Code 51: fuel system lean

Marvin threw a code 51 on me today. I am pretty good at diagnosing stuff for him but I am kinda hung up on this one.

Basically I have been getting CRAPtastic mileage my last few tanks. I normally get 300 miles to a tank, considering I use my AC in warmer weather and haul 100+ pounds of stereo equipment. But lately my low fuel lights been chiming on around 190-195 miles after a fill up.

I have checked all the logical things that can contribute to bad mileage: tire pressure (fine), spark plugs (fine), filter on intake (clean) fuel filter was recently replaced, and an O2 sensor was also replaced (i dont remember which one, but I 50% sure it was upstream).

What exactly causes your fuel to run lean and what rememdies that?

Well, you are getting to much air.

So how is the car running? Does it idle rough, does the exhaust smell funny, does it not accelerate as well, and does it ever hesitate while at cursing speeds?

I think that you may have a vacuum leak somewhere.

The exhaust does smell...not bad like rotten eggs which would be the cat, but more like the normal smell, just magnified. Idle is fine for the most part except if I sit at a red light too long it acts like it will stall momentarily. As far as performance while driving it feels fine except that my gas needle goes down quick LOL.

I talked to my service writer at the dealer...he's a good guy. He said I may need a new O2 sensor, and that the car would throw the Code 51 without the 21 or 23 for the sensor. Looking back thru paperwork, my upstream was replaced last year, my downstream is still original :eek: so I am inclined to replace that and see what happens.

As for vacuum leaks I checked everything under the hood, didn't see anything wrong and didnt hear any hissing noises....of course thats not to say I may not have a tiny leak in the system somewhere.

Still love the Neon anyway >sigh< :)

Yeah, could be an O2 sensor. If that doesn't fix it then it could be your EGR valve.

And that is another original part that's never been replaced either :eek: Guess for being at 152,000 miles I can't bitch too much, but when one sensor goes they all decide to go out at once. January was the camshaft and crankshaft pos. sensors. March was the radiator fan relay and coolant temp sensor...now this month the new coolant temp sensor blew and now this running lean crap. Well the car's paid off so a sensor here n there is still cheaper than going brand new again.

What does it mean if it idles a little rough, my winter car is an automatic and if i sit at a red light a little while in drive it starts to get a little rough, but if i slide to neutral it still sounds great.

How many miles on it? Several possiblilites are:

o2 sensor going (what I am going thru right now)
idle air controller
throttle position sensor
bad motor mounts (especially check your front mount)
bad spark plug(s) or wire(s)
throttle body may need cleaning
air filter may need to be changed/cleaned
fuel filter may be clogged


If you have a first gen. Neon you can check your OBD codes here:

http://www.neons.org/neontsb/diagnost.htm

It has 60k miles, Just got a new air and fuel filter for it, plugs are new and wires are fairly new. I pulled codes and got a 55 so i just dont know.

Could be a motor mount going then, if they are your originals. I had to replace mine at 51,000 miles. I have 152,000 now and its definitely time for a new set :eek:


Good news for me, just replaced both my up and downstream O2 sensors. No more check engine lite :D Next project new tires..probably this week or next .