IAC on EFI Blower
 

I just installed a B@M 250 EFI supercharger. It idles fine but when it is shifted from drive to neutral it surges up and then dies. This blower is known for this and their is a fix but since Holley bought B@M, and no techs are at Holley who were at B@M, no one at Holley knows what to do! I think the secondary throttle blade has to be changed on its shaft so that it can't shut all the way. Then, when the IAC motor sinces the high RPM surge and closes the throttle blade to compensate, it wont close all the way and kill the engine. I couldn't test this tonight because of weather but if anybody knows anything about this it would be helpfull. Other than this problem the blower is working fine. I can actually give it gas and keep it from dying but when my wife tries it dies. I need to correct this problem for good handling around the docks. (And to keep the wife happy)

EFI
 

Is this a Mercruiser EFI engine (454 or 502 Magnum) with the B+M blower on top of the plenum?
Dennis

This is a 454 Mag. VST fuel system. The plenum gets split in half and the B@M upper plenum gets bolted to the Mercruiser lower plenum. The stock throttle body gets bolted on ahead of the blower. A new ECM goes with it and a 2 atmosphere MAP sensor.

EFI
 

If the engine is stalling the IAC valve is possibly open to far during normal idle speed. When more air is needed to keep it from stalling it can't open any farther. Drill the hole in the throttle butterfly a little larger and the IAC valve will close down some more at normal idle. Opening the throttle blade more will make the TPS sensor reading wrong.
I also have the opinion that supercharged engines also need a heavier flywheel to keep the idle from surging (a common problem with supercharged engines). A custom built 40 pound flywheel would make the engine idle perfectly.

The instructions told me to plug the whole in the throttle blade because it causes idle irregularities so I did. I haven't checked the IAC motor on my scan tool because I didn't have a manual until yesterday. Tonight I will check the IAC and be sure it is in range. I will also look at the throttle blade alignment on the shaft and in the bore to make sure it is moving correctly. It sounds like you (Dennis) think it is a problem of the throttle blade not closing all the way being responsible for the initial surge up in RPM when it is shifted into neutral. If the IAC count checks out OK or if subsequent adjustment doesn't correct the problem I will be open to other changes to fix. I will also try uncovering the hole I covered on the throttle blade. Thanks for the input so far.

Ok, IAC count is 37. Instructions say it should be 20 -40. Throttle blades are moving good. Instructions say if it wants to stall when shifting the secondary throttle blade may need to be adjusted. It then says to call Holley Tech who told me they don't know anymore than me. I also just figured out that it has to be under a load when in gear because I don't have the problem in the driveway but I did on the lake. I am going to take it back out Sunday and screw with the secondary throttle blade and see if that does it. :rolleyes:

IAC count
 

I would open up the hole until the IAC count is 5 or less at an idle, warm engine in neutral.
Dennis

I will uncover the hole like you say if I can't fix it through the throttle blade. I want to uncover it after trying the throttle blade because it is covered with a dallop of silicon. If that doesn't fix it I will have to wait for a new dallop of silicon to dry. :D The IAC count was reading higher then I reset it and it dropped to 37. I am tending to agree with you that a lower count is going to be better. If that is right it is funny that Holley says to plug the hole in the throttle body.

iac
 

I'm really interested in the results as I have this exact same problem and still don't have it staightened out . I bought boat with this problem and was told by mech. who installed b&m blower package it could be fixed with changing the map on the the mefi 2 . no luck, I'll keep watching to see your fix, good luck.

All an IAC does, is provide a controlled vacuum leak to increase idle speed. If you set the counts @ 5 as Dennis suggests, then your ECM will lose the ability to maintain the adaptive strategy that is the heart and soul of computer controlled engine management, and what allows you to enjoy long periods of time running, without wrenching or trying to solve percieved problems when you remove that function of an ECM. I have set up a couple of these and personally spoke to the techs @ B&M to resolve the problem, successful in both cases. Opening the secondary throttle plates slightly (approx 0.40), allows extra air to enter the engine without going past the timing ports in the base plate that control your fuel timing for the transition circuit by opening up the primarys. You will also affect your fuel curve by opening the holes in the primary plates, creating a different set of problems off idle, and that is another reason B&M suggests plugging them (changes transition timing event for enrichment on Supercharged engines). You should NEVER end up with less than 12- 15 counts @ idle unloaded, and ultimately 25-30 counts loaded (or less). Adaptive strategy is the ability of the ECM to compensate for normal wear and tear, loosening of bolts sealing the motor, and other minor irregularities that are the normal function of everyday use. Putting on a 40 lb flywheel will only "mask" the problem, is not a fix but a patch at best, and will also affect other areas of your driveability. Although it is a good idea and can work, it is not the correct or recommended way to solve your problem. Also, if your fuel mixture is too rich, you will have excessive IAC counts and idle related problems. Just remember, if you want to fix this correctly... approach it using the CAUSE AND EFFECT theory.... changing flywheels fixes the effect, but does nothing for the cause, kinda like a bandaid on a bullet wound. This is why B&M included the secondary throttle plate adjustment part in the instructions.... except they didn't explain it very well, and admitted the error. Low IAC counts = a vacuum leak someplece from unmetered air (other than through the IAC passage) entering the motor... Excessive IAC counts are compensating for overly rich mixtures.... and you can take that to the bank! One last thing is, make sure your timing is on the money before doing anything idle speed related....

IAC count
 

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

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...

You got it handled
 

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

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?

Awsome?
 

YEP!

I just added two more people to my X-Mas list:p ;) :D

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?

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...

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.

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.

OK, it is 80% fixed. I will run it tomorrow again and adjust it further. It still surges 1 to 3 times before it finds idle but it doesn't die and it doesn't surge as much as it did previously.It was working pretty good where it was set with an AIC of 18. I closed the secondary down somewhat and it ran a little worse. I opened it up and it found idle pretty well. It changes based oin what we were just doing. If we are Idling around it will go in and out of gear without much surge. If we were just running at high speed and slow down, let it find idle, then shift, it will surge more. Ran 87mph at 5100 turning the 30 4 blade. Theres more left (small lake). The extra speed and prop change has changed how responsive it is to trim. (More responsive) I need more seat time to put it all together. Gonna run it tomorrow. I think it may see 90 or close with this prop. i think I will ultimately need a 32.

sounds like your almost there, we'll be waiting to here if we hit 90 and it still idles.