Mercruiser 383 MAG MPI idle rpm

[Robbanl]

Hi,

I have installed a remanufactured 383 MAG MPI into my Bayliner 2855.
What would be the normal range of idle rpm of that motor when it has reached operating temperature?

My preveous 350 MAG MPI had an idle rpm at 600.
The 383 MAG MPI idles at about 680-700 rpm.

Is that normal?
The selling company say that the idle rpm normaly is between 600-700 for that engine.
I think it sound strange that the ECM would not be able to keep the idle rpm at the specified 600?

Thanks,

/Robert

 

[tpenfield]

The things to check would be the throttle position to make sure it is fully closed. The engine temp also needs to be up to the operating temp for the engine (ECM) otherwise it will make the RPM slightly higher than spec.

Since it is remanufactured, you may want to check that the ECM is the right one for that engine.

 

[alldodge]

There should be no issue with the ECM keeping the motor at 600 rpm if everything is set correctly and there are no vacuum leaks. See if you can find a spot (fuel pressure regulator) to put a vacuum gauge on the intake. If that is good then maybe the HVS distributor was not installed exactly where it should be.

 

[Fun Times]

600 is more the normal...I recall some engine models made around 2007ish was turned down to as low as 540 RPM...But this one should be closer to 600.

There may be a small sticker on top or side of the flame arrestor cover with idle RPM info. The owners manual should have the info as well under engine specifications....The Merc owners manual found online for this engine model says 600 too along with other "performance testing website specifications" regarding this engine model.

http://www.boattest.com/engine-revie...G-Stroker_2015
https://mercurymarine-gsdesign1.netd..._864195051.pdf

 

[Robbanl]

Thanks for the feedback.
The sticker on the flame arrestor states 600 rpm.

I will double check the tension on the throttle control wire and also check if any vaccum house has become loose during the mounting of the engine in the boat. If that seems ok I will contact the company that sold the engine again as it is sold with 2 years warrenty.

 

[Robbanl]

I have checked for loose vacuum hoses and also the tension of the throttle control cable/wire but can not find anything that seems to be wrong.

Would like to ask if anyone can describe the importance of correct gap/opening of the throttle flap when idling?
I have noticed that there are a small adjustment screw that adjust the minimum gap of the throttle flap when "closed". I guess it is supposed to have a small gap even when the engine is in idling?
What is the reason for that? Is there a need for a small by-pass of air though the throttle flap because the IAC can not handle all the air volume needed in idling?

 

[achris]

A very small gap is required to allow for expansion and contraction as the throttle plate heats and cools. If it closed up tight and then expanded due to heating, it would jam.

You need to get some diagnostic software onto the ECM and read what revs it's set to idle the engine at, also to check the TPS position and what the ECM takes as 'idle position'. That will be in volts.

This is the readout from my engine. Note the 2 different idle speeds based in whether the drive is in gear. Also note the TPS voltage settings. Make sure the TPS voltage is within these settings at idle and WOT... (read the TPS voltage when the engine is running on the diagnostic software main page)...

 

[alldodge]

That adjustment should have been factory set and if it was done incorrectly then you may be on to the issue. Mine was damaged years ago and had to adjust it so it just touched closed.

 

[alldodge]

You know I have to add more to what I just posted. Adjust so it was just closed, the while it was idling I adjusted it for correct idle.