Real Rough Idle


New Member
Mar 12, 2008
Howdy Im new here but always been a mustang fan. Finally bought my friends ride.
Ive been hearing about switching from Air Density to Mass Air to solve performance issues and was wanting to know information about that. The car right now idles pretty rough. Cuts out 5% of the time. Granted it has not had a tune up in 2 yrs so thats maybe the problem. Its an 86 GT, all it has is a super chip from long time ago and shorty hedders. Just wondering if the air density thing could be my problem, any help is greatly appreciated-

thanks, Jim
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Pull the chip, douse it in gasoline and light it on FIRE. Any performance that you are even possibly seeing from the chip can be had by simply playing with (advancing) the ignition timing.

Then, give the car a full tune and check up, and I mean FULL. Clean the TB, change the cap/rotor, change the plugs, might change the wires, oil, run some Sea Foam or equivalent through a tank of gas, put a K&N panel filter in it, put a new fuel filter in it, change the PCV valve, etc. Make sure all of your vacuum hoses are in place and not rotted out. I’m sure I’m forgetting things but others will chime in, I’m sure.

If, after a full tune up, the car still runs poorly, then its time to start checking for a failed sensor, or looking for some sort of mechanical failure. The Speed Density system is likely not the cause of the problem.

At the level of performance you are seeing, Mass Air is not necessary, keep with the Speed Density. Only consider a Mass Air conversion when you install H/C/I, but you won’t be able to put aftermarket cylinder heads on an 86 motor anyway, because of the flat-top pistons.