i have a 1994 cobra 5.0 5speed and i just put new cap, rotor,180thermostat,pvc valve and new autolire xp preformance plugs in and now the idle quality isnt good, beforethe tune up it had an ocasional mis but you couldnt really tell, but after i gave it a tune up the idle is pretty bad, i tried switching to a MSD coil out of my truck and it helped, but you could still notice it. also occasionally the rpms will go from 800-1000 up and down up and down then it will even out a little, im pretty sure it has the stock cam but not postive, could it be the IAC? anyway to test it?
That dang IAC. its a bunch of crud. Ma bey its really the air pump that starts to go bad/ that's just my thought. i did a air pump job the other day on a 92 lx and i tell you, after sitting 2 days in the cold, it started right up and the exhaust was very clean. Im considering doing it to my car, because the my idle does the same thing as yours, but only in the morning. I changed the IAC twice, each time it was only better for a brief time..
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You may want to check the MAF, that has a tendency to go bad on the earlier mustangs. Mine went out earlier in the year, and I was certain that it had to be the IAC. Just a thought, good luck.
you may need to clean you throttle plate, tooth brush ,carb clean,tube off engine off,open throttle and brush and spray away, after warmed up , unplug your iac sol there is a rubber plug near iac and tps, under it is the base idle scerw , adj. it untill the idle is about 750rpm, also clean your maf sensor hot wires that are in the inside of tube, let it air dry, use crc electrical contact cleaner, let me know if this helps. big G
i have clean the throttle body iac and the whole plemium with brake cleaner, i also cleaned the maf wires, it still idles bad, ill try to adjust the base idle under that ruber plug
i thought it was the plugs so i changed them back to the motrcraft ones, still the same bad idle, i have changed the idle and it still doesnt help,so frustrated, i have herd it could be the harmonic balancer, because the idle is rough, and if you give it some gas the vibration goes away could it be the balancer?
Started after a basic tune up? Hmmm,sounds familiiar-I've done this myself,and a few others have and a few more may admit to it-but it is pretty easy to get the firing order off.look up the firing order in a book(several people know it by heart,I don't) and double check-the right 2 wires swapped will run like crap at low RPM and smooth out quite a bit as rpm increases
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My brother's MAS was causing the same exact problem. Rev it and everything would be fine but it would not idle right. Check the timing and it was all over the place. Rev it again and it was fine. A MAS can cause this problem, even if it was just bought new so don't rule it out. Wouldn't want you to start buying other unnecessary parts by thinking because the MAS is new, it couldn't be the problem. Check the function just to be sure.
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Started after a basic tune up? Hmmm,sounds familiiar-I've done this myself,and a few others have and a few more may admit to it-but it is pretty easy to get the firing order off.look up the firing order in a book(several people know it by heart,I don't) and double check-the right 2 wires swapped will run like crap at low RPM and smooth out quite a bit as rpm increases
that was the first thing i checked and al the wires were in the right place
My brother's MAS was causing the same exact problem. Rev it and everything would be fine but it would not idle right. Check the timing and it was all over the place. Rev it again and it was fine. A MAS can cause this problem, even if it was just bought new so don't rule it out. Wouldn't want you to start buying other unnecessary parts by thinking because the MAS is new, it couldn't be the problem. Check the function just to be sure.
what is MAS do you mean MAF? and i dont know what function means
1994 Cobra bad idle quality, possible iac? 
