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Discussion Starter · #1 ·
Okay sunday while I was at a friends house doing the body work on the mustang I started it up, the car is staying over there, we dropped it off saturday, and it was surging . It would idle at about 1000 rpm then drop to 500 and go right back to 1000. It did this while I was trying to pull it into the garage so we could work on it. After about two hours we did all we could do until I get some more parts so i moved it back and it fired up and idled no surging. I was befuddled. why is it doing this?? It has no air filter and the idler pulley is bent but that shouldnt cause it do that should it?


Drew
 

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I have the same problem, I can go out and start up my car when the engine is cold and it will start to surge, 200-300 rpm to 1000-1200 rpm, after awhile it will stop and run fine the rest of the day. I have ran no codes and havent tried anything, (to cold and snowing)
 

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Usually idle surging is caused by adding an aftermarket part... #1 C&L MAFs cause this. You can dial the meter and make this go away, but I chucked my C&L and bought a Pro-M. I can put the thing in upside down and my idle is still steady.
#2 Next down the list is a throttle body. The stock one has a hole in it, aftermarket ones do not. Some people drill a small hole and fix the problem. I have a 70mm on 2 5.0s. Neither has any idle problems.
#3 I set my TPS unlike everyone I know. It's been a while so i'm not sure if it's clockwise or counter-clockwise, but I start by turning the TPS as far as I can to drop the voltage down. Then I raise the idle screw up until the TPS reads .95 to .98. This usually gives me and idle around 950 to 1000rpm. I like my idle high.
 

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Mine just started that annoying crap. It ran fine for a year and now it wants to surge from 1000-1300. It will stop after awhile but it sure is annoying.
 

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I understand this is a very common problem. My car does it aswell. I've been told it is the IAC and it just needs to be cleaned. run some throttle body cleaner through it and it will clear it up.
 

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take the iac off the throttle body. 2-bolts 5/16 wrench . take vavle off controller using a screwdriver. soak valve in carb. cleaner then clean it up real good. put it back on and should fix idle surge. I did it on my 90gt and my friends 88gt as we both had this prob. if it doesnt work iac could be faulty . or you can try an idle adjustment plate(after market) hope this helps
 

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Discussion Starter · #8 ·
Hey guys thanks for all the help. I am going to try your suggestions once I get the car back to my house in a few weeks. And I will definetely need more help as I am coming froma Z28 to a GT. So it will be a big change for me.

Thanks Drew
 

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IAC is the Idle Air Control Valve. It's the tube looking part bolted to the front of the TB. As slingshot5.0 said, clean it.
 

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ive had that idle problem on all my foxes. ive had 4. many things have caused that problem on my cars. 1. vacuum leaks. somtimes if youve got a cracked hose anywhere it will surge real bad and never stop, if its an intermittent problem, thats not what it is. it will also surge real bad and shut off alot. 2. like the other guys said, the iac and tb need to be cleaned periodically, i cant tell you how many times ive had idle surge really bad at stop signs where it just turns off if i take my foot off the gas. just spray carb cleaner in there real good while keeping the motor at 3,000 and the motor will just suck in all the junk and burn it up. 3. i had a terrible surge on my 87 speed density car that wouldnt go away, no matter how clean anyting was, i finally bought new oxygen sensors for it, and i havent had a surge in 8 months. , also any of you guys know anything i can do besides these for a surging idle with a cam and supercharger, it lopes between 1000-1500, thanks mike
 
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