What exactly does a car running "lean" mean?

[coogrr99]

1.  when a car is running lean is it getting too much air and not enough fuel, or the opposite?
2.  can a dirty fuel filter cause a car to run lean?
   a.   What if fuel pressure is good?
3. could bad plugs cause the same conditon?

[Jim Fairbanks]

Yes...running lean means it is getting to much air and not enough fuel...a dirty fuel filter could cause a lean condition...bad plugs would cause it to run to rich because it isn't burning the fuel like it should be...what kind of problems are you having....Jim

[coogrr99]

lean in banks 1&2.  i already replaced maf,vac lines and hoses,dpfe,intake plenum gasket,air filter,pcv valve,. i inspected rubber intake boot(its good),replaced 2 breather hoses. i sprayed carb cleaner around gaskets and hoses to see if idle raises.  but no success. Would a faulty O2 sensor cause a lean condition? I replaced 3 but i think there is one i missed. I didnt think that could cause lean readings in both banks.

[Jim Fairbanks]

Yours should have 4 on it like the first diagram....Does it run ok?...have you checked the fuel pressure?...how about a leak in the exhaust pipe?....Jim

[coogrr99]

I tested fuel pressure(its perfect) car runs great but i cant pass ny emmissions  dtc's 0171 0174. could a bad vapor charcoal canister be the problem  i accidently drilled into it when i was mounting an amp in my trunk and i sealed it back up with silicon. that was 5 years and i have never experienced this problem before. or could that last o2 sensor be causing this situation. Is there one O2 that monitors both banks?

[Jim Fairbanks]

What was your fuel pressure?...you could have a leaking injector....did you say you changed the plugs and wires?....did any of the plugs look really wet?...Jim

[dave_X86]

I have a Tacoma truck that developed a lean condition and the fuel filter was the problem.  The fuel pressure was good however the flow must have been restricted (it would go lean going up mountain passes) I simply replace the filter every year or two as routine maintenance it has 141k on it and it runs factory fresh.   

Also since you changed the MAF this should not be the problem but I have seen where people have put on new high volume air filters (I use one) that use a coating of oil that can gunk up the MAF sensor.  What happens is that the oil gets on the semiconductor used to measure the temperature it mixes with small dust particles and causes incorrect readings.  When this happens you can sense missing of the engine when your on the highway accelerating.  If this is the case you can clean the sensor with electronics cleaner. Thanks Dave

[coogrr99]

PLUGS ARE NOT WET AND FUEL PRESSURE IS 42-43@ IDLE AND 47-49@ REDLINE(OPEN THROTTLE)

[nickb2]

What is the engine, year make and color   Is this a ford cuz you did mention changing the dpfe sensor. If it a 3.8 with intake runners, this plastic clips which hold the rods to the runners areprobably broken or missing. See this all the time and this will cause a lean mixture if runners are not being actuated by the electric motor. Wow you changed alot of parts. Fuel pressure might be nice but have the injectors been cleaned. Might be clogged. Check also for cold exhaust leaks

[roland]

lincoln navigator; just had air filter changed.  when on park or stop light, engine seems to want to stall.  could this be directly to the changing of the air filter> if so, what do I do to fix.  thanks

[nickb2]

@Roland, you should start a new thread in the domestic car forum. This thread is over two years old. You will get more hits this way. So post back in the proper form and I will be happy to help you further as will the others. Please post year,make, model and engine. Post a symptom list of your troubles and what has been done to the car. 

[roland]

Thanks Bat, I hope I was able to post correctly.  Looking forward for your advice.
Roland