I have 97 Plymouth Neon with 132K miles (2.0L SOHC). My check engine light keep coming on with trouble code 51, which means the engine is running lean. However, my fuel mileage is about 2/3 of normal. I can find no vacuum leaks and repair shop replaced my Manifold Absolute Pressure (MAP) sensor. When this didn't work, they said I needed new ECU/computer. I got used one from salvage, after about 25 miles, check engine light & code 51 show up again.
Just prior to this code 51, I had gotten code indicating my O2 Sensor was bad, so I have just replaced both upstream/downstream sensors with Bosch from Autozone. After just a day or two, the check engine light came on again with the running lean code. So, now, I'm wondering if the O2 sensors being non-OEM, I'm getting bad O2 readings. Repair shop had told me when they replaced MAP sensor that O2 sensors were reading OK. Just seems like all this started when I put in the new O2 sensors. I'm thinking of just unplugging them, I think the engine performance would be better than it is right now.
Any suggestions of other things to check? Things like fuel injectors, or Mass Airflow sensors would have seemingly come up before the bad O2 sensors.
IT would be very helpful to have a scan tool. There are various possiblities from this code. Among them are a bad engine coolant temperature sender, fuel pump, ignition coil, ignition wires, fuel filter, MAP sensor, exhaust leak between the O@ sensors, fuel injectors, fuel pressure regulator, spark plugs, fuel pump relay and other problems you have addressed already as well.
It's a lean fuel condition. Is it lean from low fuel pressure? From poor spark? Or a mangement control problem?
I would start with replacing the plugs, wires, and fuel filter unless recently done. IT may not solve it, but these are maintence items anyways.
Checking the fuel pressure would be one test that could be done to start with also.
June, 4, 2007 AT 10:51 PM
I wish I had DRB-II or other scan tool.I'm occasional mechanic type (as if you couldn't tell!) And not really wanting to invest $300+ on car that's now worth less than $2K. My repair shop put the Neon on portable scanner and test drove it about 10 miles and said no abnormal readings from O2 sensors (or anything else) Then running lean code came up again after I drove it home, that's when they replaced MAP sensor. So it seems intermittent to me.
I appreciate your advise, I replaced the plugs, they looked like they had carbon deposits, making me think the engine is actually running rich. And it just thinks it's lean. So I lean toward " management control" problem instead of ignition or fuel delivery. But, I'll also replace the wires and check the fuel system pressure. Then keep you posted. Thanks.
June, 5, 2007 AT 8:19 AM
Check real close for vacuum leaks. Also be sure you have good bright spark out of the coil.
June, 5, 2007 AT 11:01 PM
I checked vacuum level on manifold and throttle body and get about 19 in-water vacuum both places. Stays stable while running, doesn't this tell me I don't have vacuum leak, or do I need to do more checking?
My engine light came on again today, so plugs didn't fix it. I changed plug wires tonight and checked fuel system pressure -- it averages about 49psi (consistent with Haynes manual), but I notice it fluctuates wildly (about +/- 15 psi), is this normal?
Lastly, I checked my fuel injector resistances & noid light pulses. I tried to check the Throttle Position Sensor but couldn't pick up a voltage at closed. Could my TPS cause excess fuel consumption and O2 sensor reading lean fuel?
June, 6, 2007 AT 7:49 AM
Fuel pressure should stay steady, then jump a few lbs when throttle is applied (revved) Sometimes comp's will call lean when not all the fuel is being burnt. Sounds wrong, I know but if there is clean oxygen in the exhaust, the PCM will think lean because the unburnt fuel doesnt dillute the oxygen.I would do a coil and wires (OEM parts)
June, 6, 2007 AT 9:52 PM
The fluctuation in my fuel pressure (at idle) is from about 43 to 57 psi (see attached picture). Could my pressure regulator be bad? In this Neon, the fuel regulator and fuel filter are part of fuel pump assembly and the filter/regulator are sold with pump, so it's costly to fix...so I'd like to be sure.
June, 7, 2007 AT 7:54 AM
I would hate to be wrong, I know the tank module is pricey, but I cant think of anything else that would make it fluctuate like that
June, 13, 2007 AT 8:29 PM
Well, I hate to report that I've replaced the fuel pump assembly and the lean fuel error code is on again. This unit included the fuel pressure regulator, fuel filter and pump. So, I've now replaced the O2 sensors, MAP sensor, PCM, spark plugs/wires and fuel regulator/pump/filter. I notice the fuel pressure still rapidly fluctuates with new assembly, but the low side is 48psi, where it was dipping lower before.
I'm sure the system is running rich because I removed the recently replaced O2 sensor and it's already black with carbon (and is already showing low voltage error code). I can put my hand behind the exhaust and black flecks show up.
I know a restricted catalytic converter can cause bad fuel mileage (I'm getting about 21mpg compared to 29-30 previously), would this explain my other symptoms. Ruining O2 sensors and causing bad readings to go to PCM for it to run rich (and think it's running lean)?
Anything else, I'm about to give up? Thank you,
June, 14, 2007 AT 7:55 AM
If you have an exhaust leak like you say, the o2 sensor will report a lean condition, the the pcm may adjust to a richer condition.
June, 17, 2007 AT 8:54 PM
Well, I thought about my problems. And I kept coming back to the fact that I didn't know of any issues until I got the O2 sensor check engine code, then all my other problems followed after I changed out the sensors. I decided to put in some O2 sensors from salvage Neon just to see if there could be issue with the new ones. Once I replaced to the salvage O2 sensors, I ran about 100 miles with no trouble code, so I took the new ones back to Autozone who happily swapped them for new ones (Bosch). Now, I have about 70 miles on the new ones with no trouble code.
I've got my fingers crossed that this is it. My gas mileage seems to be back up and I never made it past about 30 miles without trouble light previously.