome, OSO

https://www.offshoreonly.com/forums/general-q/371779-496-mag-ho-rpm-issue.html#post4791501

 

[abvip]

first, welcome aboard

second, have you verified the tachometer? have you verified throttle travel?

Thank you! The tachometers did check fine. It's DTS and the problem is the same in dual throttle as well as one lever mode. The throttle bodies were swapped and the positioning sensors are in spec.

 

[abvip]

Welcome, OSO

https://www.offshoreonly.com/forums/general-q/371779-496-mag-ho-rpm-issue.html#post4791501

Trying to post different places to get as many ideas as I can LOL! Thanks in advance to everyone for the help. I'm extremely frustrated!

 

[alldodge]

Trying to post different places to get as many ideas as I can LOL! Thanks in advance to everyone for the help. I'm extremely frustrated!

No issue, just letting others know where others have replied.

For others in short and the key I had not noticed was

It is not a drive problem as the split occurs in throttle only mode as well as while underway.

My apologies for not taking more time when I first read the problem. This has to be an issue with the DTS calibration or a component in the controls

 

[Fun Times]

-Rinda scan tool/diagnostics show no unusual codes.

Sort of wounding what the available power percentage read on both engines? + maybe the throttle percentages... Every thing seem equal on the live engine data while scanning?
What’s the back history of this situation.. Boat new to you?
Any type of service work done then all of a sudden you noticed the change or?

One might be inclined to say swap the PCMs just to see if the issue follows over but there’s an off chance you might lose a programmed communications between the command modules near the helm and the engines PCM’s which a Merc dealer might need to resynchronize via a dealer to manufacture computer system programs... Are you working on this yourself or Merc dealer help?

 

[abvip]

Sort of wounding what the available power percentage read on both engines? + maybe the throttle percentages... Every thing seem equal on the live engine data while scanning?
What’s the back history of this situation.. Boat new to you?
Any type of service work done then all of a sudden you noticed the change or?

One might be inclined to say swap the PCMs just to see if the issue follows over but there’s an off chance you might lose a programmed communications between the command modules near the helm and the engines PCM’s which a Merc dealer might need to resynchronize via a dealer to manufacture computer system programs... Are you working on this yourself or Merc dealer help?

Both engines show 100% available power. Throttle position percentages also very close at multiple RPM. All other live engine data while running at different RPMs was reviewed by myself and a local mechanic and everything was within parameters between the two engines.

The situation developed the last two weeks or so of the season last October. No service or particular event took place that brought the problem on. I thought one of the many fixes and preventative maintenance items I performed in the offseason would have corrected the issue but they did not. I'm in my 4th year of owning this boat...never had this problem before...both engines always ran exactly the same.

I'm working the problem myself along with suggestions from my local mechanic. He is good but old school and not very familiar with DTS, Axius etc. 99% of the boats he sees do not have these features.

I did try to swap PCMs a few years ago when I had an issue with Axius acting up and the boat went haywire. Not inclined to do that again without a Merc tech and computer available. Just trying to eliminate all other possible fixes I can do myself before I call in someone with the Merc equipment...which around here is hard to get. Thanks for replying!

 

[Fun Times]

Because the available power is at 100% would start to tend to lead me off of this being a general common sensor type issue as they would normally show and reduce power as felt needed within the PCM plus sound a warning horn in most cases... With possibly 2 exceptions that sort of comes to mind as a “maybe”..,1 would be a worn down Crank shaft sensor tip area (since you mentioned Cam Shaft Sensor ) and #2 might be a harder to find Knock sensor picking up a sound of the engine or thought to be a sound that’s false which might not show as available power...

You mentioned the complete fuel system being cleaned and checked, etc, was there anything found that may have caused an issue?...
If yes (or no too) plus just so you’re aware, there is a fuel injector pulse width damper that “looks” like a fuel pressure regulator on the fuel rail (though rare) < that could become compromised/clogged with debris affecting fuel flow that may not show on a fuel pressure reading.

Thanks for reaffirming my suspicions of swapping PCM”s on an DTS system. Never done it with that system but suspected it would be a problem.

What was the fuel pressure gauge readings both with the vacuum line connected and disconnected from the fuel regulator?

 

[abvip]

Because the available power is at 100% would start to tend to lead me off of this being a general common sensor type issue as they would normally show and reduce power as felt needed within the PCM plus sound a warning horn in most cases... With possibly 2 exceptions that sort of comes to mind as a “maybe”..,1 would be a worn down Crank shaft sensor tip area (since you mentioned Cam Shaft Sensor ) and #2 might be a harder to find Knock sensor picking up a sound of the engine or thought to be a sound that’s false which might not show as available power...

You mentioned the complete fuel system being cleaned and checked, etc, was there anything found that may have caused an issue?...
If yes (or no too) plus just so you’re aware, there is a fuel injector pulse width damper that “looks” like a fuel pressure regulator on the fuel rail (though rare) < that could become compromised/clogged with debris affecting fuel flow that may not show on a fuel pressure reading.

