IAC no help, now what?

A few months ago I went to start my 95, 4.6 t-bird. It went to 2000 and died. After several attempts I was able to keep it running with the pedal, but still surging to 2000 and back. All the way home the car would buck as the motor would try to rev. and die at any rpm.

I ended up unpluging the IAC and adjusting the throttle stop and it would run ok, just no idle compensation for cold starts and a/c cycling, but it would run.

Today I bought a new IAC and installed it. At the same time I cleaned the MAF, throttle body, checked for any vac. leaks, disconnected the batt., and set the idle stop and TPS voltage to the correct .99 volt setting. The TPS is smooth from .99 to almost 5 volts.

The car runs the same as it did when I had the IAC unplugged. No cold idle up, no compensation for a/c. It idles smooth, but the IAC is not working. I am getting almost 14 volts on both IAC wires when the car is running, when the red should be almost 14 and the white around 10-11.

I took the new IAC off and cranked the car to see if the port holes were plugged, but they are not. The car would start, suck air, and die.

Now what? Am I missing something, is the new IAC bad, or should I go to Ford and let them check it.

 

Okay, try this: install the new IAC, and clear the codes. The if it still does it, check the codes. A lot of the time it's a stored code that makes the computer go wacko.

 

I left the battery unhooked all night, checked everything again, still no IAC response. I went to Autozone after the car ran for 45 minutes, and driven for 15 more. No codes at all. Still a mystery.

The new IAC showed 9.5 ohms resistance across the post, so it should be ok. They are getting another one in for me to try in case something is wrong inside the first new one.

Any ideas?

 

Did you set the idle screw back to where it was before you unhooked the original IAC ? The throttle blade may need to be opened up just a bit.

 

I have been gone this weekend, sorry for the delay. I set everything back to normal, and it would die. It acts like it did when the old valve was unplugged.

I think the new one is bad. A mechanic friend told me he got a lot of Wells electrical parts that were bad from Autozone. He even had a bad new IAC to send back to them himself. I think I will try a Borg-Warner, or maybe spring for a new Ford IAC.

 

UPDATE Still need help!

I returned the Wells IAC and bought a Borg-Warner valve just after my last post. The idle was not changing, just as if the IAC was unplugged. I had to set the throttle plate screw to make it idle. It has been set up this way since around last July. I did all the IAC changing in November. When the new IAC valves didn't help I just left the throttle set screw adjusted to where it would idle.

Yesterday I cranked up the car and it acted like it did in my first post. It would idle wide open and then quickly drop to low idle. It does this whether idling or driving down the road. Gets exciting in town when engine tries to rev up and run over the car in front of you.

I pulled the IAC air pickup hose off at the intake and stuck my finger over the hole to check the vacuum and see if the IAC was opening and shutting.

There was very little vacuum, then the valve would open up and you could feel a large increase in vacuum. It would open for a few seconds, close for a minute or two, then open quickly several times, and then close again.

I know it is not a bad IAC. This is the third one on there. It acts like the ECM has a bad connection or ground, or maybe a sensor that controls idle I don't know about. The TPS is good also. I have checked it many times. I have cleaned the MAF and checked for vacuum leaks.

This thing is driving me nuts. I unplugged the IAC so I won't run over anyone because of engine surge. Please tell me something else to check. Could it be the ECM bad? This is a 95 T-Bird 4.6.

Thanks.

 

Instead of buying more parts, run it by a Ford dealer and let them scan it with their scan tool. It will pinpoint the problem. It will cost about 1hr shop rate. Probably about 75-85 bucks, but it's better than buying parts you don't need. Sounds like a PCM problem.