Hi, in the last year my Neon's 2.0 SOHC developed a rougher idle & idle only misfire, but power has not been sacrificed anywhere in the powerband, the engine is as powerful as ever and sometimes musters up a decent idle. I've noticed this gradually get worse since a year ago November when I had no choice but to throw in old gas which eventually lead to my fuel pump having random loud humming noises that would pop up every month or two followed by fuel starvation during the noises & near stalling out and almost no forward progress out of the car until I would -shut off & re-start the car. I've since cleaned out my fuel tank & injectors completely & changed the regulator/filter and fuel pump a month ago.

After the fuel pump change I notice a raw fuel smell out of the tail pipe, I have zero leaks and fuel pressure is 53lbs steady. All cylinders compression tested in the 160-170 range and engine has 252,000 miles on it. I have deleted my cat and replaced it with a glasspack and I have a truck muffler on the rear of the car, its been this way for years so I doubt it would just recently become a problem. I have no codes that pop up minus the down stream O2 sensor code from when I deleted the cat but I've always ran the car that way since 2007 with no issues.

I think my only guess at this point would be blown intake valve seals...I use a quart of oil a week if I pound it over 6,000 Rpm every day for that week which is probably 500 miles of poundings. It doesn't use any major amounts of oil if I treat it nice. Exhaust also has a slight oil smell. I changed my exhaust valve seals 2 years ago but due to the small spaces I could not pivot my 19mm wrench off the intake side rocker shafts to do the intake valve seals so I never did them.

Right now I'm stuck at thinking its probably a crappy pcm or intake valve seals is what I'm betting on. Engine past idle (1000rpm+) runs smooth & has a sack on it, enough to spank a 2000 2.5 V6 Avenger from a 25mph kick down to 90mph by a few cars. Pleased with it other than its using more gas lately & has a rough idle. I probably get 105 miles out of 4 gallons of gas. That was always an easy 130 miles last year.

 

I would pull the pull the injectors and make sure one isn't leaking. Then I would run a compression test, followed by a leak down test if the compression test shows poor results.

 

Compression test numbers were between 165-173 for all 4 cylinders. Wide open throttle tested & no oil added to the cylinders to get those numbers.

I had swapped injectors from a 2001 Neon into it, soaked them in Mystery Marvel oil first for an hour or two to disolve any possible rust or debris. Injector swap didn't make a difference in idle that I could tell. Fuel Pressure is 53psi at idle.

I did notice something kind of odd when I was testing injectors... I decided to try pulling injector connectors off while the engine was running and all of the cylinders except cylinder 2 makes the engine bog down regardless of what injector I've swapped in its place. I've swapped known working ones in and also swapped them back to their original positions and still the same thing with cylinder 2. Its just weird that cylinder 2 being unplugged still sounds as if 4 cylinders are running but intermittent roughness, while unplugging the other 3 one at a time makes it run rough & lower idle as expected.

Motor runs like a champ while driving, just idle sucks & exhaust has a raw fuel smell, maybe it is cylinder 2's injector harness being bogus? lol.