Lafuerza

I have a Mercruiser 454 Mag Horizon EFI with 254 hours but its from 2001 and been on a boat that I restored and cleaned up a lot of corrosion. Put about 8 hours on it and it recently started doing very high idle if I shut it off once its warmed up. When cold idle is around 800rpm. When hot it jumps to 1600rpm and then won't come down. If I get lucky it hunts between 1100 and 1400rpm. I play with throttle, turn on and off, and still does it forcing me to put into gear at that rpm which results in a nasty clunk. Any ideas on what to trouble shoot. Since motor is older I am open to just replacing more than less to limit future failures. IAC, throttle body, etc?

I plan on removing the injectors and having them serviced since I am not getting full power out of the motor. Doesn't get past 4300rpm lately and to me lacks torque when getting on plane.

Thanks in advance. Links to video or tutorials greatly appreciated. Been on youtube and lots of stuff for 5 and standard 7.4L motors but not mags.

F-2 Speedy

You need to have someone hang a scanner on it and see what its doing, if you're shifting a bravo in those higher rpm ranges you are asking for problems

AllDodge

Have you disconnected the throttle cable to see if it will idle back down?

Trash

Check throttle linkage as suggested. May be hanging up. It may be hanging up and not closing causing the high idle AND it may not be opening all the way so top end performance suffers as you also noted.

You may also have a vacuum leak when warm which will cause the same idle issue. Trick is finding the leak.

Lafuerza

Will start there. Where are typical vacuum leaks found?

F-2 Speedy

so the throttle cable only sticks when its warmed up..........check around the intake manifold for possible leak

oconnor marine

where the intake bolts to the head. Spray flammable brake cleaner around all gaskets areas and tgrottle plate shaft. If idle changes you have found the problem.

Griff

If it was vacuum leak, it should do it all the time, not just when hot. My guess is IAC. You really need to put it on scanner when its happening.
The more you shift a Bravo at over a 1000rpms, the more likely it will destroy itself

Lafuerza

Yes. Idles fine when cold. Starts right up. Once warmed up if i shut down the idle goes to 1400 or 1600 and wont cone down. If i play with it might get to 1000rpm but when i put in neutral it jumps again.

Trash

I would suggest thermal expansion could possibly be causing either throttle linkage binding (remote chance) or a vacuum leak again due to thermal expansion in a suspect gasket area.

The gasket around the top of the manifold hat and or any vacuum lines would be my first look.