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engine is idling very low around 100-200 on the tach. cel is code p0171.

it seem to drive fine, acceleration seems to have a weak spot in the power band in the 1700-2000 rpm range, but i dont expect much power at a low rpm.

where do i start. when the engine cools down im going to get under and spray with soapy water looking for vacuum leaks, tomorrow fuel filter from napa. after that if no improvements i dont know what to look for.

mpg has previously been recorded low 30s and occasional high 20s. i dont drive aggressive, slow accelerations and a 70/30 split highway/city. otherwise no previous problems fuel related.

all stock 97 civic lx

edit:Another detail i should add: cold start up it runs fine, around 1500 rpm. after engine is warm and coolant gauge reads operating temperature the idle drops very low.

 
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45 System too lean/rich Fuel supply system, Primary O2 sensor, MAP sensor, contaminated fuel, valve clearance, exhaust leakage


i guess this is my to do list to trouble shoot. is there anyway for me to test my o2 sensor just replacing it? what tools would i need to test?

 
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does anyone have any ideas?

I cleared the memory today after a couple attempts. Drove a few miles. idle rpm is still 100-200 range. no surging, just steady low idle. The CEL isn't on yet, but it will be soon.

here what it isnt.
clogged fuel filter. pcv valve. vacuum leak.

02 sensor, map sensor, fuel, valve clearance, fuel pump and exhaust leak unknown.

half of these i have no idea how to check. The pump primes but i haven't tested pressure. I'm leaning towards a bad 02 sensor since my previous mpg has been a lower than it should. Does anyone know how to test the rest of the list?

valve clearance is the only thing i know how to work on, i'll check tomorrow. I'm at a half tank, i'll fill up tomorrow with lc fp plus and cross my fingers.

 
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you know the starter cranks at 250rpm right? or idling at 100-200rpm is not enough to sustain combustion...i think.

 
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yes an the stock tach only goes down to around 500rpm, when my car has full heat and the outside temp is cold, it will idle at extremely low rpm until i turn the blower fan on and then it jumps to normal idle

 
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well, i guess if that is correct, then my tach isnt reading properly. its settles just above the 0 line about 1/4 (guessing the 100-200). Idle is considerably lower than it used to be and will stall now almost immediately after release of the clutch pedal if i dont rev it up near 1000 first.

the CEL persists though. its still reading p0171.

Another detail i should add: cold start up it runs fine, around 1500 rpm. after engine is warm and coolant gauge reads operating temperature the idle drops very low.

 
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Update:

first off i didnt troubleshoot enough to eliminate 1 thing at a time, i just did a whole mess of cheap fixes first. Fuel filter, checked all plugs condition even though all are near new, replaced a couple of vacuum lines, pcv valve, and searched for exhaust leaks w/o finding any. I cleared code+ idle relearn drove it with no improvement.

I cleared the code again, added fuel injector cleaner (lc fp plus) and filled the tank from 1/2 to full. I've driven about 30 miles including freeway and its idling fine, no CEL.

I dont know if it will stay off or idle will remain normal but for the time being i have a couple conclusions.

1. bad batch of gas.
2. fuel cap (if it was this would it be a fuel system rich code though)
3. plugged fuel injector (i have my doubts injector cleaned this quickly)
4. I dont think its an o2 sensor. I assume code would have come back by now if it were bad. MPG hasnt taken that huge of a hit to convince me either.
5. fuel pump/pressure regulator- unable to test pressure
6. map sensor-same as o2. i would assume additional code plus return of CEL

I dont know how the fuel tank vents it self, but is there a possible ventilation issue? as gas burns, if air doesn't take its place can that cause a fuel lean/starvation situation? I haven't owned the car long enough to go through a couple tanks yet to find a pattern.

any thoughts what to add to list or disagree with my conclusions?

 
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sounds like an o2 sensor problem, you need a solus so you can monitor the o2s

 
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if it were an o2 sensor, wouldn't the low idle and CEL come back almost immediately after ecu reset? I guess i would assume continuous idling issues if it were the o2 sensor. Mpg is down for my driving style, but not horrible at 30 mpg.

 
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Problem solved! i think the major contributing factor was ignition timing off by 4-5 degrees. Other fixes definitely helped, but this alone made it idle smoother & acceleration with out hesitation. MPG is the only thing left on my list after a couple tanks, i'll know more.

 
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how the hell did you get your timing that far out?

 
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bought the car 1 month ago. came that way... runs great now though

 
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ah, well glad you got it running right.