OkieTunnel

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)

Dennis Moore

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

OkieTunnel

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.

Dennis Moore

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.

OkieTunnel

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.

OkieTunnel

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.

Dennis Moore

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

OkieTunnel

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. 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.

J.V.IRONHANDS

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.

FindMe

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....