'98 neon r/t misfire

[magtec]

alright, i realize that most of you don't have a thing for neons, but this is driving me absolutely crazy and the org has been no help so far... consider this post an act of desperation.

'98 neon r/t, 2.0 dohc, 5-speed

issue:
engine misfire (or so the pcm says). the ses light flashes constantly while cruising on the highway, it will also come on during aggressive acceleration. in general it seems kind of slow and boggy, and gas mileage isn't all that great. the idle is a bit lumpy (this is a 2.0, not a trans4) and it does pop a bit sometimes. other than that i haven't really noticed any issues. in fact, the car is dead reliable - i've driven it thousands of miles with no problems.

codes:
p0300 - random/multiple cylinder misfire
p0301 - cylinder 1 misfire
p0302 - cylinder 2 misfire
p0303 - cylinder 3 misfire
p0304 - cylinder 4 misfire

it never throws any other codes.

the diagnostics:
- swapped coil packs (twice)
- new wires
- new plugs (standard champions)
- new crank position sensor (replaced for unrelated issue)
- reattached the grounds to the chassis
- seafoam and fuel injector cleaner
- 185-185-195-190 compression
- fuel pressure at the rail is steady at ~50psi, even while the ses light is flashing

here's where it starts to get fun: in the process of troubleshooting, i installed what i thought was a '97-99 dohc mtx pcm. it turned out to be an atx pcm, but i ran it for a few weeks anyway. it never set a misfire code once. great, i thought - the old pcm must be bad. so i got my hands on a '99 r/t pcm and installed that. first test drive... flashing ses light again. seriously, wtf?

any suggestions would be greatly appreciated.

[TurboRon25]

Pins on connectors to the injectors and coil.

Since it went away and came back with computer swaps, check the pins on the PCM side too.

Shake the wiring harness to see if you can cause misfire.

Are the motor mounts good? Engine moving around can pull on some wires if they're not routed correctly. IDK, maybe not on a Neon.

Ron

[magtec]

ron, thanks for the quick response.

the pins on the pcm and harness look great, no corrosion or anything. all of the wiring is very clean (73k on the car). one suggestion that i haven't gotten to yet was to remove the wires from the coil pack plug and squeeze the tabs together, i'll try to get on that asap. i've moved the wires at the coil pack while it's running, but not the injectors. the engine harness isn't all that easy to grab and shake on the 2.0.

the front motor mount is bad. i've been itching to buy some prothane inserts for the car, but i wanted to figure out what was causing it to misfire before throwing more money at it.

[turbovanman²]

I would pull the fuel rail and injectors, due to Dodge and others being cheap azz's and deleting the external fuel filter, alot of crap gets into the fuel rail and plugs up the injectors. Had this happen on multiple Neon's and other products,

Once you remove the injectors, its pretty obvious if that's your problem.

[BadAssPerformance]

Its junk, sell it to me 4 cheep

[magtec]

alright, i'll pull the injectors off and take a look. might have to grab a set from the yard for test purposes.

yeah, jt, it's a real piece of junk.

[shadow88]

I would pull the fuel rail and injectors, due to Dodge and others being cheap azz's and deleting the external fuel filter, alot of crap gets into the fuel rail and plugs up the injectors. Had this happen on multiple Neon's and other products,

Once you remove the injectors, its pretty obvious if that's your problem.

After that, You'll probably be looking into a fuel filler neck being rotten out and allowing water and crap into the gas tank, causing all the rust and stuff.

[magtec]

the car is essentially rust-free. the fuel filler neck is still covered in glossy black paint. thanks for the suggestion, though.

[Dodge Aries K]

Eeew snow

[Endo]

Mine used to do this, turned out to be a sucked intake manifold gasket. Spray some carb cleaner at the surface where the head and the manifold come together, if the idle changes, thats your problem. Pretty common problem on those.

[shadow88]

the car is essentially rust-free. the fuel filler neck is still covered in glossy black paint. thanks for the suggestion, though.

That was to be checked after the injectors were found to be full of rust and junk. Were they?

Don't let good looking mean good.

[magtec]

replaced the injector o-rings while it was apart and triple checked the spark plug gap. the rail ends of the injectors are clean (and a bit harder to take good pictures of).

[turbovanman²]

No, you have to REMOVE the injectors from the fuel rail to inspect.

[magtec]

i did.

the rail ends of the injectors are clean (and a bit harder to take good pictures of).

i replaced both o-rings on each injector, gotta remove them from the rail to do that.

[magtec]

wiggled the harness at the injectors, coil pack, pcm, and anywhere else it could be moved. no change in idle. took the injectors off again to try to get pictures of the rail ends, but it's nearly impossible with my camera. from what i can see, there is no debris or buildup of any kind.

[dodgeshadowchik]

Check the wiring under the main fuse block two connectors a gray and black. The wires from these two connectors have been known to rub and become chaffed and short out.

Also check for vacuum leaks but the misfire would be there all the time if this was the problem. Any rough idle ect...

[shadow88]

You have a 99 pcm in a 98 car? For all the trouble you've gone through, I'm guessing you can't find the right one for testing purposes.

I'm liking the intake gasket idea, even though I've never seen it before.

[Dodge Aries K]

Neon PCM from 97-99 are virtually identical.

[magtec]

'97-99 neon PCMs are interchangeable. '95 and '96 are unique to their model years.

the harnesses under the fuse block look fine. the pins are all clean, pulled off the tape and exposed the wires about six inches back. no pinched wires, chafing, etc.

unplugged the knock sensor, it still sets all of the codes within a few miles of highway driving.

i'll grab a can of carb cleaner and check for leaks.

[magtec]

sprayed the intake cleaner around where the manifold meets the head and where the two pieces of the manifold join. also sprayed it around the throttle body and various vacuum connections. no change in idle.