Hanging idle issues

[HydraX]

As some may have seen in my post on "the welcome wagon" thread, I referenced some idle issues I've been having with my C head 98 Cobra.

The main problem I'm having is the rpms take their sweet time coming back down to idle after coming to a stop. I'll be driving down the road cruising at around 1800 rpm for example, and if I was to put the car in neutral while rolling the rpms will shoot up to 2000 rpm and stay there until I either put the car back in gear or come to a complete stop, and if I just simply let off the gas while in gear the car will accelerate until the rpms reach the 2000 mark. Even after coming to a stop, the rpms will sometimes continue to hang for up to a few minutes until finally coming back down.

So far I've discovered that the idle set screw has been tampered with by the previous owner. I purchased a new motorcraft IAC to see what changes, and discovered the car will not stay running unless given throttle. While I will have to go back and do it again to verify I did it correctly, I did do the idle relearn procedure with no new results. I've also had 2 separate well liked shops check for vacuum leaks, and nothing came up.

Anyone have insight as to what could be the issue? I'm confident the tampered with idle set screw is part of the problem, but I'm not sure if that is the only problem. One thing I will mention is I have yet to check the voltage on the TPS, I still need to get my hands on a voltmeter and check that off the list of potential causes. Thanks!!

 

[Noobz347]

What are your EEC codes?

 

[HydraX]

What are your EEC codes?

That's the fun part, there are no codes. No codes, no dash lights, I'm not sure what the cause of that is but it's pretty much got me in the dark with any issues I'm having with the car.

 

[Noobz347]

That's the fun part, there are no codes. No codes, no dash lights, I'm not sure what the cause of that is but it's pretty much got me in the dark with any issues I'm having with the car.

If the EEC checks out, you should have at least 1 code.

You cannot rely on the dash lights. That will only illuminate for emissions codes.

 

[HydraX]

If the EEC checks out, you should have at least 1 code.

You cannot rely on the dash lights. That will only illuminate for emissions codes.

I've checked for codes myself and also had my local auto parts store scan for codes, no dice. Could the EEC itself be on the fritz? Any issues I've had with the car that I'm almost certain would throw a code would never do so. Hell, my low fuel warning doesn't even show up lol. I doubt that has anything to do with it but it just shows that my car doesn't seem to want to tell me when things aren't right.

 

[squeak93]

C headed car on b head tune or else? Stock throttle body, what intake? I once did a 96 cobra with an aviator engine (c headed motor) and it didn't like idling until we put a tune on it.

Also, these cars overall tend to hang the idle up there (my 03 mach does odd things idle wise at times). You can restrict the IAC by placing a brass plug in the line and drilling a hole (start small) until the desired idle characteristics are met. I've had to do that in 2 different 2v cars after I got tired of throwing all the same parts at them you did.