350 Mag MPI low idle

[duped]

Hi all, I am having a little annoying problem with one of my engines. It's a 2000 Mercruiser 5.7 MPI Horizon straight inboard S/N 0L394846.

The idle is steady at about 500 RPM instead of the 600 listed on the flame arrestor cover. The other engine is at exactly 600. Ordinarily I wouldn't care because the engine otherwise runs great, but these velvet drives have a tendency to rattle if the idle RPM is too low, or so Im told.

Anyway, I have some rattling noises on this side when it's idling at 500. The rattle goes away completely if I bump the throttle up to 600 rpm so I'm pretty sure this would fix the problem if I could get it to idle at spec. Unfortunately it says "non-adjustable"...any ideas? I'm thinking clean or replace the IAC and see where I'm at?

 

[alldodge]

Most likely case is the IAC needs to be cleaned or replaced, part number 803149. The IAC is mounted on the throttle body but is not shown in the link
http://www.mercruiserparts.com/bam/subassembly/31594/4835/120

 

[alldodge]

You know sometimes I just spit an answer out to fast.

The great thing about inboard transmissions is it will tell you if the motor is not firing evenly on all cylinders. This is the rattle you hear at idle.in neutral. The motor may also be idling at 600 rpm but the helm tach is not reading accurately. So make sure to look at all things

 

[harleyman1975]

Yes check to make sure it is running on all 8. loose plug wire, bad plug, burned valve, bad dist cap, dirty injector...or IAC.

 

[duped]

Great advice as always guys. I will start with cleaning the IAC. I had an old truck that needed that thing cleaned constantly for whatever reason, so it's not a big deal for me to try.

I also ordered plugs, wires, cap and rotor to do tune ups on both motors. I'm assuming the stuff is all original, or at least pretty old so it's just a good thing to do anyway. We will see what happens once this stuff is installed.

Thanks

 

[Bondo]

The motor may also be idling at 600 rpm but the helm tach is not reading accurately.

Ayuh,..... Agreed,..... Until cross-checked with a known shop tach, don't believe the dash tach,....

 

[duped]

Any reason I couldn't just move the grey wire on this engine over to the other engine's tach and compare the result?

 

[alldodge]

Any reason I couldn't just move the grey wire on this engine over to the other engine's tach and compare the result?

Yes, but its not just moving the gray wire. When the key is turned on all gauges for that side get power up. So if your going to try this, need to disconnect the wires off the other side and makes jumpers to go to the other gauge

 

[QBhoy]

Id defo be replacing the IAC. Mine failed recently. I tried cleaning it up and made no difference. I almost ruled it out as the problem. For the price of an aftermarket one, I'd just replace it.
Failing that, the diz cap will need at least cleaned up or replaced. Standard MPI problems.