my son and I both own 02's and have both come up with the problem of when braking for a light and coming to a stop the engine drops down to about 450 to 500 rpm's and shakes like nothin else, but if you shift car into nuetral and let the r's come back up and then shift back into drive the car is fine.
It's not shooting any trouble codes and since it does it to both of our cars I'm hoping this is something that is something that others have already figured out and fixed so I don't have to go on a goosehunt looking for it.
Also let me mention that it only happens after the car is warmed up and also only on an average to harder stop, but if you have a long drawn out glide time to the light it is fine.

Any thoughts on what to zero in on to look into?

Thanks in advance

 

Idle Air Control Valve? Had a similar problem with a nissan maxima and it ended up being that. Sorry but I forget how we diagnosed that off the top of my head.? also check vacuum lines?

 

Happens once in while on my 02 as well. Never got to the bottom of it though. Checked the IAC valve and it was fine and all vac lines as well. Most of the time the A/C or defrost is on so idle stays up so no worries.

Sometimes if your spark plugs tubes come loose and move up oil gets down in there and causes misfires or erratic idle if the oil rises enough to get inside spark plug boot. Every older Neon eventually has the tubes come loose and pop up. Easy check, yank plug wire off and see if a tube is up higher than others and look down in the hole and see if oil is in there also covering spark plug. There is a sticky video on here how to fix those tubes.

 

I had the same problem with my '04 SXT. Did a tune up and it hardly does it anymore, and when it does, the RPMs don't drop nearly as much as they did. Before it would drop to ~5kRPMs. Now, if it does it, the RPMs don't go under 600. I think Briann510 is right about the spark plug thing. When I did my tune up I found plug #2 was pretty loose. I changed my plugs, wires, air filter, and oil/filter.

 

I have an 04 se and it does this exact same thing no matter what i do, it does it, warm or not!!!I've changed plugs, wires,cleaned intake thouroughly replaced stock air filter housing with a CAI and K&N ,both belts , the engine light doesn't/hasn't came on, ive put inserts on my motor mounts, new brakes ect. This has to be something that every neon does but the big ? is how do we fix it because its really irratating Also i have no rpm gauge so i dnt know exactly what rpms it idles at but if anyone has any more input please preech on!.. another thing ive started to notice a suttle knocking in the mornings when i start my car and its more often during the winter any suggestions

 

The knocking on cold startups is carbon buildup on the pistons. Mine had it horribly around 30K sounding like whole engine was shot and I took it and they covered it under warranty. They pulled the plugs and filled each hole up with top end cleaner Mopar sells let it sit over night and sucked it out then it was fine for another 30K and started doing it again. I just pulled the plugs myself and poured a bunch of top cylinder cleaner in each hole again and let it sit overnight, sucked it out next morning and it was fine. Its a common problem with these engines they said.

 

so jsut pull the plugs and fille the tubes , then just suck it out with a vaccum huh?

 

also could i buy this cleaner at autozone maybe?

 

This is just a shot in the dark here, but try checking your alternator belt. I noticed when my RPMs would drop, it was from some kind of fan kicking on under the hood, at least that's what it sounded like. Next time you're at a light, roll down your window and listen. I say to check your alt belt because if it's slipping at idle, you may be losing some electrical power. I forgot to mention that when I did my tune up, I changed the alt and accessory belts because one or both would squeal upon starting. And like I said before, I had the same problem before I did the tune up, and now, allthough I still get a slight decrease in RPMs when whatever it is under my hood starts humming, it's not nearly as bad as it was.
Hope this helps, good luck.

 

Dont fill the tubes themselves or overflow out of the plug hole. Just fill into the spark plug holes only. I read you can also use the top end cleaner sucked into the intake as well since its a foam it will coat everything and let it sit 30 minutes.

But yes once you pour it into each spark plug hole then you vacuum it out. I used a Mighty Vac. The dealer told me this is the best way to really get it cleaned up.