I'm new here to forum and have searched quite a bit but cannot findthesame problem that I am having with my Neon.
I have a 97 DOHC with mtx. Hereare my problems: If the car sits for 6 or more hours, I have to put my foot on the gas for about 5-10 seonds to get it started and keep it running - once it is goingI can take my foot off the gas and it works fine.......for a while. If it isa warm start it starts right up with no problems. The second issue I have is that once I get the car started from a cold start it operates properly until I reach operating temp on the guage and then my RPM's tend to hang around 1500 for 2-4 seconds when I come to a stop and then slowly decrease to idle (900). So far I have cleaned the throttle body and made sure that the butterfly andthrottle cable are not sticking; installed and tested output of TPS; cleaned IAC; replaced PVC valve; checked all vacuum lines and grounds, and made sure the heater was not on defrost. Would having no cat cause this problem because mine ended up in muffler? I read in the Haynes manual that if the PCM does not see an O2 sensor that it operates in closed loop....so if I disconnect the downstrem O2 sensorit should tell me that the cat is causing this right?
Oh yeah - I'm not getting any codes.
Any help would be appreciated.

 

You cleaned the IAC, but it's possible it's bad.

 

I did change the IAC with one from a junkyard as well and there was no change. How do I check to see if its working right? Is it a DC out put..if so what are the voltages? It is not in the Haynes manual.

 

You need a DRB scan tool to the test the IAC, from what I understand.

 

I changed the IAC with a new one and the car still acts the same. What systems or sensors start to work when the car gets up to temperature?

 

Sounds like you may have an electrical gremlin on your hands. Have you had the battery checked and/or looked at the fuel pump/filter?

 

The battery is good but I never had fuel pump or filter checked. It almost seems that it may be the alternator/voltage regulator causing the engine RPM's to increase. Where is the voltage regulator and what does it regulate at - 14 volts?

 

I had my Neon do the revving thing before the alternator went out on it. I would be going, the engine would rev around 1500-2000rpms, as soon as I came to a stop, it would go back down. It went away once I changed out the alternator. You can pick one up at the junk yard for around 20 to 40 bucks. I wouldnt buy a new one to trouble shoot, considering they are close to 200, if I recall correctly

 

Found my high idle trouble. I had a broken wire between my IAC valve and PCM. If you are having an idle issue and have tried everything this is how I narrowed it down to my IAC. I removed the TB and pulled out on the IAC to make sure it was closed on the TB so that no air would pass by. I then left the electrical connector to the IAC unplugged and started the car.....and it stalled, which meant that it was the IAC or its circuit. I then disconnected the electrical connector at the PCM and checked for continuity betweenthe IAC and PCM. Sure enough I had no resistance between the two points on one wire. I found the broken wire under the power distribution box. Spliced and soldered and put everything back together and I still can't believe it - my Neon works like it should.
I'm not sure if I mentioned this in the earlier posts but I had no CEL; Dodge; and another private garage scanned the car andcouldn't find this problem. It took 15 months of frustration to find this and lots of parts.
If this post helps you, let me know as I have seen a lot of idle issues on this forum