Thanks for reaffirming my suspicions of swapping PCM”s on an DTS system. Never done it with that system but suspected it would be a problem.

What was the fuel pressure gauge readings both with the vacuum line connected and disconnected from the fuel regulator?

I am not getting any warning horns. I did attempt to swap the crank sensors however I could not get the port engine one out so I did not want to force it and cause another problem. I did not look into the knock sensor. No codes were present for either item in my scan but as I think you are suggesting they might not cause a code or warning.

Nothing really abnormal was found with the fuel system overhaul...very minor debris. Boat is running about the same after cleaning, maybe at slightly higher RPM on both sides but the split has remained and is the same. I did not look into the fuel damper...forgot about it...I believe those can be removed from the rail and swapped...will try that.

I did not check fuel pressure with the fuel regulator disconnected. With it on It was steady at idle on just over 40 on both engines. Next time I am able I will check with it off.

 

[Fun Times]

I did not look into the fuel damper...forgot about it...I believe those can be removed from the rail and swapped...will try that.

Service Bulletin Bulletin No. 2006-09 is a guide for you if desired to try,

http://ww2.justanswer.com/uploads/keysync/2010-05-08_005804_EN_09.pdf

I did not check fuel pressure with the fuel regulator disconnected. With it on It was steady at idle on just over 40 on both engines. Next time I am able I will check with it off.

Sounds about right with the fuel pressure but the vacuum hose is an easy enough thing to try to do of knocking down the list of possibilities.

How is the exhaust output set up on your boat...Maybe the Y-Pipe is clogged.

First thinking external components before worn internal engine components.

Could also try calling Mark or Ed at RindaTech and ask about their version of Mercury / MerCruiser G3 engine diagnostics system to see if they have a program for command modules to PCM communications. https://www.rinda.com/marine/diacommarine.htm

 

[dubs283]

i'll play...

first, verify the tachometers are reading correctly, can be done with an advance timing light with rpm capability attached to plug lead

second, verify both throttle plates are opening evenly and fully

third, agree with fun and verify exhaust system is open and allowing full exit of gasses/water. ensure flappers are in place and operating correctly

a KV meter on the plug leads will show you precisely the amount of voltage each lead/coil is sending to the plugs. 496 coils have been known to have varying outputs

after that my guess is there is an issue with the dts binnacle, servo, wiring or a software issue

further investigation would include a leakdown test, however ime something obvious reveals itself before this test is necessary

 

[abvip]

Service Bulletin Bulletin No. 2006-09 is a guide for you if desired to try,

http://ww2.justanswer.com/uploads/keysync/2010-05-08_005804_EN_09.pdf

Sounds about right with the fuel pressure but the vacuum hose is an easy enough thing to try to do of knocking down the list of possibilities.

How is the exhaust output set up on your boat...Maybe the Y-Pipe is clogged.

First thinking external components before worn internal engine components.

Could also try calling Mark or Ed at RindaTech and ask about their version of Mercury / MerCruiser G3 engine diagnostics system to see if they have a program for command modules to PCM communications. https://www.rinda.com/marine/diacommarine.htm

Thanks for the bulletin. I am going to try that.

I did swap the fuel regulator hoses as well which resulted in no change. I'll get pressure readings with the hoses off.

I have a Corsa exhaust and the problem persists with the thru hull open or closed.

I'll inquire with Rinda as well to see if they have something...thanks for the suggestion.

 

[abvip]

i'll play...

first, verify the tachometers are reading correctly, can be done with an advance timing light with rpm capability attached to plug lead

second, verify both throttle plates are opening evenly and fully

third, agree with fun and verify exhaust system is open and allowing full exit of gasses/water. ensure flappers are in place and operating correctly

a KV meter on the plug leads will show you precisely the amount of voltage each lead/coil is sending to the plugs. 496 coils have been known to have varying outputs

after that my guess is there is an issue with the dts binnacle, servo, wiring or a software issue

further investigation would include a leakdown test, however ime something obvious reveals itself before this test is necessary

I do know the RPM on the helm and with the scan tool match but I'll try and verify them with the method you suggested. I'm pretty sure my tachs are pretty accurate tho because you can actually hear the difference in RPM and them not being in sync.

I did verify the throttle plates opening evenly. I ran with two other guys and had the engine hatch cracked and flame arrestors off. We verified even movement and fully open at WOT for both engines.

I have no change in the split with the Corsa open or closed. I had the diverters rebuilt just a year ago so I'm pretty confident they are set correctly and working properly.

I did not measure coil output. I did have an even RPM drop on my misfire test under load. I will try and get each coil measure precisely.

I'm trying to eliminate all potential culprits I can work on before I move on to DTS and software which will require a certified tech/computer, downtime and $$$ LOL

I have not done a leakdown test but my compression test was spot on.

Thanks for all the great suggestions!