496 MAG high idle

[Knot 4 Me]

What can cause a 496 MAG to idle at 800 - 900 RPM out of gear at all the time? My drive has really been banging into gear lately and I noticed the motor is idling much higher than it used to (about 650 - 700). I haven't been able to verify the true RPM yet with a scan tool...just going by tach and banging hard into gear. TIA.

 

[cigdaze]

The first few things I would check.
(1)Throttle linkage / cabling (throttle not closing all the way).
(2)Bad IAC (Idle Air Control).
(3)throttle body plates not closing all the way - check for debris, etc.

 

[Raylar]

Leaky Teaky??

Add any type of intake manifold, throttlebody, injector vacuum leak or such and it will raise the idle rpm sometimes beyond the range programmed in the ecm. The ECM will try to bring the idle rpm back down to normal levels if the system is operating properly. Take a squirt bottle with spray head and spray entire top and sides of manifold and fittings with engine running.
If engine seriously stumbles or stalls, then you know you've found a leak that will easily raise the idle rpm. Repair or plug the leak and the factory set idle should be installed When a stock 496 mag or Ho engine is idling out of gear it should register about 650 to 700 rpms and the scan tool should show the TPS sensor voltage at about .57 to
.64 volts.

Best Regards,
Ray @ Raylar

 

[MOBILEMERCMAN]

Check the fitting on the throttle body where the hose connects from the IAC, It is a press fit. I have seen them loose and leak air. If it will wiggle it will leak. Loctite it back together.

 

[Knot 4 Me]

Thanks everyone. Hopefully I can get some decent weather soon to drop the lift down and fire the motor up and start down a checklist made from all of your suggestions. Looks like I need to invest in a scan tool as well as our marina's Merc certified wrench decided to take another job earlier this summer.

 

[MOBILEMERCMAN]

On smart craft motors a series of beeps every 2 minutes is an indication the IAC may be bad and power is reduced to 90%. That may also raise and vary the idle. Depends where the valve hangs up.

 

[Knot 4 Me]

No beeps and motor still pulls to 4,800 RPM at WOT.