Throttle Control

Hello All... new here...

I have a 2002 Searay Amberjack 290, with twin 350 MPI shaft drive engines. My question today is this:

The port engine is the worst but to some extent the starboard engine is similar... I can idle the engines (troll or slow through marina waters) in which case the engines sit at about 630rpm... If I am gentle on the throttles I can increase the rpm to about 850rpm but then it hits a definite change and the engine jumps to like 1200-1500 rpm, nothing in between.

I assume the initial engine control is with the IAC and not the main throttle body but my question is should I not be able to increase the RPM slowly up through the lower 1000's, why the jump.

I tested it today after changing the IAC on the starboard engine (had two suspect idle loss of power situations) and at the dock with no load on the engine it jumped from 850rpm right up to 1500rpm and it was virtually impossible for me to get anything in between. (same jump on the port engine 850 right up to 1500rpm)

Thoughts please.

Additional and likely important info: The throttle and gear controls are separate on my engines. Two knobs for the gear and two for throttle.

Cheers

Welcome aboard.

shaft drive = inboards

The throttle cable connection should go right to the butterfly and is the source of air control for engine speed. The IAC control should only control the idle speed with the throttle is closed. (Some may have a separate throttle closed switch.) Guessing this is a used boat and relatively new to you and the problem has been there and now being addressed.

I'd start with a scanning tool and evaluated the throttle position sensors' outputs. You didn't mention the brand name but I'd suspect it has the MEFI-4 computer. You may wanna check with the local dealer for that brand; Service bulletins address this type of issue if it occurs with modest frequency.

You may also wanna consider getting the factory service manual(s).

Hi There Mak...

Thanks for the info. You are correct that this is a used boat which is new to me. The engines have been doing this for some time, basically just the port engine but as indicated above I am not sure about the starboard engine now after changing the IAC.

The engines are Mercruiser 350MPI Horizon, I have found a service manual online for it Number 31 which is dated Sept 2000. Unfortunately the manual does not include much in the name of trouble shooting this kind of stuff.

I will try to follow your suggestions and see what I can find.

Cheers

That may be new enough that you need to get another manual for the PCM (ECU, computer, etc). If it is a PCM555 controller, I know they have they own manual. The MEFI4's are in the thick manual.