Idling problem, but the IAC appears to be okay?
 

My 96 DX hatchback (automatic) has been sitting a while (we drive a Prius as our regular car). When I started it up it had a strange idling problem:

During warmup, the car appears to run fine.
Once at operating temperature, the idle starts surging badly, every couple of seconds
It surges while in Park or Neutral, but not in Drive/Reverse (though in Drive it revs higher than normal)

Having searched the forum, it appears that the two things to look for are the Idle Air Control (IAC) valve, and a vacuum leak. I followed the testing procedures outlined in a Chilton's guide to check these (see image), and here are the results:

1. I tested the IAC valve by unplugging it's electrical connector. The idle immediately dropped and the car seemed to idle normally. But Chilton's didn't explain whether this means that the IAC is okay or not. From the way they wrote the paragraph, it seems like unplugging the connector is supposed to cause the idle to change, in which case the IAC valve is okay? They didn't give me a very good explanation about what it means.

2. I tested for possible vacuum leaks by plugging the forward hole inside the throttle body inlet. The idle dropped, as it's apparently supposed to, so I guess that means there are no vacuum leaks.

3. I also checked for any unplugged hoses, but there were none.

Anyone have any ideas about what may be wrong here? The car is not displaying any codes (well, it wasn't until I unplugged the IAC connector).

Thanks,

Vince


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RE: Idling problem, but the IAC appears to be okay?
 

it could be something as simple as needing to bleed off your radiator coolant level, because air in the system will cause surging or stumbling.

the worst possible scenario is a bad ECU.

try bleeding off your coolant following the chilton's instructions if it has it. it's easy to do.

RE: Idling problem, but the IAC appears to be okay?
 

What about dirty injectors?

RE: Idling problem, but the IAC appears to be okay?
 

I've had this problem before. Hopefully for you its just a dirty throttle body. A small film of dirt inside the throttle body will throw off the air intakeyou are receiving. Remove your air filter and clean with throttle body cleaner and it should be fine.

RE: Idling problem, but the IAC appears to be okay?
 

Check the bottom of your throttle body and see if there is an FITV. If so, that may be the culprit

search for my "Idle Surge Issue" thread, lots of info there