3.1L

I know this is a group for Chevy trucks but I can't seem to find an answer anywhere for this problem. I just rebuilt a 3.1L Pontiac Grand Am motor , auto trans, and it idles great, but when I take it up to 55 it starts to
shutter until I let off or stomp on the gas, I read two codes, MAP voltage too low and IAC inproper setting, I changed the MAP no luck, I don't have any vacuum leaks and at idle a vacuum test shows no probs, any ideas? TIA
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Year and type of injection system? Was the motor rebuilt with stock compression, stock cams and stock exhaust?
Doc
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Thanks for the reply, everything is stock, SFI, my scan tool says Quad drivers 2&4 are bad, also code set for MAP and IAC, I replaced the MAP with known good one and cleaned the Idle sensor, any help? TIA

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I'm also having trouble with my news server! To your reply Doc It's a 3.1L SFI with auto trans, all stock , bearings, cam, rings etc. Exhaust is stock. Now it's starting to idle a little rough, I was thing IAC maybe, any thoughts? Thanks
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I'm also having trouble with my news server! To your reply Doc It's a 3.1L SFI with auto trans, all stock , bearings, cam, rings etc. Exhaust is stock. Now it's starting to idle a little rough, I was thing IAC maybe, any thoughts? Thanks
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what sounds like a stupid question, but may kead to the trouble:
When it idles rough, does the brake pedal get 'harder' ?

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