If your 5 speed 03/04 Mach 1 has the slow to return to idle problem, there has been a fix found. Originally posted on mach1registry I posted pics on stangnet.com of where this restrictor plate goes. And I sell these plates made of Stainless steel. Visit
nice mod. Wonder what it does for emissions and if it would show up with the way they test emissions these days.
The 'hanging' rpm's is to reduce emissions. What happens is this: when you would no have the slow rpm drop there would be a sudden vacuum (well lower pressure) after the throttle valve is closed. what this does is it will pull any fuel remainders of the walls of your intake-path. this will cause a sudden short increase in emissions since there's too much fuel and less air. Trick is to drop the rpm's slowly and this won't happen. I remember this happening on the very first efi car's we had. I guess there is no other way around it.
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Here where I live if you dont have the check engine light "on" you will pass. This restrictor plate dont affect the A/F ratio, due to it being on the IAC which is ONLY in effect during idle.
It *will* affect the A/F ratio during that sudden throttle closing. This is what Rein is trying to say. There's still fuel there, but no air... the slow decent to idle is to decrease some of the emissions. There is no harm in this... the amount of fuel being burned is minimal at best. Not only that... if you turn you A/C off, you'll notice that the idle doesn't hang nearly as long - the PCM keeps the engine idling faster to power the A/C compressor.
JS
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