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Discussion Starter #1
Everyone:

Good morning. The mileage on my 2003 Neon has been steadily getting worse. I'm looking for ideas as to what's causing this issue and how I can fix it.

I own a 2003 Neon. 2.0 SOHC with an automatic transmission. Starting this past winter the mileage started getting worse. At first, I didn't mind because the change was so gradual. It went from 24 combined to 22. Now, I'm barely getting 20 mpg combined. I've been running my air conditioning continuously, but I don't remember the mileage dropping off this much in previous summers.

I do have a misfire on cylinder 3 and the engine idles roughly. I suspect this is the culprit. Again, my Neon has had this issue for years and the mileage has always been in the 24-28 combined range.

Any ideas or suggestions will be appreciated.

Ray
 

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Couple items could be attributing to this. One would be the obvious plugs and wires needing changed, they don't last forever. Second thing could be dirty injectors. When injectors get dirty, things like mileage will go down, but they can also attribute to burned valves and cause head gasket problems from the difference in temperatures between the cylinders and warp the head. It may be a good idea to do the plugs and wires, and while changing them, do a compression test to ensure nothing is damaged, cleaned injectors or at least fuel injector cleaner added to the tank to help that out. If you do find low compression (10 percent between the four cylinder pressures is normal), and ensuring the sparkplugs look the same (color and gap), all should be OK. As far as sparkplugs go, over time the gap gets wider from firing, too wide a gap can make mileage go down, a dirty injector will make the other cylinders run richer to compensate the gasses coming out the tailpipe, so start with the tune-up first, and add a new air filter to the list, it can choke the engine and cause gas mileage to go down, too.
 

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A misfire will kill fuel economy. With unburned oxygen going past the O2 sensor, the PCM will see this as a lean mixture and add more fuel to an already bad situation.
 

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Discussion Starter #4
Thanks, guys, for your replies. I will definitely look into changing the spark plugs. I'm sure the misfire is causing mileage problems. No one has every figured out, in my opinion, why my car misfires. It's done this since I got the car in 2008. It's been temporarily fixed by replacing the cylinder valve or replacing the catalytic converter, but a few miles down the road the engine begins misfiring again. I believe this is caused my a manufacturing flaw in this particular engine.
 

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Well, this information helps improve a little bit of the diagnosis. A replaced valve was from breaking a timing belt and having a bent valve, so at that point I would say it was also a replaced head gasket and possibly a grinding of the valves, at least on the bad cylinder, so all that can be taken out of the equation. The catalytic converter being changed, now that is an indication of there being an injector problem believe it or not because as noted with the fuel/air and computer compensation system, running too rich to offset running lean from one cylinder not having enough fuel, yes, a catcon will burn out faster. If the mileage was good and is slowly getting worse, that isn't a manufacturing flaw, that is simply something that is wrong slowly getting worse. Any check engine light by chance? True misfires will show up in the computer, and bad sensors (non factory sensor) can show up as well. I am thinking real hard on a dirty/bad injector not actually operating at its full capacity.
 

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Raphael said:
Thanks, guys, for your replies. I will definitely look into changing the spark plugs. I'm sure the misfire is causing mileage problems. No one has every figured out, in my opinion, why my car misfires. It's done this since I got the car in 2008. It's been temporarily fixed by replacing the cylinder valve or replacing the catalytic converter, but a few miles down the road the engine begins misfiring again. I believe this is caused my a manufacturing flaw in this particular engine.
My 2003 Dodge Neon with 2.0 liter 4 cylinder engine will develop an intermittent misfire at 12,000 - 15,000 mile intervals. I replaced the spark plug wires and and spark plugs and the misfire disappeared. But then it would return. A tip from Imperial Crown on this site sent me in the proper direction. Remove the spark plug wires and spark plugs and look at the creamic insulator on the plug. If you see a very small carbon track from the connector tip to the metal body, there is arcing that is occuring from inside the spark plug boot. Use coarse sand paper and remove all traces of the carbon arc. Reinstall the spark plugs (don't forget to use anti-sieze compound on the threads) and that should solve the problem for 12,000 - 15,000 miles.

Also make sure you use the proper specification Champion spark plugs. I tried to use other brands and there was immediate misfire. Seems strange that an engine only likes one particular brand of spark plug but that is the situation with the Neon I drive.
 

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Cleanliness while installing new Champion RC9YC spark plugs is important. Any dirt or oil on the spark plug insulator will give electricity a possible means to form an arc path to ground.
I smear a dab of silicone die-electric grease inside the rubber boot before I slip it over the spark plug. This gives some added measure of water-proofing and prevents the boot from sticking to the plug at the time of removal.
I'm not saying that plugs and wires are the cause of your misfire, you may want to do some investigation and diagnosis first.
Pay attention to spark plug tip color when removing them and keep them in order. The misfiring plug tip may look different from the other 3.
 
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