AAX86

I've had this problem since i bought the car. Common problem, idle surges between 600 and 1400 rpm. I learned to deal with it but now it's preventing my stang from passing smog. Or even testing smog at all. Guy said the idle was just too erratic so they wouldn't even test it. I'm just stuck now. I don't know what to do but find someone who'll "pass" it for a little extra. Is that my best bet? I messed around with the timing yesterday to no avail. I know for a fact that the tps isn't set correctly cause i lost my multimeter. Could the tps be the sole cause of a surging idle?

So far i've:
Tried resetting the base idle, not sure if i did it right
*Thoroughly* cleaned the iacv and tb
Set the timing (currently around 12*)

I would check for vacuum leaks but i wouldn't even be able to tell if the idle changes cause the damn thing won't stay steady.
Any suggestions? What would be your next move? I'm lost

Car is '86 stang with '90 5.0 pretty much stock

dillon1340

When you set the base idle you had to disconnect the idle air motor. did it still surge when that was disconnected?

and a little side note...I'm so glad we don't have smog tests where I live. I would never have this hobby if they did.

AAX86

You mean just the electrical plug on the idle air motor, right? Not the iac itself? That's why i wasn't sure if i did it right. Cause i unplugged the electrical connection to it and when i tried to run it without the plug, it started for just a second then died. I tried it a few times but it kept dying without the plug hooked in. So i just connected the plug and proceeded with the other steps.

Is it supposed to run with the iac plug disconnected?


You're lucky with no smogging. It suuucks in CA...

dillon1340

I'm pretty sure if you google 'base idle mustang 5.0' you'll come up with some results. but yes, you do need to have the iac unplugged then turn in the screw until you get your base idle. there is a plate you can buy that goes between the idle air motor and throttle body, it was the cure for my surging idle I got after installing an E cam.

http://www.latemodelrestoration.com/...l-Iac-Adjuster

mattdel

You can test for vacuum leaks just fine. Using the carb cleaner method, the engine will stabilize and possibly raise in idle if you do have vacuum leaks.

"Setting" the tps is a myth. The computer automatically adjusts the CT-WOT values depending on the TPS startup value. Makes 0 difference where it is.

Cleaning the IAC doesn't always work. The motor inside can weaken.

I'd say check for vacuum leaks and replace the IAC.

AAX86

A myth?? I thought that was on the rough idle checklist. It'd make absolutely no difference to try and mess with it? I did a key-on engine-off test just now and only got one code, 23, "tps signal out of range".

I'll try to check for leaks today and report back
Thanks

dillon1340

did you adjust your tps? what happened with the surge, did you fix it yet?

AAX86

Woops.. forgot about this thread. But yes i did fix the surge. It was the TPS. It was showing twice the amount of voltage it should have at idle so i got a new one from a local o'reilly's. Easy $40 fix and the car runs perfect now.

Going to smog again today. Wish me luck