Mercruiser 3.0 low rpm under load

[Drobb*]

This is a 2003 model. Mechanic did tune-up and the result was it still ran 3400 RPMs with wide open throttle. Mechanic then said the tachometer was probably wrong. Replacing the tachometer make no difference either. Mechanic says the compression was 150 across the board however I have little confidence that he is telling me the truth about this does anyone have any idea why this engine would run 3800 RPMs with the number one plug wire disconnected and only 3400 with it connected.

 

[Scott Danforth]

welcome aboard

first, verify the tach with a diagnostic tach

second, do your own compression test.

next, verify your fuel system is up to snuff.

there are stickies in the DIY link at the top of the forum for low WOT

other questions

what prop do you have on your 3.0.
what boat is the motor in
has the boat been weighed to make sure its not waterlogged?

 

[Bt Doctur]

And find another mechanic

 

[Drobb*]

This engine is in a 22' fisher party barge and has had the same prop for years. The boat is always taken out of the water when not on use. We only use it abut 5 times per year. It used to run about 4300 at wot. I am really puzzled as to why the engine runs faster (3800 rpm) with number 1 plug wire disconnected. They also rebuilt the carb. Ive personally checked for a crossed plug wire. Full speed now is about 18 mph with 2 people on board and it used to be about 22. Thanks for any ideas. And yes I need a new mechanic.

 

[TurtleTamer]

Why on Earth would you end up running it at any speed with a plug wire disconnected?

 

[achris]

Why on Earth would you end up running it at any speed with a plug wire disconnected?

It's a legitimate diagnostic tool. As a temporary test, quite acceptable.

I once had a boat that had no power with the battery switch on the 'BOTH' position. It had power at '1' and '2'... I checked for power in the 'OFF' position and found it working. Diagnostic... the wiper in the switch had slipped. Pulled the switch apart and fixed it. The owner just shook his head, saying he "why would anyone test it in the OFF position?"... Now he knows why.

Chris........

 

[TurtleTamer]

Oh, believe me, I've pulled my share of plug wires trying to isolate dead cylinders but have never thought to go above idle or start pulling wires without doing any other sort of diagnostic first.

So far we have, as a diagnostic process, the replacement of the tach and a check of the firing order. So either we're missing something or OP started poking around and decided to start pulling plug wires before doing much of anything else.

 

[Drobb*]

I am a bit of an amateur mechanic myself so the only explanation I can come up with that an engine runs better on 3 cylinders than four cylinders is that either I have a blown head gasket or the valves are leaking therefore I am going to get my own compression tester and test the compression myself I will let you all know what I come up with