Dennis Moore

I have scanned at least 100 Mercruiser EFI engines and have found the IAC counts for a normal engine will be between 0 and 20 counts. It seems that the engines that perform the best have low IAC counts of around 5.

An EFI engine that will die periodically is usually one of those engines that has a high IAC count at idle.

It seems that the IAC motor can not respond as quickly as is sometimes needed, it can not open fast enough to keep the engine from dieing (especially when shifting around docks or the launch ramp when the engine is cold). Having a larger hole in the throttle plate will always assure that some air will be delivered to the engine no matter how slow the IAC motor responds. The larger the hole in the throttle plate the less the engines idle stability is dependant upon quick IAC opening speed. This can be particularly important for a modified engine that idles poorly because of a larger camshaft.

The only advantage to a higher IAC count would be to return the engine speed to idle faster. With a larger hole in the plate the IAC will take longer returning to a 5 count than returning to a 20 count. This could be important to Mercruiser warranty repairs, it may cut down on drive repairs from customers shifting into reverse before the engine speed returns to idle (something Bravos are touchy about).

The new Mercruiser 555 PCM (used on the 496) has a different style IAC motor that solves the problem with a faster idle air control system.

Sincerely
Dennis Moore

FindMe

Dennis, whereas what you say may be correct to some degree, you are comparing apples to pistons aren't you? Are all 100 of those you scanned equipped with a B&M supercharger? Have you worked through those with the B&M techs with positive results? Each application is different, and if you like I can post the diagnostics and related bulletins from Mercruiser, as I have all of their manuals for all applications, including to date bulletins sitting in front of me... point being, if you ever need to know or see something, feel free to ask. My info is your info...

Dennis Moore

Dear Find Me,
Sounds like you really have it together! I will be asking you from time to time for advice!
Sincerely
Dennis Moore
Mercruiser Master Technician

FindMe

Well Dennis, I don't exactly claim to have it together, or be the advise guy either, but I do have a wealth of information available for anyone who asks, and solving complex electrical problems, and computer engine management incompatability issues are what I do for a living, including having the equipment and software to flash reprogram GM ECM's from 1995 to current, and do it daily.... and going to be attending 10 days of advanced programming classes @ the GM training center in October. Being a credentialled advanced GM and FoMoCo electrical and enginemanagement tech, and being lucky enough to usually be the #1 guy in these classes, I am able to get the instructors to give me info I am not supposed to have (or anyone else for that matter) and you can bet your ass, I will be working as hard as I can to aquire whats needed to reflash the 1,2 & 3 Mercrusier ECM's, or anything related... I already am well versed in the issues these classes will include, and my motive is gaining anything possible to get into the marine ECM's, and be able to manipulate them to suit the needs of what you are running... At best... wish me luck, because I will share whatever I can... Sounds like between the 2 of us, there isn't many issues out there that can't be solved, I am a sponge for knowledge and will take all I can get... Look forward to maybe bumping mellons with you to help some of these guys out... what an awesome group of guys heh?

Dennis Moore

YEP!

mxz800

I just added two more people to my X-Mas list

OkieTunnel

Thank you Find Me and Dennis. This is what I have been looking for, someone who knows the specific fix for this problem. It was eluded to in the instructions but not explained at all! I know there are many of these blower setup's out there running perfectly. Find Me, where should I measure the .040? Or should I just open it until I have 25 to 30 counts on the scan tool?

FindMe

Try a half a turn at a time on the secondary throttle shaft stop, and look at your counts until you are in the ballpark with the counts. Personally, I prefer to look at the loaded counts, (in gear in the water) since this is where you need idle control. If you have trouble with returning to idle fast enough, lower the counts as Dennis suggested with a little more opening and see how that works, but not too far as it will hammer the IAC motors... It's no bigge, but B&M did fail to explain it in a manner that makes sense to guys like us...

J.V.IRONHANDS

Did you get it straightned out yet? Sounds like Dennis and Warlock know there stuff. Okie, did your blower system come with the B&M PowerPlus Ignition Enhancement? Mine did but the mech decided to remove it saying it wasn/t necessary.

OkieTunnel

J.V., i opened the secondary throttle blade and was able to lower the count on the scan tool. I opened it last night about two turns and the count went to 18. According to Find Me and Dennis a lower count will fix the problem. I am taking the boat to the lake later today and will fine tune it under load and see if it fixes the problem. Yes, I do have the Power Plus installed. I've never heard of a downside to a powerful ignition system! I finally got a fuel pressure guage installed and a bigger prop (30). Today will be the first day I "open it up. " I will post later and report on all.