Idle air control valve

Still trying to fix the off idle hesitation on this engine. After doing some reading a number of people suggested checking the IAC valve. I removed what I believe is the assembly and now I'm wanting to look at the actual seating surface plus I want to look at how the motor turns when I cycle the ignition power.

Engine is a 2005 Mercruiser MPI 315hp

One of the ports goes to the intake manifold and the other goes to the intake plenum just above the butterfly valve.

I did clean the filter in the intake plenum.

Questions:
1) Can I split the module right where the pencil is pointing?
2) Is there another way to check for proper operation?

To answer your question, yes, but it's not your problem. The IAC controls Idle speed, as soon as the throttle is moved, the IAC is out of the picture.

Could be a dead spot in the TPS (throttle position sensor). You'll need a scope to see it.

I've got an off idle hesitation in my black scorpion i've been trying to track down for over four years with no luck...I just deal with it.. I've replaced the TPS, IAC, Wires, Cap, Plugs etc. I've replaced all the fuel filters multiple times and have put a pressure gauge on fuel system and everything checks out perfect. Mine sounds like the engine is trying to suck in a buch of air chokes a little bit then goes. It only happens if you really stap it off idle. If you roll into the throttle you're fine. Some of my hotrod friends have suggested drilling a small 1/8th inch hole into the butterfly plate to allow for more airflow. They do this on all their big cam LS1 motors. I've taken the leave it alone approach and roll into the throttle a hair smoother and its fine.

My 350Mag motor does the same. About once per day or a little less. I've found it doesn't do it if I run hi octane fuel but for the $$$ I'll cope with the miss every now and then.

Timing is either too low or too hi by 2-4 degrees. I need to dig through some of my old TSB's and see. Too many 7'n 7's tonight to remember.

TigeMike - You mean "yes, it can be separated"? If so, any suggestions on a method?

About 2 years ago I got fed up after my dealer's mechanics couldn't figure out the problem. I was convinced that it was the TPS so I used both a scope and a data logger to investigate the TPS signal throughout the entire range but specifically at and around idle. Unfortunately the plot from the data logger showed no abnormalaties in the signal.

It used to be that this would only show up when the engine was being run hard like when wakeboarding and I was picking up the rider. But starting this year it hesitates nearly every time I go to take off. It is always in the same region of the throttle and I've also noticed it if I engage the neutral button.

I have a mechanic friend with the same motor and we swapped bits back and forth to try and isolate the problem but no dice. We tried ECM, Dizzy Cap thingy, rotor, leads, plugs. It feels electrical but we gave up in the end, its not much of an issue and the motor is so strong and gives such good gas mileage that it stopped bothering me. It seems to be happening less and less as the motor runs in (700hrs now)

Paul, yes, it will separate there, it's just the gasket holding together. Use a 1" putty knife and work it around the mating surface. Dont pry too much, or you can break a bolt hole boss off.

Shorten the plug gap about .004"

If the engine is rated for 87oct and you are using 89oct or higher - increase the timing

If the engine is rated for 89oct or higher and you're using 87oct, drop the timing.

It sounds like timing advance to me. Check the MAP sensor and its vacuum hose to the manifold.

Mike - What would you expect shortening the gap to do? The engine is rated for 87 oct and that's what I normally fill with. Side note, the gas in MN is a minimum of 10% ethanol.

Trace - Anything in particular to look for on the MAP sensor? Is this a sensitive surface or can I clean off any accumulation?


All - Unfortuately I do not have a shop manual for this engine (yet). I do have a timing gun but have never checked/changed the timing on this engine. Any tips on this? Check it at idle or higher RPM or both?

Thanks for the help so far...

Shortening the plug gap lowers the Kv (Kilovolts) required to arch the gap of the plug.

I've never had a problem with a MAP sensor, but I assume testing it would involve a vacuum pump, ohm meter, and shop manual. I doubt there is anything you can clean or service like the IAC.