1998 Plymouth Neon Check Engine Light Problem

I have a 1998 Plymouth Neon that I got from my grandfather a couple of months ago, and I have been doing my best to keep it running. Starting last week, I noticed that at constant speeds of 60 mph, and at other times going past 60 mph, that the check engine light would come on. When I am driving below 60 mph, the check engine light will not come on, and the car runs smoothly. I have replaced all 4 spark plugs, spark plug wires, and the ignition coil and my car runs more smoothly now, but the check engine light still comes on greater than or equal to 60 mph. The engine is a 2.0L L4 SOHC, and I never noticed it before, but it looks as though my Uncle some time in the past added extra lights that were installed from Alamo Auto Supply, the battery cables look like they need to be replaced also, and I am working on ordering them right now, but I am not sure whether or not this could be the problem. While the check engine light is on, I had it diagnosed at auto zone, and it stated a cylinder misfire, and the guy there was trying to get me to replace the spark plugs, spark plug wires and the ignition coil again, but I already replaced those with new ones, and I was wondering if anyone would be able to help me pin point the problem, or if you have figured out for yourself the solution that perhaps your vehicle had the same problem as mine does. Another thing is my dad wants to replace the wiring because he believes that Alamo Auto supply dipped into the wiring, and I am still trying to persuade him from going to the junk yard to get that wiring, because I would rather have new. Any help would be greatly appreciated.

Have the “check engine” code read again, and this time write it down in “P0123” format (not just what the guy tells you the code means) and post it here. There are several different misfire codes, and without knowing the exact code no one can make an educated guess as to what’s happening.

Meanwhile, you’re going to need a competent, trustworthy mechanic who won’t take advantage of your youth. Check the “mechanics file” section of this website for a recommended mechanic near you. And stay away from “quick lube” places and repair shop chain stores.

The code that pops up is P0300 when I go past 60 mph, or at least that is what it reads.

That code indicates a random misfire.

I would first try adding a can of SeaFoam to the gas tank to see if the code is being caused from dirty/sticking injectors.



Here is one of a number of videos you can refer to for this issue. You may have a vacuum leak causing the issue.

I checked all hoses for leaks, checked the battery fuses and the alternator along with the starter, and they all registered to be fine. Now a new symptom has formed, it was easy to start the car when I first got it, but yesterday and today, I went to turn it over and it sputtered, I tried again and it finally started. I thought perhaps it was my fault, as in I might not have held it long enough, but my dad just tried to started it this morning and the same result happened. Any ideas?

You might think about changing out the crank sensor.

If you’re now having problems during a cold start, you might want to have the fuel pressure tested.

A weak fuel pump will not only cause the engine to be hard to start when cold, it can also cause a random misfire code.


Weak fuel pump was my thought too based on the high speed misfire. Now that it’s turned into a “low speed” misfire (ie hard starting) the fuel pump may be on its last legs. But before replacing the pump you might want to try changing the fuel filter first in case it’s clogged and causing the problem.

A little background info might be helpful to the OP. A “misfire” is when the engine computer notices the crankshaft doesn’t speed up like it should when a spark plug fired and should have ignited an explosion in the cylinder. P0300 means it isn’t happening on just one cylinder, the same cylinder in other words, but it is happening on several or all of them, with no rhyme nor reason which one. That the computer can figure out anyway.

So what causes the explosion? Fuel and air go into the cylinder. It gets compressed. A spark fires the compressed mixture. Bang. The crankshaft should speed up. Then the exhaust gas goes out the tailpipe.

If it doesn’t speed up the crankshaft, it has to be something wrong with one or more of those events.

Fuel problem, intake air problem, compression problem, no spark, weak spark, or spark at the wrong time, exhaust gas not being expelled quickly enough.

When I read your post, my immediate thought was the latter, something is preventing the exhaust gas from moving through the exhaust system and out the tailpipe quickly enough. An exhaust restriction would manifest itself this way, as more volume flow is required the faster you go. So while it is something of a wild guess, given the number of things that could cause this symptom, that’s my guess, some kind of exhaust restriction.

Well I bought myself three things, the Crankshaft position sensor, the Camshaft position center, and a fuel pressure tester to see if the Fuel pump is working or not. Let’s hope the sensors work. “fingers Crossed”

Should have also bought a fuel filter. Sounds like you have a lean fuel condition that could be caused by bad pump, plugged filter, bad MAF, etc. Fuel issue would be the first to look at though IMHO.

I was replacing the camshaft sensor, and I noticed something peculiar, does anyone know what this is? and what hose is supposed to connect to it? it is the black plastic part missing a hose, I do not recall removing the hose.

I don’t know what that gadget is for. hmmm … It’s on the far passenger side of the firewall it appears. And has a large hose connected to it. And a place to connect a vacuum-size hose, but the hose is missing. Do you see any electrical connector on it?

Stuff on the firewall tends to be for the HVAC system, or the brakes or clutch. Sometimes they’ll put a MAP sensor in that location as it is close to the intake manifold. Does all the stuff w/ your passenger compartment heating and cooling work?

The only thing that doesn’t work is the air conditioning, it blows warm, like there is no freon in the system.

The missing hose from the vacuum brake booster is for the heater/AC control.

That’s why there’s no AC.


Any thoughts on where the end of the hose connects to?

The extra port on the brake booster check valve is for the cruise control servo. If you don’t have cruise control replace the missing cap, they split and fall off.

That’s the thing we are supposed to have cruise control and it doesn’t work either.

It has to go to some sort vacuum reservoir for the HVAC system.

These are usually a black plastic ball. They sometimes locate these behind the front fender in front of one of the front tires.