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Discussion Starter #1 (Edited)
hello everyone, I have a 1990 GT thats heavily modded. I am having problems with the my car idling. After I start the car and let it warm up for 30 or 40 seconds the car idles fine, after that, then the idle surges up and then drops, lets say from 750-800 (due to cam and other mods) to 1500rpms. Then it drops down bellow 800 then back up and then back down. It does this a few times then shuts off. I have been reading the forums for what the idle air control valve does and my question about the iac is, if you block it off or disconnect it does it make the cars computer act different, like lean the motor out or riches it up? Does the iac have any other affect on the car to affect how it runs? Im not sure if it has that affect on it or not.
 

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Discussion Starter #3
i have tried to unplug it but it idles funny still. im not sure if the iac is unplugged it will make the motor run funny like richen it up or lean it out or mess with the timing to compensate for it being unplugged. i do think it is bad but i would like to eliminate it all together.
 

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I dont thnk on our cars the iAC controls anything in the ecm just holds idle. On newwer cars it controls the a/f mixture.
 

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Discussion Starter #6
ok where to begin. 5.0 block bored .030 over, stock stroke, ford crank and rods, speed pro flat top 4 relief pistons, clevite bearings, all fel-pro gaskets, brodix alum race heads milled .008, larger valves, ported heads and upper and lower intake, rolloer rockers, lunati 51017 cam, all arp studs, high pressure fuel pump, old school paxton sn92 supercharger, paxton fuel control unit, 24lbs injectors thats about it for the motor. it runs great and pulls hard. i was weary of disconnecting the iac cause i didnt want to make the computer act funny and make it run lean or rich, and spark plugs for those heads are not cheap! lol.
 

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After seeing your mods has this car ever been tuned? A S/C app has to be tuned the stock ecm cannot support those mods and you will most likely ruin that engine
 

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the only way to fix your problem unless you have done this already is your IAC dashpot setting within ECM needs to be calibrated to match the difference with the .030 over bore and i assume you have an aftermarket intake so if your still running a stock setting for the IAC thats why your having this problem, but i must ask did this just recently become a problem, or has this been on going since your mods?
 

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Discussion Starter #11
the idle seems to be getting worse. but my question is, does the iac have an effect on the fuel settings? like if the loop is open it uses more fuel or vice versa. the motor runs fine, it does not lean out on top end, i have a paxton fuel control unit so when it senses pressure it closes of the return fuel line for every pound of pressure it senses.
 

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the moates chip i have requires extensive Fuel and air measurements and datalogging, the new quarterhorse looks awsome, it datalogs and the whole nine much better then what i have, moates is just awsome, product and customer service!!!!
 

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You may have a VAC leak, I just had the same problem, I had to remove the upper and lower intake and replace all the gaskets.

Pull you IAC wire and if the car stalls you don't have a VAC leak. If your car still runs you have a vac leak. I went through hell trying to figure out what my problem was.


YouTube - 1990 mustang gt Dyno
 

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When I unplug my 351s it dosent act like you did anything and it idles high only in park and neutral, it idles great in all drive gears with brake pressed
 
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