The IAC experiments - Idle on a 2003 SXT - Why oh why?

webkris · Post by **webkris** » Mon Mar 25, 2013 1:19 am

2003 Dodge Neon SXT MTX (Rally Car)
Bone stock right now.

When I first acquired this car I noticed that I couldn't rev match. I would be cruising in 4th gear - clutch in - blip the throttle...
BLIP... nothing
STAND ON THE THROTTLE... nothing
After a 2.5 - 3 seconds the RPM's would go up, and the transmission was already slowed and I had already completed my braking. Later I would coast to a stop and the car would just keep on at 1,300RPM's. This car suffered a front end crunch and the wiring on the passenger side had been chewed up by the AC. Needless to say it needed some TLC - So I chalked this rev / idle issue up to that.

Car was prepped to be raced. This meant pulling the motor, unbending the radiator support, and meticulously fixing all the wiring. Motor was re-installed. All emissions and lines, etc. As this car is in the Country of California and needs to pass. Which it did... Idle, rev, issue still present.

Just before going to a test event, the PCM stops fueling Cylinder 1 - and I scramble to get the last PCM in the country. I bring it to my dealer friends - they take care of me. I think - "Well at least that damn idle problem will go away..." Nope - same problem. No rev-match, motor continues to run on after rev. What the hell?!

This weekend I went looking for a junkyard IAC (Idle Air Control Valve) and I struck out, but it got me thinking: What is this and what does it do? Then I started an interesting experiment. First I looked at the voltage going to the IAC:
http://www.youtube.com/embed/CT-3FrFs3dQ

Notice that after revving hard - the car opens up the IAC and just lets it slowly trickle down - for a while... Was this an attempt to OVER SOLVE the Idle Under-run issues of the past by Chrysler? Not sure what other sensors besides Throttle and RPM control the IAC. Not even sure why it's needed after doing the next experiment...

I pulled it off the car and blocked off where it was with aluminum tape. I added a small hole to compensate. This allowed me to see the IAC in action. It was in fact moving in accordance with the voltage it was given. I noticed however that my idle seemed better. Why would the idle be better with the part that controls idle off the car? hmm...

I tried tricking the car into thinking it was there when it wasn't for about a minute and got bored resetting IAC voltage low codes with my laptop scan tool. I then carefully bypassed the IAC by drilling a hole in the aluminum that went from the filter box air to the other side of the valve (there is a lot of meat on the underside of the IAC). I then blocked off the large hole to the valve with aluminum tape, etc. Effectively: A small amount of air was always escaping past the throttle (1/8" hole), and the IAC no longer did anything, but the PCM saw it and didn't complain.

I can heel toe rev match the car now! The throttle response is still laggy, but I know that it's probably from the tiny plastic throttle body and not the PCM keeping the friggen IAC valve open for days. Throttle still seems to settle down slowly when I coast to a stop. It settles out at 700ish just like it did before. It kicks it up okay when the fans kick on.

Your Mileage May Vary
I've been reading about 100 "Idle issue" posts over the last 2 days - so what's your experience? Let's discuss.

- Kris

Post by **occasional demons** » Mon Mar 25, 2013 7:58 am

The slow return to closed used to be for emissions. Well, it is more of a carry over from carbureted days. At least on carbureted cars, it was to keep the manifold vacuum from pulling too much fuel from the carb on hard deceleration.

I imagine now it is for better driveability. I would think more of an MTX issue than ATX, except when coming to a fast stop. I know on the 1gn, with the IAC froze, (in the nearly closed position) it was a PITA to drive it at low speed stop and go. Kind of gave me a new appreciation for just how much it actually does.

I can agree with the way you did it, still giving it some control, but not full on. I know it can easily let the engine idle at 4000+ rpm just with the flow through the valve.