2001 340 Sundancer, twin 6.2 Liter...

[Guye926]

2001 340 Sundancer, twin 6.2 Liter MPI, 377 CID, Mercruiser with Vortec heads, Inboard, V-Drive 

Mystery RPM problem that's been looked at for two months by two or more reputable mechanics

WOT should be 4500 range

Boat Cruises at 3600 RPM with no problem 

added speed between 3600-3800 RPM

However, when throttles pushed to the final 1/3 to max throttle, NOTHING

No add'l response to get past 3800 RPM, no changes to boat since prior to loss of power.

All of the usual have been tested;

Engine RPM's fine out of water

Fuel pressure, good

fuel, good

props unchanged from prior to issue

timing/firing, good

compression, good

hull, clean, no growth

boat weight, normal, no excess water/cargo

Mechanics are frustrated as they don't have further suggestions as to where the missing 700-800 RPM is going?

Any prior experiences, thoughts or comments would be appreciated.

-Guy

[chameleon]

I had this issue on an 8.1 Merc in a 340. But it was just a spark plug jacket that had popped off. Seems like you have checked that though. Other than that, altitude change? That's all I got.

[Guye926]

Thanks chameleon, no spark issue or altitude change. Thanks for the suggestion.

[Summer_Wings]

I had this issue on our 2007 310 Sundancer, twin 6.2 Mercruiser V-Drive. Turned out to be partially clogged fuel injectors. Had them all cleaned and rebuilt. Runs fine now.

[Captain_Jeff]

I had a similar problem and it was the fuel injectors. Your mechanic should be able to take the boat for a sea trial with his computer hooked up and shut down some of the cylinders while cruising. He can then determine if some are not contributing full potential. Once my injectors were rebuilt, the boat runs perfectly.

[Guye926]

Thanks Capt Jeff. Will purse the injector angle a little more, will let you know. Appreciate the feedback.

[Guye926]

Thank Summer Wings, will do the same. Hoping that's it.

[Bret]

Sounds like what we had with our 240. Mechanic hooked up his computer and found a faulty switch that would stop the engine from going over 3500 rpm. The switch was $30 and the guy put it in for $100.

[juggernaut]

Had a similar problem on my 8.1 which lost rpm during a trip. After having the "experts" look at it to no avail, I discovered it was a spark plug wire shorting out against the exhaust manifold. New spark plug wires and rpm's came back.

[JWR]

OI had a similar problem coming home from the Bahamas this summer, only getting 2500 RPM from port engine, 2005 Sea Ray 340 with Merc 8.1 Turned out to be a flame arrestor was clogged with salt due to water leak!! Best of luck. Let me know.

[Summer_Wings]

You may want to check for a clogged fuel injector. I had the same problem with my 6.2s last year. One clogged injector shut one cylinder down at 3000RPM.

[sinking]

If engine is running smooth, it may be your tach guage. On a recent mechanical survey of my 260 Sundancer, the engine sensor was reporting higher rpm, then the gauge. The surveyor said the solution was to replace the gauge.

[Wyoming_White_C]

Sounds like a fuel volume or deliverability problem. Just because the pressure is good does not mean that there is sufficient fuel being delivered. Was the fuel pressure measured with the engines running or off. I have had this happen to cars before. They run fine until you get a load on the engine, such as hard acceleration or going up a hill and more fuel is required, then that becomes the limiting factor.