Help! Racing idle after IAC replacement 2.7L

Hey guys, I have read a lot of threads out there about high idle, throttle body cleaning, IAC, TPS, vacuum leaks but I haven't seen one talking about engine racing up over 3000 rpm when you go into neutral at a stop light.

Previously I was having rough starting and engine misfires and knew my IAC was original 388,000 miles and throttle body was never cleaned. I replaced IAC and cleaned throttle body. Also found the port inside intake manifold to EGR was almost crusted over so I cleaned that off as well.

After reassembly with a new throttle body and IAC gasket my idle starts at 2000 rpm and when I run in gear and stop and put in neutral idle keeps racing higher and higher. Tapping gas only makes go higher even over 3000 rpm.

Is this just vacuum leak? Worn butterfly valve? Seems no control over throttle.

 

Reset the TPS

 

Ok by reset do you mean disconnect battery? While doing repair I had negative cable off and tried again for about 4 hrs after idle was screwy

 

According to Haynes manual did both tests on my TPS the 3 contact version which is not adjustable. I don’t get the range they give in the manual for the test. In fully closed manual says resistance should be 0.2 - 5.7k ohms. Why such a huge range?? At fully closed mine showed 0.36 is that ok?

On the other test using needles to back probe with the key turned on at fully closed was about 0.53 volts tested the cycle to fully open and voltage seemed steady up and down at each position. Thinking that means my TPS is ok? Also was getting 5volts as I should be from the harness.

I went back over the vacuum lines and replaced the one that looked cracked. Was insane trying to drive home like someone else had their foot on the gas. If this wasn’t a manual would be totally not driveable

 

Ok so for now I had to MACGYVER a solution to get me to work. First I bottomed out all the set screw adjustments on the throttle since the butterfly is worn and needs to close farther. Then I bunjeed the throttle closed all the way with one of my wife's hair ties. It worked! Proved my theory that even though the butterfly valve is worn down it will close enough to get proper idle. The problem is it's so far from normal adjustment the TPS is messed up. Since this year has the non adjustable TPS how about I slot those holes and rotate that thing a little bit? So far until I have time to send the throttle to Maxbore to be rebuilt that's the best I can do.

I have now replaced the hair tie with a spring and it's working. Seems like the computer is learning what to do with this crazy worn throttle body!

 

^ genius ingenuity, seriously. I love it as a temporary fix. Glad you installed the spring as a replacement.

My 1965 Mustang had a carburetor that would not return to idle without a blip. During investigation, I discovered that a push on the end of the butterfly shaft would allow it to close completely and idle normally. So I installed a spring on the opposite end of the butterfly shaft (effectively the same as a push from the original side). That spring was anchored to a bracket on the firewall Worked perfectly.

 

Thanks! Yes here is my spring which was a little heavier than I needed so I have been adjusting it. Seems like the computer is learning and needing less and less of the spring or at least that's what I'm hoping

 

Just a final update on this experiment. Once I started to feel like the idle when truck was warmed up was sitting a little too low I took the spring off and then it was totally perfect. Idles at stop lights about 800 rpm and no rough starts!

This is still with the set screw stops totally bottomed out. So yes the TPS did need to be reset...

But in addition to disconnecting the battery for a while a throttle body with 388,000 miles on it needs to be bridled back into submission. But does not necessarily need to be replaced!