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merc twins 496HO will not exceed 2800

Old 10-14-2014, 12:33 PM
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Post merc twins 496HO will not exceed 2800

These engines have low hours and ran well. Suddenly this summer we have not been able to get them over 2800. Was getting low water pressure alarm but that was corrected with a new sensor and now no alarms and no codes but no ability to go to WOT. I cannot understand what is making both engines behave identically. . What we get from Mercury is "I dunno" Can any one help?
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Old 10-14-2014, 01:15 PM
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1. Can you get a scan tool?
2. Whats the battery voltage? How old are the batteries?
3. Check grounds.
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Old 10-14-2014, 07:54 PM
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You are reporting that both engines are doing the exact samething as far as problems correct ? You need to find the common problem area that links both engines together,

Fuel tank so take that out of the system and run ur boat / engines on outboard portable gas tanks with fresh gas and new fuel ine hooked to the outboard portable gas tanks. Next it might be too late because your fuel system is jacked from crap that was in your fuel tank. Fuel injectors restricted or plugged fuel filters or fuel rails or electric fuel pumps or everything mentioned. I would still separate your area of the problem as is it the fuel tank side & its related components or the motor side ? Your fuel vent(s) are not plugged correct ?

Next common link between engines could be battery cables and batteries and possible grounds or hopefully you have all of that completely separate for each engine.

next run the scan and see what your available power is for both motors. These motors could be in guardian mode ( power reduction mode).

if you have the gen III cool fuel module - YOU did post what you have as far as year model of engines and serial #'s of engines. A 2001 496 is not the same as a 2006 or a 2010 496. Different coils, fuel pumps , fuel module, intake gaskets, fuel filters, and so on. The gen III cool fuel module internally will peel paint and or form corrosion causing fuel restrictions.

next is your motors DTS or mechanical shift ?

What have done to both motors that would make them do the exact same ? changed part(s) - had them worked on, added anything inline with any wire harnesses to the engines and or key switch wiring / harnesses. Did you splice into any of that ?

More info is needed plus how do the engines start and idle and run even at 2800rpms - do the engine back fire or stall or run smooth at all rpms achieved ?

Last edited by BUP; 10-14-2014 at 07:59 PM.
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Old 11-02-2014, 02:02 PM
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We have had them connected to a scan that showed the bad water pressure sensor on starbord, that was corrected, now no codes show.
The batteries are new and showing 14 volts
We have gone over every inch of the wiring and cannot find a fault.
Thanks for the input. If any one can come up with a solution it will be this group!
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Old 11-02-2014, 02:26 PM
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The fuel system was the first thing we checked after we got no code readings on the computer. We changed the Racor cartridge and when that did not change anything we did what you suggested and hooked each engine to a seperate outboard tank but again, dead end.
We are sure they are in guardian mode but do not understand why, especially with the computer showing no codes. Does the darn thing lie?
I have no idea if they a cool fuel module and do not know where to look to find out.
My bill of sale from FOUNTAIN lists them as 2004 496 Mag HO with serial #OM683286 and OM687151
The shift is mechanical
The engines start at first kick and purr like kittens at idle and run very smoothly until they hit 2800 and then stay there no matter where you put the levers.
I have a sick feeling that the only thing I cannot check is the EMC buut why both engines and without spending several thousand dollars for new ones I know of no way to check them.
I have a gut feeling that somebody in this group will come up with a solution
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Old 11-02-2014, 02:38 PM
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Did you check to make sure the throttle s are opening all the way . It happened to a buddy of mine after spending. a ton of money on diagnose,s turned out to be a bad throttle cable
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Old 11-02-2014, 09:41 PM
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Hmm interesting I'm going to look at one.doing same thing this week I.figured it was going to be a simple fuel supply issue , one tank . Did you try full ecm reset , ? Fuel sample passed ,and your remote tank wasn't fed from a outboard 1/4 quicky fitting ,true ? Keep us posted this sounds like a.fun one
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Old 11-02-2014, 09:41 PM
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vent on fuel tank clear and free flowing?
scratch this,,see you ran it on a separate tank.water and air temps in line,can you monitor the knock sensors when running?

Last edited by sutphen 30; 11-02-2014 at 09:46 PM.
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Old 11-02-2014, 09:47 PM
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Originally Posted by airjunky
And I think some 496 ho ecms will cut the rpm around 3000 or so if it thinks drives are not in gear
thats a good thought.
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Old 11-02-2014, 09:48 PM
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Originally Posted by airjunky
And I think some 496 ho ecms will cut the rpm around 3000 or so if it thinks drives are not in gear
thats a good thought.
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