I purchased a '03 Neon SXT back in November, 67K w/A/T. It had 2 problems of which the first one, vibration, I took care by getting the 4 mount kit on eBay, and replacing all 4 mounts. The second problem is the head scratcher. When the car reaches operating temp and I come to stop at light or traffic the RPM's drop to about 550 without any check engine lights. The engine sound like it wants to stall but doesn't. If I turn on the A/C or defroster engaging the A/C compressor, the idle comes back to 750 RPM. I've replaced the TP sensor, MAP sensor, IAT sensor, IAC valve, checked to see if the throttle body was clean (it is) and even bought a refurbished PCM, that has the same results. There has to be someone out there who has had the same head scratching experiences I'm going thru. Anyone???

 

Welcome to the 2nd Gen Neon forum. I see you don't have any replies, so will see if I can get things started.

Good feedback on how the idle speed acts out. Seeing as how it idled up when turning the AC on, suggests the IAC & ECM were ok.

So lets think about this some more. When coming to a stop & it seems to want to stall, quickly shift to "N", or "P" & if that cures the problem it might be suggesting the torque converter isn't unlocking??? Thats sorta like stopping a straight drive without pushing the clutch in all the way.

SO, have you scanned for trouble code clues, if so, post All code Numbers, as they can help focus a trouble shoot. If you don't have a code reader, or scan tool like the inexpensive ELM & running a diagnostic software on the viewing device of our choice, to retrieve codes, or monitor live data, you can trigger the ECM to put up codes on the Dash display by cycling the ignition from Off to On 3 times, remaining in On the Third time & Posting All code Numbers. Some beginning thoughts for consideration, let us know what you find.

 

Well, I forgot to add that there are no codes. And to further add to the mystery, it seems the ambient temperature also plays into it. When the temperature is 40 degrees or lower the idle does not drop. When the temperature is 40 degrees or above that's when the "fun" starts. But as I posted, the IAT was replaced. This reminds me of my '86 Mustang GT with "Speed Density" which always gave me idle problems. I think that if the Neon had "Mass Air", this problem wouldn't exist.

 

Ok, that's good & interesting feedback about the idle acting out being temp sensitive. OBD-2 diagnostics is rather sophisticated compared to OBD-1, so if you can borrow or come by the ELM scan tool & running a good diagnostic software on the viewing device of your choice, so you can monitor whats going on with fuel trim, you might come by some viable candidates for your suspect list.

Were all those sensors you replaced checking bad, or were you just replacing them on a hunch???

 

Since I did not have a check engine light, and my scanner indicated no codes, I assumed that the problem was intermittent at best. I read postings on this and other forums, and replaced the components I previously mentioned. So, as you put it, "on a hunch". Unfortunately, my hunches were incorrect. I'll see if I can obtain or purchase a ELM scan tool and hopefully it will lead me in the right direction. Thanks!

 

Ok, sounds like you have some spares, so don't toss those replaced parts.

Lots of possibilities on the ELM scan tool & folks on the TCCA & FTE forums suggest considering Amazon vendor BAFX for the ELM, as they have been there for & back up the product they sell & warranty it to work. They've also been there in case of application problems too, so seem to be willing to go the extra mile for their customers, thus don't hesitate to ask them questions before purchase if need be.

The inexpensive ELM & a good diagnostic software can put up all kinds of live sensor data on the viewing device of our choice, so we can real-time monitor whats going on with fuel trim & other things that for instance affect idle quality on a problem like yours, so can help narrow down the suspect list.

The ELM comes in 3 models, corded UHB cable & cordless Bluetooth & WiFi models.
I came by mine back in 2014 & it helped me quickly find & fix my 99 Ranger suddenly appearing ABS trouble light & me working my way into a corner & not being able to go further because my Actron scan tool couldn't access the ABS, GEM, or Body computer / controllers, just the engine computer / controller. To have a Dealer or Brake shop perform a ABS scan was going to set me back Way more than the ELM, so I began to read up on them & the available diagnostic software & come by such. Nice addition to my electronic tool box as it covers all OBD-2 & CAN diagnostic systems since about 1995.
I opted for the cordless WiFi ELM & am running FORScan diagnostic freeware on my old WIN'-7 ACER laptop, as I wanted to graph some data on the screen, so to do that I was advised to consider opting for a WiFi model ELM, as it can handle more data & broad cast it over greater distance without dropping any data out, so keep those thoughts in mind when making your choice. Being Cordless, not having that UHB umbilical cord, the Bluetooth & WiFi models give Way more move about freedom. Some thoughts for consideration. When you get up & running, let us know what you find.

 

Well Gabriel how is the trouble shoot going, 256 curious minds would like to know???

 

have same prob. 05 sxt with 89000 miles. Always been that way so figured the norm.

 

Sit in car with doors closed and turn key to start position without starting. Do this 3 times and on the 3rd your dash will show codes needed. My obd did not show what my car did.

 

My 2005 neon does the same thing. I think it is the ngc trying to save fuel when at idle I always give it a little gas when it does that and it takes it right back to 800 rpms