I just registered to see if anyone can help me with a problem.
I have an '03 Neon SXT that has a misfire when the weather is warm. Yes, I said the weather. When it's below about 80 degrees outside the car runs like a top, but above 80 degrees it misfires. The temp of the engine makes no difference. It scans as p0300 and occasionally also p0301, multiple misfire and cylinder 1 misfire. It also shows codes for the leak detection pump and the power steering pressure sensor but I wouldn't think either would cause a misfire. In the last 5 months it's had a full tune up, new cam position sensor and harness, nearly all vacuume hoses replaced, new intake gaskets and injector o-rings, new gas cap, and new wiring harness for the coil pack, and shortly before I bought it last fall the timing belt & water pump were replaced and they performed a compression test and all cylinders were with 10 psi.
I can't for the life of me figure out what's wrong or could be wrong that would be affected to much by outside temp, I've even had a mechanic friend look it over and he's not coming up with anything either and admits if I took it in they'd probably throw parts at it until something worked (which I don't want to pay for!)
Any help would be greatly appreciated!
I bought this car for a song because it needed a heater core and planned on flipping it after I changed it, but it's really grown on me, so I've decided to keep it but this misfire has me at wits end!

 

Welcome to the 2nd Gen Neon forum. Good idea to look for & post the misfire code Numbers. Also post the other code Numbers.
Seeing as how the plugs & wires have been replaced & the computer is unhappy with cyl-1, focus your touble shooting there.

We have a "Waste Spark" ignition & it works it's components Twice as hard as the old distributor ignition did, by firing our plugs on Every rotation of the crank shaft, once on the compression/power stroke & again on the exhaust stroke. This system also pairs two plugs on each coil, such that when one plug is firing on it's compression/power stroke, it's companion is firing on it's exhaust stroke. One of the 2 plugs also get a Reverse polatrity spark, that eminates from its ground lug to the center electrode, while its companion plug gets a normal polarity spark, that eminates from it's center electrode to the ground lug. All this info to outline the importance of using plugs & wires that can tolerate the Double work load.

I use fine wire iridium enhanced center electrode, with platinum pad side wire design plugs, as iridium has a higher melting point than standard nickel, or platinum electrodes, so the spark gap will stay in spec longer & the fine wire center electrode can fire at a lower voltage & make a hotter spark on those cold winter starts when the battery voltage is at its lowest & the ignition is working its hardest to get us going.

SO, all that Double work load puts extra electrical stress on our plug ceramics, plug wire insulation & the coil pack. Seeing as how the plugs share a coil but you don't have a misfire code for #1 cyl companion & the misfire is temp sensitive, it makes me want to suggest you look closely at #1 plug & plug wire, for damaged/cracked ceramics, cut plug wire insulaton, the plug wire not being Fully fastened, on Either end.

If you suspect a insulation problem but don't see any visable damage, try opening the hood after dark & with the engine at warm idle look for a arcs & sparks light show along the #1 plug wire run. Maybe consider doing a "Wet Down" test with a spray bottle of water, to see if you can induce an arcs & sparks light show from faulty insulation.

If no joy, remove #1 plug for a Close look at the internal & external ceramics for hair line cracks or chips.
If no joy there, maybe consider swapping the #1 & #4 plugs & see if the trouble code follows.
If no joy swap #1 & #4 spark plug wires & see if the misfire code follows.
If no joy, maybe perform another compression test on #1
If still no joy, maybe consider trying a one ounce per gallon Techron Concentrate Plus in the tank, at the pump before filling, so to get good mixing & run most of the treated tank out brfore refilling. Do daily rpm run up's above 3500 for a mile or two during the treatment.

A bunch of thoughts for consideration, let us know how it goes.

 

I've never heard of it on a Neon, but it could potentially be some sort of heat soak issue. I'd swap plugs, and then injectors, with another cylinder and see if the misfire moves. If so, you know what's bad.

 

I should have specified it was doing it before the new plugs, wires and coil pack too so they aren't bad.
I actually think I found the issue. I noticed last week I've got a slow coolant loss with no apparent leak, and a bit of white smoke from the exhaust. So I'm thinking my head gaskets going bad, which would explain the misfire (coolant diluting the fuel causing weak or no spark) but I'm not sure why it wouldn't do it all the time rather than just at higher ambient temps!?
So I've got a top end kit ordered and plan to pull the head and take it to the machine shop this week, and with luck I'll be back on the road and running fine by next weekend.

 

Ok, let us know what you find.

 

Got the top end finished, and while the head gasket was going bad I found the actual cause of the misfire on teardown, while unhooking the injectors I noticed some of the exposed wiring was discolored and brittle and after unwrapping the harness I found most of the wiring for injectors 1 & 2 was bad. Glad to have that FINALLY figured out, but I should have thought of it sooner considering most issues I've had have ended up being wiring.

 

Ok, way to hang in there on your trouble shoot & good find, fix, feedback & to hear the trouble shoot & rework has proved positive.
How do you suppose the fuel injector wiring got so over heated that it damaged the insulation? Had the factory positioned wiring harness been rerouted, or had something like a heat shield been removed? A curious mind would like to know if I too may have a problem cooking, so to speak?

 

I'm not sure how it got so hot, but it appears to me that rather than an outside source that the wiring itself got hot. I've actually had to replace the pigtails for the coil pack and the cam position sensor for the same reason.
I'm think I'm going to do some research as to exactly why this is happening though. I'm wondering if bad grounds are to blame?

 

[QUOTE=33lions;3351985]I'm not sure how it got so hot, but it appears to me that rather than an outside source that the wiring itself got hot. I've actually had to replace the pigtails for the coil pack and the cam position sensor for the same reason.
I'm think I'm going to do some research as to exactly why this is happening though. I'm wondering if bad grounds are to blame?[/QUOTE

If you suspect voltage drop grounding problems, you could use your DVM to make some under load battery to engine & body & engine to body voltage drop tests to see how those connections check out.
If your not familiar with voltage drop testing, have a read here on how to do it & what to look for. http://www.aa1car.com/library/voltage_drop_testing.htm