Rough Idle- 2001 Plymouth Neon

I'm sure this is a common problem...

I own a 2001 Plymouth Neon, 96,000 miles.

When I start my engine cold, it's as smooth as glass. But when it gets to normal operating temperature, I have a rough idle at stop lights. When I shift from Drive to Neutral, the rough idle stops. When I shift from Neutral back to Drive, the engine still runs smooth.

Now here is what has been replaced new in the last 2,000 miles:

Spark Plugs
PCV Valve
O2 Sensors, downstream and upstream

Repair history shows that the following were repaired/replaced in the last year:

Engine Mounts
Belts

I think it's really strange that the engine stays smooth when going back to Drive from Neutral, AND that the car is smoother when it's cold. Do these factors help point to a specific problem?

Thanks for your help.

Mike

What type of plugs did you put in there?

The plugs I have in there are NGK.

I'm not real familiar with the 2nd gen neons, so I can't offer alot of help.

Rough idles, could be a number of things though. Plugs, wires, coil pack, vacuum lines and so on. Your CEL isn't on is it?

No, the CEL has never come on. The rough idle is more of annoyance than anything- the car runs great, especially after putting the new plugs in. I was just trying to rule out serious repair options like replacing the EGR valve, fuel filter, get the fuel injectors cleaned/replaced, etc. I also want to make sure it's not a serious issue like compression.

Now for vacuum lines, is there any way to *hear* hissing or is there a specific method for testing leaks? My simple visual inspection finds that the hoses look pretty cherry, but obviously I can easily miss something.

Also, did the clues I provide help at all, 1) car runs smooth cold and 2) stays smooth when shifting from N back to D? I'm pretty sure number 2 rules out engine mounts if I'm not mistaken.

I was considering getting the throttle body cleaned, but there's a little sticker on there that says to stay the heck away from it. Also I have already used a fuel injector cleaner bottle in the last 2,000 miles, so I probably shouldn't do that again for fear of damaging the injectors.

Thanks for your help, much appreciated.

Mahalo,

Mike

Cleaning the throttle body could help, and I don't know why there would be a sticker saying to stay away. It's a fairly simple removal, cleaning and installation.

So I cleaned the Throttle Body with Throttle Body Cleaner and the Mass Airflow Sensor with MAS cleaner. I notice MUCH better horsepower (sweet) but the rough idle is still there, *might* be a bit better. Progress nonetheless.

Other thoughts...the fuel filter for my year and model is listed in the books as "not serviceable" - which I guess it means that I can't get in there and change it out? I've also heard people say that the EGR valve sometimes causes trouble with idle. And then maybe it's the fuel injectors.

Man, there's so much crap that can go wrong with a car!

Wondering if anyone has any theories? Otherwise I guess I'll just leave it alone because it really is a non-issue. But if a few simple DIY fixes can make my car run better, I'm up for it!

Michael

PS- the sticker on my throttle body basically warned me about the special coating- so I didn't use a toothbrush, just some soft shop towels to clean inside.

Do you still have the rough idle after filling up? I am wondering if you just got some trashy gas.

I get 87 octane, Chevron Techron. I have heard stories of Hawaii gas stations adding water to mix, but these are usually the cheapo stations and I wouldn't expect it from Chevron. I do know that there's a certain level of ethanol required so that's probably kicking around in there too.

I don't really notice any difference before/after a fill up.

New idea- could it be a worn Transmission Mount, or the exhaust pipe touching the frame? This might explain why it only happens after warming up and why it remains smooth after doing the shift from Drive to Neutral to Drive again.

Wow since 2007 This thread has been up. I recently came across a 01 Neon and it has been great but i have now this exact issue which is amazing to see. Drives great but at the lights it really idles rough.

The poster never mentioned if he had fixed it with the transmission mount that was recommended or any way at all. Hmmm I wish I could figure it out. I had tried all the suggestions you all gave him as well.

I'm having the same issue in a 2005 Dodge Neon SXT. It seems like it's been getting worse and worse lately. 150k miles on the car. The timing belt was bad so I was hoping that fixing that would fix this problem but no dice. Just replaced a shot motor mount, still no luck. I tried higher octane gas... If anyone got this fixed could you please post a solution? Thanks so much!

No one else seems to have mentioned this: perhaps it has to do with the Transmission? Maybe after the engine warms up, the idle goes down and it struggles with moving parts in the Tranny. When was the last time the fluid/filter was changed in the tranny? Any hard shifting? Any gear change issues?