Modified 496Mag IAC Problem
12-18-2003, 03:32 PM
#1
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Modified 496Mag IAC Problem
My 496Mag was modified by Tyler Crockett, the modifications included a MEFI4 computer. I sent the computer back about a half dozen times to Tyler for adjustments because the engine wouldn’t idle properly. The idle problem couldn’t be solved so I decided to drive the 600+ miles to his shop so he could work on the computer right on the water with the engine running. Tyler drilled out the throttle blade hole several times and still no improvement. He ultimately realized he had to cut out a portion of the throttle body mounting plate which covered the IAC passage on whatever the throttle body is bolted too. I guess the Volvo (MEFI4) and Mercruiser (PCM555?) activate the IAC differently. The IAC responded properly and we went for a quick test drive, everything seemed fine. I’ve been on the water several times since then at Lake Winnipesaukee in NH. Several things I’ve noticed. I go to fast in no wake zones (my boat idles at about 825 rpm). When I’m idling in neutral the engine has a rup rup rup sound. It isn’t a nice high lift cam lope, it sounds like the engine is surging. Also, there is a slight loss of power slightly off idle, after you pass this point everything is fine. Last thing I noticed accidentally was if you increase throttle while in neutral then return the throttle to idle it takes an abnormally long time to come down to idle rpm again. I’ve been reading other IAC posts and these issues I have seem IAC related. It is mostly due to my ignorance that these issues were not addressed when I was at Tyler’s shop. I was hoping to get some expert advice from fellow OSO members before calling Tyler. Could my problems be partially related to the throttle blade being drilled out? I have no idea. I’d appreciate any advice. Also, besides these relatively minor issues the engine runs beautifully. It does however really suck down the gas!!!!!!
12-19-2003, 07:59 AM
#3
Was this one of Tyler's 496 upgrades,i.e. 530hp or 600hp?
I spoke to him when he was developing these but have not made the change yet. I really didn't like the idea of paying for a different computer when there is a perfectly good one already on there that can be programmed (unfortuntly not by him though).
Sorry to hear it isn't running as smooth as it did before. Seems like if he had the same setup on the dyno he would see those problems on there, did he??
Rick
I spoke to him when he was developing these but have not made the change yet. I really didn't like the idea of paying for a different computer when there is a perfectly good one already on there that can be programmed (unfortuntly not by him though).
Sorry to hear it isn't running as smooth as it did before. Seems like if he had the same setup on the dyno he would see those problems on there, did he??
Rick
12-19-2003, 02:52 PM
#4
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I have the 530HP upgrade, it dynoed at 550HP. I was hoping someone with some knowledge would either confirm my suspicions or just give me some good advice. He definately didn't tune idle on the dyno because the IAC would not have worked. Advice anyone????
12-20-2003, 08:51 AM
#6
You need to start with the TPS and make sure it is at zero and not opartially open. A volt meter or scan tool will help you. Once it is at zero then you can adjust the IAC. You need to see the IAC count. If you think that the holes are too big then put a screw in it to fill it and try again. Make sure the IAC is working and not clogged. Unfortunately it is no more exact than that.
12-20-2003, 09:33 AM
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My guess would be the drilled throttle blade is allowing too much air at idle now. What are the IAC counts at idle? You need a scan tool to get some data. I'm thinking the IAC may be totally closed at idle since you have an adequate supply of air from throttle blades and it cannot idle down any more. The idle surge may be because of it being a little lean. It sounds like the only way to really trouble shoot it is with the laptop though. Everything you need to do and see will be there. Call Tyler....
12-21-2003, 02:26 PM
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The 555 computer uses a Ford IAC, the MEFI4 uses a GM style IAC. Completely different drivers. The FORD is much faster and responsive. We run into the same problem sometimes with the idle sound when we use the MEFI3. The rup, rup, rup sound is the timing controling idle rpm. It's probably swinging low and high to maintain rpm. We max out the open speed of the IAC and almost maximize the closing speed, but it's still not as fast as timing control for the desired idle rpm. The only way to change this is to lower the gain for spark control as well as lowering the maximum spark per event. You would then have to go and adjust the IAC to do more control (take a bit of trial and error testing), there is no set adjustment you can do to without the boat.
To understand how the computers keep motors running, the system is looking at rpm, it has a desired idle speed. When the rpm is lower than the set speed, it will add timing and open the IAC, if the rpm is lower, it does the opposite. You input how much the IAC will open and how much timing it will add/subtract and how fast. The sound is usually from the motor going from 20 advanced to 10 retarded, back and forth. The rpm is never steady, always has a span of at least 25-50 rpm so its constantly modifying.
The power loss off idle is one of a few things, first I would think that the IAC follower is probably not following the throttle. The follower helps at the low rpm to start opening the IAC in correlation of the TPS opening. If not this, it may not have enough spark at that rpm, might be due to the idle system retarding timing to keep rpm down.
To change the sound, you can play with the set screw that adjust the throttle blade stop. This will allow more air in or less air. If the ECU is retarding timing to keep the motor running at it's desired idle speed, then more air will allow it to run more timing at idle. Vice versa for low rpm. One thing Tyler can do easily is lower the desired idle speed to say 750 (500HP EFI) but this could create other problems.
I'm sure Tyler can fix the power loss, the sound is something more common. Don't you remember the motor stock? The 496's run -5 degrees spark advance at idle so they always sound bad.
Thanks,
Dustin
To understand how the computers keep motors running, the system is looking at rpm, it has a desired idle speed. When the rpm is lower than the set speed, it will add timing and open the IAC, if the rpm is lower, it does the opposite. You input how much the IAC will open and how much timing it will add/subtract and how fast. The sound is usually from the motor going from 20 advanced to 10 retarded, back and forth. The rpm is never steady, always has a span of at least 25-50 rpm so its constantly modifying.
The power loss off idle is one of a few things, first I would think that the IAC follower is probably not following the throttle. The follower helps at the low rpm to start opening the IAC in correlation of the TPS opening. If not this, it may not have enough spark at that rpm, might be due to the idle system retarding timing to keep rpm down.
To change the sound, you can play with the set screw that adjust the throttle blade stop. This will allow more air in or less air. If the ECU is retarding timing to keep the motor running at it's desired idle speed, then more air will allow it to run more timing at idle. Vice versa for low rpm. One thing Tyler can do easily is lower the desired idle speed to say 750 (500HP EFI) but this could create other problems.
I'm sure Tyler can fix the power loss, the sound is something more common. Don't you remember the motor stock? The 496's run -5 degrees spark advance at idle so they always sound bad.
Thanks,
Dustin
12-22-2003, 06:42 AM
#10
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Hi Dustin,
I had a feeling I would have to go on another 600 mile road trip! My stock 496Mag seemed fairly smooth running to me, definately better than it is now. Thanks for all the great info. If there ever is a Supercharger in my future you will be the person I call!
AWelsh
I had a feeling I would have to go on another 600 mile road trip! My stock 496Mag seemed fairly smooth running to me, definately better than it is now. Thanks for all the great info. If there ever is a Supercharger in my future you will be the person I call!
AWelsh
