Mercuiser 496 MAH HO holding back

[jbp51]

I am a Dutch owner of a Cobalt 242.
I sail these boats on the Dutch rivers and lakes and every year I take the boat to a beautiful Italian lake.

As far as I know I am the second owner of this boat.
The original engine was replaced by the previous owner with the current MERCRUISER 496 MAG HO.

I have sailed here the past few seasons without any problems.
However, now I have a problem and until now no one can find the cause. Hence I appeal to your knowledge and experience.

The engine delivers 100% of its power up to 3500 rpm
From 3500 rpm the power becomes less, at 4000 rpm only 75%.

We have connected the computer to the engine but no error code comes up.
Does anyone have any idea what could be causing this malfunction?

Thank you for your help.

JBP

[kas232ca]

Have they done a compression test? My 496 Volvo started missing and it was the spark plugs. Could be a fuel flow issue as well.

[jbp51]

Their first thoughts were the fuel injectors but that seems not to be the problem.

[AsLan7]

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Hey jbp. So glad you found us!

Surprised the computer is not showing an error. Somethings not right with that program.
Compression, fuel pressure, injector pressure, timing, etc should be reading on the laptop while running on the water. Did you spin a prop?

Tough to diagnose from the internet. Any pics?

Others will chime in shortly.

Let’s get you fixed.

And welcome aboard mate!

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