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502 mag mpi idle issues

Old 05-10-2020, 09:51 PM
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Default 502 mag mpi idle issues

I have some of you have been following my venture with my crownline but long story short 454 mag mpi blew andi replaced it with a 502 mag mpi short block. I used all 454 mag mpi parts and had the ECM tuned to a 502 gen vi stock tune.

Engine fires up beautifully and runs fine but after it warms up I get a issue im not sure how to deal with. It will idle all day around 750 rpm until i rev the engine. Then it idles at 1500 rpm and stays there until I cut off the engine. Soon as I start it back up its idling at 750 rpm again. I checked the timing to make sure it's at 8° btdc. If I advance it to 5° atdc it will surge a bit but settle back to 750.

IAC comes to mind but I wasnt having problems with it last summer.... only on this engine. Any chance one of the 454 mag sensors are throwing off the ecm? I've done some research but everyone with similar issues never post the fix.

Video of the problem. I cant cut it off and it starts back all over.
https://youtu.be/riLgfxmrovo

Last edited by cableguy1979; 05-10-2020 at 09:56 PM.
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Old 05-10-2020, 10:13 PM
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I was driving when you messaged me earlier, with FA off, put your thumb or finger over Iac hole or air feed horn when its idling high or "uncontrolled", IF it immediately drops, your iacs not cycling/sticking etc. Set timing at 8 degrees with timing tool or paper clip plugged in and leave it. Make sure you have no vacuum leaks, especially the hose going to PCV, for now you could just put a cap on intake port to eliminate it, if THAT makes a difference, you found your issue
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Old 05-10-2020, 10:27 PM
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I will try it tomorrow. She is sounding to good not to go on a lake test... lol
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Old 05-11-2020, 10:22 AM
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So to re-iterate, I think you told me before you DID have your injectors flowed and cleaned by someone? If not, you SHOULD (they are NOT causing your high idle BUT can melt a piston IF you have a low flowing one. On the early Tb you will find a little brass horn thing that feeds the IAC, newer ones just have a hole. This is where you will block it, running too high to see IF idle drops. You also asked "where you cap the intake pcv vacuum port", On front, you should have a 5/6 or 3/8 vacuum port attached to your rubber hose going to PCV, just plug the port wiith a cap for now
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Old 05-11-2020, 10:51 AM
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Do everything they said above. You have a vacuum leak due to 1) IAC that may be stuck or hangs up, 2) a true vacuum leak, cracked vacuum line, bad gasket on manifold etc. or 3) check to see that your throttle linkage is allowing the throttle body blades to return fully to idle. It may be binding.
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Old 05-11-2020, 11:30 AM
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Does it have a TPS to inform the ECU engine is in idle mode?

Only and idea, what if the IAC is unplugged when hi idle?

Last edited by PA.WOODCHUCK; 05-11-2020 at 11:36 AM.
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Old 05-11-2020, 02:06 PM
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Originally Posted by articfriends View Post
So to re-iterate, I think you told me before you DID have your injectors flowed and cleaned by someone? If not, you SHOULD (they are NOT causing your high idle BUT can melt a piston IF you have a low flowing one. On the early Tb you will find a little brass horn thing that feeds the IAC, newer ones just have a hole. This is where you will block it, running too high to see IF idle drops. You also asked "where you cap the intake pcv vacuum port", On front, you should have a 5/6 or 3/8 vacuum port attached to your rubber hose going to PCV, just plug the port wiith a cap for now
I have not had the injectors out or flowed. Never had a problem with them before. Technically I don't have a PCV just a elbow and baffle that are routed together but does let into the flame arrestor like the picture in the manual. The 5/16th line on the intake is already capped and has always been that way that I can remember.

I will try closing off the hole in the feed horn or throttle blades this afternoon. Then Ill try unplugging the IAC to see if that helps. Seems very plausible it could be the IAC resetting after I cut off the engine so if I disconnect it while its running high shouldn't it close and idle return to normal?


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Old 05-11-2020, 02:10 PM
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I confirmed the throttle linkage is not causing high idle by disconnecting it. I guess its possible there could be a vacuum leak but it should run that way all the time if that was the case. I can kill the engine and start it back up and it idles fine again. Soon as I rev it its back at 1500.
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Old 05-11-2020, 02:12 PM
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Old 05-11-2020, 02:17 PM
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Check the secondary throttle blade/shaft for slight dragging. Or possible spring not applying enough force.

I run into this with vac secondary carbs with light diapragm springs and GM tbi units with light shaft springs on bigger than OE engines. So....just a suggestion.

And yeh, shut off engine, restart the rpm goes back to normal idle.
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