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Discussion Starter #1 (Edited)
I've had my 86 FSB w/5.0 EFI for 12 years and have had this issue from day one where randomly then idle will suddenly drop down and hover briefly at 200 rpm then shoot up two like 1000/1200 the kind of search for a sec then settle back at normal 7/800. Other times it'll do that and stall. It'll start right back up and continue. It's been easy to manage with the auto just ride brake temporarily and quick throttle. Well I just completed my 4 speed swap and it's much more difficult to deal with, for obvious reasons.

So I took a vacuum reading this afternoon and based on some research vacuum seems to be ok? Sorry about the key buzzer noise but looking for opinions on thoughts to the video. It always stays at about the 20 mark. The needle swing is me tapping or revving engine.

To also answer questions, I've replaced the map, tps, icm, fpr, entire dizzy, ect, iat, ign coil, fuel pump, filter, fuel res with new filter and entire new unit, intank pump as well. I've replaced them all with oem parts. Like said I've had this issue for 12 years and never been able to nail it down.

Thanks for any help 馃槉

Video of the vacuum test.

 

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Does it have a vac actuated 'dashpot' on the side of the carb responsible for maintaining idle?
I had one of the earlier 5.0L mustangs with a carb and that item went and caused the same problem
 

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Discussion Starter #3 (Edited)
Its EFI truck so no carb :). Sorry I'll update post
 

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1986 Eddie Bauer Bronco 5.0L AOD Full length headers, no cats annoying Flowmasters
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I have an 86 302EFI, and had a very very similar issue... chased it for months, replaced everything just like you did.

Then one day I looking at my cruise control that doesnt work, & it is run by vacuum, and mine was rusted at the plastic to metal seams allowing air to leak in. I pulled the vacuum line that went to the cruise control at the intake and put a vacuum cap on it, and it solved the idle problem.

Still havent replaced the cruise control yet...
 

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Discussion Starter #5
I'll check that out, thanks!
 
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